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1
BRONNIKOV, KIRILL A., und ALEXEI A. STAROBINSKY. „ONCE AGAIN ON THIN-SHELL WORMHOLES IN SCALAR–TENSOR GRAVITY“. Modern Physics Letters A 24, Nr. 20 (28.06.2009): 1559–64. http://dx.doi.org/10.1142/s0217732309030928.
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It is proved that all thin-shell wormholes built from two identical regions of vacuum static, spherically symmetric spacetimes have a negative shell surface energy density in any scalar–tensor theory of gravity with a non-ghost massless scalar field and a non-ghost graviton.
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MORDANE, Soumia, Kamal L. MAROIHI, Abdellatif ORBI und Mohamed CHAGDALI. „Une formulation parabolique pour la propagation en profondeur finie des ondes de gravité en surface“. Oceanologica Acta 24, Nr. 3 (Mai 2001): 287–94. http://dx.doi.org/10.1016/s0399-1784(01)01147-1.
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Longuet-Higgins, M. S. „The propagation of short surface waves on longer gravity waves“. Journal of Fluid Mechanics 177 (April 1987): 293–306. http://dx.doi.org/10.1017/s002211208700096x.
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To understand the imaging of the sea surface by radar, it is useful to know the theoretical variations in the wavelength and steepness of short gravity waves propagated over the surface of a train of longer gravity waves of finite amplitude. Such variations may be calculated once the orbital accelerations and surface velocities in the longer waves have been accurately determined – a non-trivial computational task.The results show that the linearized theory used previously for the longer waves is generally inadequate. The fully nonlinear theory used here indicates that for longer waves having a steepness parameter AK = 0.4, for example, the short-wave steepness can be increased at the crests of the longer waves by a factor of order 8, compared with its value at the mean level. (Linear theory gives a factor less than 2.)The calculations so far reported are for free, irrotational gravity waves travelling in the same or directly opposite sense to the longer waves. However, the method of calculation could be extended without essential difficulty so as to include effects of surface tension, energy dissipation due to short-wave breaking, surface wind-drift currents, and to arbitrary angles of wave propagation.
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Bechtel, S. E. „The Oscillation of Slender Elliptical Inviscid and Newtonian Jets: Effects of Surface Tension, Inertia, Viscosity, and Gravity“. Journal of Applied Mechanics 56, Nr. 4 (01.12.1989): 968–74. http://dx.doi.org/10.1115/1.3176198.
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The motion of inviscid and Newtonian jets issuing from elliptical orifices is analyzed. The analysis is not confined to small departures of the jet free surface from a circular cylindrical mean surface, but rather is fully nonlinear. Two types of behavior are predicted: (1) In the presence of surface tension the major axis of the elliptical jet cross-section alternates between perpendicular directions with distance down the jet. In this case the system is described as a single-degree-of-freedom nonlinear oscillator, conservative for the inviscid elliptical jet in the absence of gravity, and nonconservative for the Newtonian jet. (2) When surface tension is neglected, the transformation occurs only once, after which the jet flattens into a sheet perpendicular to the major axis of the orifice. The effect of gravity is discussed both for downward flowing jets and fountains.
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Benilov, E. S. „Oblique liquid curtains with a large Froude number“. Journal of Fluid Mechanics 861 (19.12.2018): 328–48. http://dx.doi.org/10.1017/jfm.2018.925.
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This paper examines two-dimensional liquid curtains ejected at an angle to the horizontal and affected by gravity and surface tension. The flow in the curtain is, generally, sheared. The Froude number based on the injection velocity and the outlet’s width is assumed large; as a result, the streamwise scale of the curtain exceeds its thickness. A set of asymptotic equations for such (slender) curtains is derived and its steady solutions are examined. It is shown that, if the surface tension exceeds a certain threshold, the curtain – quite paradoxically – bends upwards, i.e. against gravity. Once the flow reaches the height where its initial supply of kinetic energy can take it, the curtain presumably breaks up and splashes down.
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Mouyen, Maxime, Philippe Steer, Kuo-Jen Chang, Nicolas Le Moigne, Cheinway Hwang, Wen-Chi Hsieh, Louise Jeandet et al. „Quantifying sediment mass redistribution from joint time-lapse gravimetry and photogrammetry surveys“. Earth Surface Dynamics 8, Nr. 2 (22.06.2020): 555–77. http://dx.doi.org/10.5194/esurf-8-555-2020.
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Abstract. The accurate quantification of sediment mass redistribution is central to the study of surface processes, yet it remains a challenging task. Here we test a new combination of terrestrial gravity and drone photogrammetry methods to quantify sediment mass redistribution over a 1 km2 area. Gravity and photogrammetry are complementary methods. Indeed, gravity changes are sensitive to mass changes and to their location. Thus, by using photogrammetry data to constrain this location, the sediment mass can be properly estimated from the gravity data. We carried out three joint gravimetry–photogrammetry surveys, once a year in 2015, 2016 and 2017, over a 1 km2 area in southern Taiwan, featuring both a wide meander of the Laonong River and a slow landslide. We first removed the gravity changes from non-sediment effects, such as tides, groundwater, surface displacements and air pressure variations. Then, we inverted the density of the sediment with an attempt to distinguish the density of the landslide from the density of the river sediments. We eventually estimate an average loss of 3.7 ± 0.4 × 109 kg of sediment from 2015 to 2017 mostly due to the slow landslide. Although the gravity devices used in this study are expensive and need week-long surveys, new instrumentation currently being developed will enable dense and continuous measurements at lower cost, making the method that has been developed and tested in this study well-suited for the estimation of erosion, sediment transfer and deposition in landscapes.
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Konoplya, R. A., und A. Zhidenko. „How general is the strong cosmic censorship bound for quasinormal modes?“ Journal of Cosmology and Astroparticle Physics 2022, Nr. 11 (01.11.2022): 028. http://dx.doi.org/10.1088/1475-7516/2022/11/028.
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Abstract Hod's proposal claims that the least damped quasinormal mode of a black hole must have the imaginary part smaller than half of the surface gravity at the event horizon. The Strong Cosmic Censorship in General Relativity implies that this bound must be even weaker: half of the surface gravity at the Cauchy horizon. The appealing question is whether these bounds are limited by the Einstein theory only? Here we will present numerical evidence that once the black hole size is much smaller than then the radius of the cosmological horizon, both the Hod's proposal and the strong cosmic censorship bound for quasinormal modes are satisfied for general spherically symmetric black holes in an arbitrary metric theory of gravity. The low-lying quasinormal frequencies have the universal behavior in this regime and do not depend on the near-horizon geometry, but only on the asymptotic parameters: the value of the cosmological constant and black hole mass.
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Garstang, Michael, Steven Greco, George D. Emmitt, Tricia A. Miller und Michael Lanzone. „An Instrumented Golden Eagle’s (Aquila chrysaetos) Long-Distance Flight Behavior“. Animals 12, Nr. 11 (06.06.2022): 1470. http://dx.doi.org/10.3390/ani12111470.
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One-second-processed three-dimensional position observations transmitted from an instrumented golden eagle were used to determine the detailed long-range flight behavior of the bird. Once elevated from the surface, the eagle systematically used atmospheric gravity waves, first to gain altitude, and then, in multiple sequential glides, to cover over 100 km with a minimum expenditure of its metabolic energy.
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Welker, William V., und Donald L. Peterson. „A Surface-Roller Herbicide Applicator for Weed Control in Turf“. Weed Technology 3, Nr. 3 (September 1989): 472–74. http://dx.doi.org/10.1017/s0890037x00032590.
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A surface-roller wiper that rotates by direct contact with the ground was developed to apply herbicides to broadleaf weeds in turf. The free-wheeling roller 1.8 m wide consists of a 30-cm diam PVC pipe covered with a 1.3-cm thick carpet The herbicide is delivered by gravity to the carpet through a pipe manifold. Broadleaf weeds in turf were effectively controlled with 2,4-D roller applied once as a2%solution (2.2 kg ae/ha) or twice as a 1% solution (1.1 kg/ha). Herbicide drift was avoided with the roller applicator as indicated by lack of injury to tomato plants downwind from 2,4-D roller applied as a5%(5.6 kg/ha) solution.
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TJAN, K. K., und W. R. C. PHILLIPS. „On impulsively generated inviscid axisymmetric surface jets, waves and drops“. Journal of Fluid Mechanics 576 (28.03.2007): 377–403. http://dx.doi.org/10.1017/s0022112007004648.
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The evolution of an unbounded inviscid free surface subjected to a velocity potential of Gaussian form and also to the influence of inertial, interfacial and gravitational forces is considered. This construct was motivated by the occurrence of lung haemorrhage resulting from ultrasonic imaging and pursues the notion that bursts of ultrasound act to expel droplets that puncture the soft air-filled sacs in the lung plural surface, allowing them to fill with blood. The tissue adjacent to the sacs is modelled as a liquid and the air–tissue interface in the sacs as a free surface. The evolution of the free surface is described by a boundary-integral formulation and, since the free surface evolves slowly relative to the bursts of ultrasound, they are realized as an impulse at the free surface, represented by the velocity potential. As the free surface evolves, it is seen to form axisymmetric surface jets, waves or droplets, depending upon the levels of gravity and surface tension. Moreover the droplets may be spherical and ejected away from the surface or an inverted tear shape and fall back to the surface. These conclusions are expressed in a phase diagram of inverse Froude number Fr−1 versus inverse Weber number We−1. Specifically, while axisymmetric surface jets form in the absence of surface tension and gravity, gravity acts to bound their height, rendering them waves, although instability overrides the calculation prior to its reaching that bound. Surface tension acts to suppress the instability (provided that We−1 > 0.045) and to form drops; if sufficiently strong it can also damp the evolving wave, causing it to collapse. The pinchoff which effects spherical drops is of power-law type with exponent 2/3, and the universal constant that relates the necking radius to the time from pinchoff, thereby realizing a finite-time singularity, has the value ${\mathfrak{K}} \,{=}\, 0.45 \pm 0.025$. Finally, drops can occur once the mechanical index, a dimensional measure used in ultrasonography, exceeds 0.5.
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Juárez-Aubry, Benito A., Bernard S. Kay, Tonatiuh Miramontes und Daniel Sudarsky. „On the initial value problem for semiclassical gravity without and with quantum state collapses“. Journal of Cosmology and Astroparticle Physics 2023, Nr. 01 (01.01.2023): 040. http://dx.doi.org/10.1088/1475-7516/2023/01/040.
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Abstract Semiclassical gravity is the theory in which the classical Einstein tensor of a spacetime is coupled to quantum matter fields propagating on the spacetime via the expectation value of their renormalized stress-energy tensor in a quantum state. We explore two issues, taking the Klein Gordon equation as our model quantum field theory. The first is the provision of a suitable initial value formulation for the theory. Towards this, we address the question, for given initial data consisting of the classical metric and its first three 'time' derivatives off the surface together with a choice of initial quantum state, of what is an appropriate 'surface Hadamard' condition such that, for initial data for which it is satisfied it is reasonable to conjecture that there will be a Cauchy development whose quantum state is Hadamard. This requires dealing with the fact that, given two points on an initial surface, the spacetime geodesic between them does not, in general, lie on that surface. So the (squared) geodesic distance that occurs in the Hadamard subtraction differs from that intrinsic to the initial surface. We handle this complication by expanding the former as a suitable 3-dimensional covariant Taylor expansion in the latter. Moreover the renormalized expectation value of the stress-energy tensor in the initial surface depends explicitly on the fourth, 'time', derivative of the metric, which is not part of the initial data, but which we argue is given, implicitly, by the semiclassical Einstein equations on the initial surface. (The rôle played by those equations also entails that the surface Hadamard condition subsumes the constraints.) We also introduce the notion of physical solutions, which, inspired by a 1993 proposal of Parker and Simon, we define to be solutions which are smooth in ħ at ħ = 0. We conjecture that for these solutions the second and third time derivatives of the metric will be determined once the first and second time derivatives are specified. We point out that a simpler treatment of the initial value problem can be had if we adopt yet more of the spirit of Parker and Simon and content ourselves with solutions to order ħ which are Hadamard to order ħ. A further simplification occurs if we consider semiclassical gravity to order ħ 0. This resembles classical general relativity in that it is free from the complications of higher derivative terms and does not require any Hadamard condition. But it can still incorporate nontrivial quantum features such as superpositions of classical-like quantum states of the matter fields. Our second issue concerns the prospects for combining semiclassical gravity with theories of spontaneous quantum state collapse. We will focus our attention on proposals involving abrupt changes in the quantum field state which occur on certain (random, non-intersecting) Cauchy surfaces according to some — yet to be developed — generally covariant objective collapse model but that, in between such collapse surfaces, we have a physical semiclassical solution (or a solution of order O(ħ) or a solution of order O(ħ 0)). On each collapse surface, the semiclassical gravity equations will necessarily be violated and, as Page and Geilker pointed out in 1981, there will therefore necessarily be a discontinuity in the expectation value of the renormalized stress-energy tensor. Nevertheless, we argue, based on our conjecture about the well-posedness of the initial value problem for physical solutions, that, with a suitable rule for the jump in the metric and/or the extrinsic curvature, the time evolution will still be uniquely determined. We tentatively argue that a natural jump rule would be one in which the metric itself and the transverse traceless part of the extrinsic curvature will be continuous and the jump will be confined to the remaining parts of the extrinsic curvature. We aid and complement our discussion by studying our two issues also in the simpler cases of a semiclassical scalars model and semiclassical electrodynamics.
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Al-Habahbeh, Osama M. „Sustainable Design of an Artificial Lake in Jafer Basin Based on Gravity Flow“. Mathematical Modelling of Engineering Problems 9, Nr. 1 (28.02.2022): 101–10. http://dx.doi.org/10.18280/mmep.090113.
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Jordan is a dry country which badly needs water bodies. This work proposes a design of a sustainable artificial lake in Jafer basin (JB). The design involves excavating a tunnel to connect Gulf of Aqaba (GoA) and JB, and building a pumping station at the end of the tunnel. The tunnel will have a downslope gradient to allow gravity flow of seawater from GoA to JB. Once the water arrives underneath JB, a series of pumps along an inclined shaft uplift the water into the basin creating an artificial Lake, with total water volume of 2,000 Million Cubic Meters (MCM). The time required to fill up the basin is three years. The tunnel diameter is 3 meters and the depth of the tunnel and the shaft is 1000 m. 2% of the lake surface will be covered by Photovoltaic (PV) panels to produce electricity for pumping and to reduce evaporation rate. The total investment cost is 10.1 B$ (Billion Dollar) spent over three years, after that, all the pumping power needed to counteract evaporation is provided by PV panels. Once completed, the project is expected to pay back by encouraging human settlement, tourism, agriculture and industry.
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Smith, Frank T., und Phillip L. Wilson. „Body-rock or lift-off in flow“. Journal of Fluid Mechanics 735 (22.10.2013): 91–119. http://dx.doi.org/10.1017/jfm.2013.464.
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AbstractConditions are investigated under which a body lying at rest or rocking on a solid horizontal surface can be removed from the surface by hydrodynamic forces or instead continues rocking. The investigation is motivated by recent observations on Martian dust movement as well as other small- and large-scale applications. The nonlinear theory of fluid–body interaction here has unsteady motion of an inviscid fluid interacting with a moving thin body. Various shapes of body are addressed together with a range of initial conditions. The relevant parameter space is found to be subtle as evolution and shape play substantial roles coupled with scaled mass and gravity effects. Lift-off of the body from the surface generally cannot occur without fluid flow but it can occur either immediately or within a finite time once the fluid flow starts up: parameters for this are found and comparisons are made with Martian observations.
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Ogiriki, Shadrach Olise, Jennifer Oyindamola Adepoju, Adeyinka Sikiru Yusuff und Victor Anochie. „Physical Properties of Agbabu and Yegbata Bitumen in Nigeria“. Journal of Applied Science & Process Engineering 5, Nr. 1 (30.03.2018): 227–41. http://dx.doi.org/10.33736/jaspe.427.2018.
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This experimental work evaluates the physical properties of Abagbu and Yegbata bitumen from Nigeria with comparison with that of Canada being the world largest producer of crude oil from bitumen. This study employed the American Society for Testing and Materials (ASTM) methods in conducting laboratory experiments in order to determine the viscosity, specific gravity, API gravity, pour point and flash point. A Gas Chromatograph (GC) was used to determine the hydrocarbon content of the bitumen samples. Test results show that the bitumen samples have specific gravity of 1.01 with a 0.9962 OAPI for Yegbata, while the Agbabu bitumen sample had 8.599 specific gravity and 10.54 OAPI. Kinematic viscosity ranges of 1.0×102 – 3.3×104 and 1.6×103 – 5.6×104, flash point of 288oC and 282oC and pour point of 44oC and 47oC. The gas chromatography analysis showed that the samples contained 46.35% and 7.59% saturates, 21.63% and 64.39% aromatics and 32.03% and 28.01% resins for Agbabu and Yegbata respectively. In comparison with Athabasca bitumen, the results were similar. The assessment and comparison of these properties with the properties of bitumen from Athabasca in Canada reveals that any surface or subsurface crude bitumen gotten from Agbabu and Yegbata in Ondo State, Nigeria can be exploited using similar technologies, if not the same as the technologies being used in Athabasca, Canada
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Snyder, Chris, David J. Muraki, Riwal Plougonven und Fuqing Zhang. „Inertia–Gravity Waves Generated within a Dipole Vortex“. Journal of the Atmospheric Sciences 64, Nr. 12 (01.12.2007): 4417–31. http://dx.doi.org/10.1175/2007jas2351.1.
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Abstract Vortex dipoles provide a simple representation of localized atmospheric jets. Numerical simulations of a synoptic-scale dipole in surface potential temperature are considered in a rotating, stratified fluid with approximately uniform potential vorticity. Following an initial period of adjustment, the dipole propagates along a slightly curved trajectory at a nearly steady rate and with a nearly fixed structure for more than 50 days. Downstream from the jet maximum, the flow also contains smaller-scale, upward-propagating inertia–gravity waves that are embedded within and stationary relative to the dipole. The waves form elongated bows along the leading edge of the dipole. Consistent with propagation in horizontal deformation and vertical shear, the waves’ horizontal scale shrinks and the vertical slope varies as they approach the leading stagnation point in the dipole’s flow. Because the waves persist for tens of days despite explicit dissipation in the numerical model that would otherwise damp the waves on a time scale of a few hours, they must be inherent features of the dipole itself, rather than remnants of imbalances in the initial conditions. The wave amplitude varies with the strength of the dipole, with waves becoming obvious once the maximum vertical vorticity in the dipole is roughly half the Coriolis parameter. Possible mechanisms for the wave generation are spontaneous wave emission and the instability of the underlying balanced dipole.
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Jensen, O. E. „The spreading of insoluble surfactant at the free surface of a deep fluid layer“. Journal of Fluid Mechanics 293 (25.06.1995): 349–78. http://dx.doi.org/10.1017/s0022112095001741.
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The unsteady spreading of an insoluble monolayer containing a fixed mass of surface-active material over the initially horizontal free surface of a viscous fluid layer is investigated. A flow driving the spreading is induced by gradients in surface tension, which arise from the nonuniform surfactant distribution. Distinct phases in the flow's dynamics are distinguished by a time T = H02/v, where H0 is the fluid depth and v its viscosity. For times t [Lt ] T, i.e. before the lower boundary has any significant influence on the flow, a laminar sub-surface boundary-layer flow is generated. The effects of gravity, capillarity, surface diffusion or surface contamination may be weak enough for the flow to drive a substantial unsteady displacement of the free surface, upward behind the monolayer's leading edge and downward towards its centre. Similarity solutions are identified describing the spreading of a localized planar monolayer strip (which spreads like t1/2) or an axisymmetric drop (which spreads like t3/8); using the Prandtl transformation, the associated boundary-layer problems are solved numerically. Quasi-steady sub-layers are shown to exist at the centre and at the leading edge of the monolayer; that due to surface contamination, for example, may eventually grow to dominate the flow, in which case spreading proceeds like t3/4. Once t = O(T), vorticity created at the free surface has diffused down to the lower boundary and the flow changes character, slowing appreciably. The dynamics of this stage are modelled by reducing the problem to a single nonlinear diffusion equation. For a spreading monolayer strip or drop, the transition from an inertia-dominated (boundary-layer) flow to a viscosity-dominated (thin-film) flow is predicted to be largely complete once t ≈ 85 T.
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Sampietro, Daniele, und Martina Capponi. „A Novel Approach for Bathymetry Estimation through Bayesian Gravity Inversion“. Geosciences 13, Nr. 8 (26.07.2023): 223. http://dx.doi.org/10.3390/geosciences13080223.
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The bathymetry is the most superficial layer of the Earth’s crust on which it is possible to perform direct measurements. However, it is also well known that water covers more than 70% of the Earth’s surface, so an enormous expenditure of acquisition campaigns should be performed to produce a high-resolution map of this layer. Currently exploiting mainly commercial navigation routes, the sea floor coverage with shipborne sounding is only at 11%, and the remaining part is currently modeled by classical interpolation techniques or satellite-based gravity inversion methods. In the present work, a new method to refine bathymetry modeling based on the exploitation of global gravity field models is presented. In the proposed solution, once modeled and removed from the observed gravity field, the gravitational signals related to the deepest structures, a 3D Bayesian inversion algorithm is used to improve the actual knowledge of bathymetry. The proposed inversion method also enables limiting the solution to shipborne sounding measurements in such a way as to improve the seafloor grid where no local, high-quality information is available. Two test cases are discussed in the Mediterranean Sea region. Promising results are presented, opening the possibility of applying an analogous method to refine the bathymetry modeling at larger scales up to the global one.
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Dicezare, Marília T., und Eder C. Molina. „CHARACTERIZATION OF THE RIO GRANDE RISE FROM ELEMENTS OF THE TERRESTRIAL GRAVITY FIELD“. Revista Brasileira de Geofísica 36, Nr. 3 (06.09.2018): 1. http://dx.doi.org/10.22564/rbgf.v36i3.1950.
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ABSTRACT. The aim of this paper is to investigate the structural characteristics of the Rio Grande Rise, South Atlantic, through the analysis of the elements of the terrestrial gravity field. We used sea surface height (SSH) data and calculated sea surface gradients (SSG) from the ERS1-GM, Geosat-GM and Seasat satellite missions. By analyzing the sea surface heights it was possible to identify larger structures, such as the rift of the rise, some fractures and large seamounts. Sea surface gradients provided greater details of the features characterized by the SSH and, additionally, of the entire area, also revealing several other structures related to short wavelengths. The positioning of the features identified by both SSH and SSG is fairly accurate. Factors such as the direction and the orientation of the satellite tracks and the presence of adjacent structures may influence the SSG response to a given tectonic feature, making it important to analyze both ascending and descending sets of tracks from several missions to obtain better results. The study also allowed us to identify possible structures with a characteristic response of seamounts on SSH descending tracks, which were not previously characterized in the literature and do not have a similar correspondent in topographic/bathymetric models.Keywords: Sea Surface Height (SSH), Sea Surface Gradient (SSG), Rio Grande Rise (RGR), Satellite Altimetry.RESUMO. Este trabalho teve como objetivo investigar as características estruturais da Elevação do Rio Grande, no Atlântico Sul, através de elementos do campo de gravidade terrestre. Para isso, foram utilizados dados de altura da superfície do mar (SSH) e gradientes da superfície do mar (SSG) provenientes dos satélites das missões ERS1-GM, Geosat-GM e Seasat. Através da SSH foi possível identificar estruturas de maior porte, como o rift da elevação, algumas fraturas e montes submarinos maiores. A SSG forneceu maiores detalhes sobre as feições já caracterizadas pela SSH e de toda a região, revelando também diversas outras estruturas relacionadas aos comprimentos de onda curtos. O posicionamento das feições identificadas por ambas as grandezas é bastante preciso. Fatores como a direção e a orientação das trilhas dos satélites e a presença de estruturas adjacentes podem influenciar a resposta da SSG para uma determinada feição tectônica, sendo importante analisar os dois conjuntos de trilhas, ascendentes e descendentes, de várias missões para obter melhores resultados. O estudo também permitiu identificar possíveis estruturas com uma resposta característica de montes submarinos, nas trilhas descendentes de SSH, que não foram caracterizados anteriormente na literatura e não possuem correspondente nos modelos topográficos/batimétricos.Palavras-chave: Altura da Superfície do Mar (SSH), Gradiente da Superfície do Mar (SSG), Elevação do Rio Grande (RGR), Altimetria por Satélite.
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Chandler, Thomas G. J., und Philippe H. Trinh. „Complex singularities near the intersection of a free surface and wall. Part 1. Vertical jets and rising bubbles“. Journal of Fluid Mechanics 856 (04.10.2018): 323–50. http://dx.doi.org/10.1017/jfm.2018.708.
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It is known that in steady-state potential flows, the separation of a gravity-driven free surface from a solid exhibits a number of peculiar characteristics. For example, it can be shown that the fluid must separate from the body so as to form one of three possible in-fluid angles: (i) $180^{\circ }$, (ii) $120^{\circ }$ or (iii) an angle such that the surface is locally perpendicular to the direction of gravity. These necessary separation conditions were notably remarked upon by Dagan & Tulin (J. Fluid Mech., vol. 51 (3), 1972, pp. 529–543) in the context of ship hydrodynamics, but they are of crucial importance in many potential-flow applications. It is not particularly well understood why there is such a drastic change in the local separation behaviours when the global flow is altered. The question that motivates this work is the following: outside of a formal balance-of-terms argument, why must cases (i)–(iii) occur and furthermore, what are the connections between them? In this work, we seek to explain the transitions between the three cases in terms of the singularity structure of the associated solutions once they are extended into the complex plane. A numerical scheme is presented for the analytic continuation of a vertical jet (or alternatively a rising bubble). It will be shown that the transition between the three cases can be predicted by observing the coalescence of singularities as the speed of the jet is modified. A scaling law is derived for the coalescence rate of singularities.
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Filho, Nelson Ribeiro, Cristiano Mendel Martins und Renata de Sena Santos. „A NOVEL REGIONAL-RESIDUAL SEPARATION APPROACH FOR GRAVITY DATA THROUGH CRUSTAL MODELING“. Revista Brasileira de Geofísica 36, Nr. 4 (21.12.2018): 1. http://dx.doi.org/10.22564/rbgf.v36i4.1980.
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ABSTRACT. Gravity anomalies normally contain information of all sources beneath Earth’s surface. Once residual anomalies exhibit information about the main target, the knowledge of this specific residual signal is extremely important to interpretation. To find this signal, it’s necessary to perform regional-residual separation. We present here a new approach of separation by using gravity crustal modeling. We divide the surface in prisms, with density given by GEMMA. We calculate the regional signal, assuming Earth’s crust can be the source of observed anomaly. This methodology was applied on Barreirinhas basin-Brazil. Its formation is related to geologic events in South America-Africa break. Besides, the complex geology is the main obstacle on finding the residual anomaly. We compare our methodology with robust-polynomial fitting and spectral-analysis. They were not able to identify the residual anomaly. Main trouble relies on absence of crust information. Those kind of environment usually requires forward modeling and/or gravity inversion. On the other hand, our approach considers all crust’s parameters. Then the difficulty on choosing the residual no longer exists. The residual anomaly follows a geologic pattern. The crustal depocenter was mapped between structural faults. Therefore, our results satisfies the main expectation and are extremely linked to Barreirinhas basin’s geological background. We recommend this separation procedure, once Earth’s crustal model and gravity data are available for all planet.Keywords: Gravity modeling; GEMMA model; Barreirinhas basin; residual anomaly. RESUMO. Anomalias gravimétricas contêm informações de todas as fontes na superfície terrestre. Uma vez que anomalias residuais exibem informações sobre alvos principais, o conhecimento desse específico sinal residual é extremamente importante para interpretação. Para encontrá-lo, é necessário realizar separação regional-residual. Apresentamos aqui uma nova abordagem de separação utilizando a modelagem gravimétrica crustal. Discretizamos a superfície em prismas, com densidade fornecida pelo modelo GEMMA. Calculamos o sinal regional, assumindo que a crosta terrestre é a fonte da anomalia observada. Aplicamos esta metodologia na bacia de Barreirinhas - Brasil, que tem sua formação relacionada aos eventos geológicos de separação da América do Sul e África. Além disso, a complexidade geológica é considerada o principal obstáculo para encontrar esta anomalia residual. Comparamos nossa metodologia com Ajuste Polinomial Robusto e Análise Espectral. Essas técnicas não foram capazes de identificar a anomalia residual. O principal problema se dá pela ausência de informações acerca da crosta. Para esse ambiente, geralmente requer modelagem direta e/ou inversão geofísica. Por outro lado, nossa abordagem considera todos os parâmetros crustais e a dificuldade em escolher o residual deixa de existir. A anomalia residual apresenta um padrão geológico. O depocentro crustal foi mapeado entre falhas estruturais. Nossos resultados satisfazem a expectativa principal e estão extremamente ligados ao cenário geológico da bacia. Recomendamos este procedimento de separação, uma vez que os modelos crustais e dados gravimétricos estão disponíveis para todo o planeta.Palavras-chave: Modelagem gravimétrica; modelo GEMMA; bacia de Barreirinhas; anomalia residual
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Carpenter, Chris. „Machine-Learning Method Determines Salt Structures From Gravity Data“. Journal of Petroleum Technology 73, Nr. 02 (01.02.2021): 70–71. http://dx.doi.org/10.2118/0221-0070-jpt.
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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201424, “Machine-Learning Method To Determine Salt Structures From Gravity Data,” by Jie Chen, Cara Schiek-Stewart, and Ligang Lu, Shell, et al., prepared for the 2020 SPE Annual Technical Conference and Exhibition, originally scheduled to be held in Denver, 5-7 October. The paper has not been peer reviewed. In the complete paper, the authors develop a machine-learning (ML) method to determine salt structures directly from gravity data. Based on a U-net deep neural network, the method maps the gravity downward continuation volume directly to a salt body mask volume, which is easily interpretable for an exploration geophysicist. The authors conclude that the ML-based method from gravity data complements seismic data processing and interpretation for subsurface exploration. Introduction In subsurface exploration, seismic is the dominant method used to reconstruct the underground image for geophysicists and geologists to locate possible hydrocarbon reservoirs. Seismic acquisition is carried out by human-induced sound waves (by airgun or vibrators) that are recorded, once reflected, on the surface. Through the iterative waveform inversion process, a subsurface image can be reconstructed for reservoir location and property determination. Nonseismic (gravity and magnetic-measurement) methods, on the other hand, are passive measurements and not intrusive to the environment. In gravity data acquisition, gravimeters measure the change in the gravitation-al field, which can be used to determine the density variation on the subsurface. Compared with seismic acquisition, gravity acquisition is cheaper and introduces a much smaller carbon footprint. Gravity data resolution is, in principle, worse than that of seismic. However, especially in areas of salt structures, gravity data provide a unique addition because the density contrast between salt and the surrounding sediments in-creases with depth, while the velocity contrast decreases with depth. Therefore, gravity data provide valuable additional constraints in salt delineation for interpretation and seismic processing. Recently, ML and deep-learning (DL) applications in hydrocarbon exploration have been studied extensively. The authors note developments such as use of ML/DL on seismic data noise attenuation, salt interpretation from seismic stack, least-square inversion, rock-facies classification, and 4D seismic in reservoir management. To the authors’ knowledge, no literature exists that explores use of ML on nonseismic data. The authors’ method can map the gravity downward continuation volume directly to a salt body mask (0/1 for nonsalt/salt) volume, which saves iterative effort of the conventional gravity inversion process and is easily interpretable for explorational geophysicists and geologists. Gravity Data Processing Raw gravity data are measured as a 2D Bouguer anomaly (the difference between measured gravity and theoretical gravity value) grid. The first step of gravity inversion is to perform a downward continuation calculation to generate a 3D volume so that the depth of the density anomaly can be estimated. The equivalent source technique is one of the more-stable downward continuation calculations and is a preferred method for making downward continued volumes used in in-field reference drilling.
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Forslund, R. R. „The power function as a simple stem profile examination tool“. Canadian Journal of Forest Research 21, Nr. 2 (01.02.1991): 193–98. http://dx.doi.org/10.1139/x91-023.
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Formulae are presented for calculating the volume, surface area, centre of gravity, centre of mass, form, taper, and instantaneous slope of the power function. A profile plotting technique is also used that allows visual comparisons between average stem profiles independent of the size and taper of individual stems. The technique is used to demonstrate that the position near 30% of the height from the base of the stem is a position of form stability for volume estimation. Using the paracone profile model (a power function midway between a paraboloid and a cone) with the diameter at the 30% position and the total stem height, individual stem volume estimates within 10% of the true volume (95% confidence) are obtained once again using a sample of 50 yearling aspen (Populustremuloides Michx.). The plotting technique is then applied to the aspen sample, and the average profiles support the hypothesis that juvenile stems may begin life as paraboloids. The average centre of gravity, the average centre of mass, the sectional form, the average form, and the taper of the stems are also examined and are shown to be quantitative indicators of the plotted profile characteristics.
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Lindsay, Mark D., Sandra Occhipinti, Crystal Laflamme, Alan Aitken und Lara Ramos. „Mapping undercover: integrated geoscientific interpretation and 3D modelling of a Proterozoic basin“. Solid Earth 11, Nr. 3 (24.06.2020): 1053–77. http://dx.doi.org/10.5194/se-11-1053-2020.
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Abstract. Gravity and 3D modelling combined with geochemical analysis examine the subsurface within and below the poorly exposed Palaeoproterozoic Yerrida Basin in central Western Australia. Understanding the structure of a region is important as key features indicating past geodynamic processes and tectonic activity can be revealed. However, in stable, post-depositional tectonic settings only the younger sedimentary units tend to be widely exposed, rendering direct observation of basement and intrusive rocks impossible. Geophysical imaging and modelling can reveal the structure of a region undercover. High-magnitude density anomalies around the basin cannot be reconciled with current geological knowledge in the case presented here. The gravity anomalies infer an abundance of buried and high-density material not indicated by the surface geology. A hypothetical causative source for the high-magnitude gravity anomalies is mafic rocks that were intruded and extruded during basin rifting. The simplest and plausible stratigraphic attribution of these interpreted mafic rocks is to the Killara Formation within the Mooloogool Group. However, geochemistry reveals that the Killara Formation is not the only host to mafic rocks within the region. The mafic rocks present in the Juderina Formation are largely ignored in descriptions of Yerrida Basin magmatism, and results indicate that they may be far more substantial than once thought. Sulfur isotopic data indicate no Archean signature to these mafic rocks, a somewhat surprising result given the basement to the basin is the Archean Yilgarn Craton. We propose the source of mafic rocks is vents located to the north along the Goodin Fault or under the Bryah sub-basin and Padbury Basin. The conclusion is that the formation of the Yerrida Basin involves a geodynamic history more complex than previously thought. This result highlights the value in geophysics and geochemistry in revealing the complexity of the earlier geodynamic evolution of the basin that may be indiscernible from surface geology but may have high importance for the tectonic development of the region and its mineral resources.
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Karampelas, K., T. Van Doorsselaere und M. Guo. „Wave heating in gravitationally stratified coronal loops in the presence of resistivity and viscosity“. Astronomy & Astrophysics 623 (März 2019): A53. http://dx.doi.org/10.1051/0004-6361/201834309.
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Context. In recent years, coronal loops have been the focus of studies related to the damping of different magnetohydrodynamic (MHD) surface waves and their connection with coronal seismology and wave heating. For a better understanding of wave heating, we need to take into account the effects of different dissipation coefficients such as resistivity and viscosity, the importance of the loop physical characteristics, and the ways gravity can factor into the evolution of these phenomena. Aims. We aim to map the sites of energy dissipation from transverse waves in coronal loops in the presence and absence of gravitational stratification and to compare ideal, resistive, and viscous MHD. Methods. Using the PLUTO code, we performed 3D MHD simulations of kink waves in single, straight, density-enhanced coronal flux tubes of multiple temperatures. Results. We see the creation of spatially expanded Kelvin–Helmholtz eddies along the loop, which deform the initial monolithic loop profile. For the case of driven oscillations, the Kelvin–Helmholtz instability develops despite physical dissipation, unless very high values of shear viscosity are used. Energy dissipation gets its highest values near the apex, but is present all along the loop. We observe an increased efficiency of wave heating once the kinetic energy saturates at the later stages of the simulation and a turbulent density profile has developed. Conclusions. The inclusion of gravity greatly alters the dynamic evolution of our systems and should not be ignored in future studies. Stronger physical dissipation leads to stronger wave heating in our set-ups. Finally, once the kinetic energy of the oscillating loop starts saturating, all the excess input energy turns into internal energy, resulting in more efficient wave heating.
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Anderson, Jeffrey L., Bruce Wyman, Shaoqing Zhang und Timothy Hoar. „Assimilation of Surface Pressure Observations Using an Ensemble Filter in an Idealized Global Atmospheric Prediction System“. Journal of the Atmospheric Sciences 62, Nr. 8 (01.08.2005): 2925–38. http://dx.doi.org/10.1175/jas3510.1.
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Abstract An ensemble filter data assimilation system is tested in a perfect model setting using a low resolution Held–Suarez configuration of an atmospheric GCM. The assimilation system is able to reconstruct details of the model’s state at all levels when only observations of surface pressure (PS) are available. The impacts of varying the spatial density and temporal frequency of PS observations are examined. The error of the ensemble mean assimilation prior estimate appears to saturate at some point as the number of PS observations available once every 24 h is increased. However, increasing the frequency with which PS observations are available from a fixed network of 1800 randomly located stations results in an apparently unbounded decrease in the assimilation’s prior error for both PS and all other model state variables. The error reduces smoothly as a function of observation frequency except for a band with observation periods around 4 h. Assimilated states are found to display enhanced amplitude high-frequency gravity wave oscillations when observations are taken once every few hours, and this adversely impacts the assimilation quality. Assimilations of only surface temperature and only surface wind components are also examined. The results indicate that, in a perfect model context, ensemble filters are able to extract surprising amounts of information from observations of only a small portion of a model’s spatial domain. This suggests that most of the remaining challenges for ensemble filter assimilation are confined to problems such as model error, observation representativeness error, and unknown instrument error characteristics that are outside the scope of perfect model experiments. While it is dangerous to extrapolate from these simple experiments to operational atmospheric assimilation, the results also suggest that exploring the frequency with which observations are used for assimilation may lead to significant enhancements to assimilated state estimates.
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Taverna, R., W. Zhang, M. Dovčiak, S. Bianchi, M. Bursa, V. Karas und G. Matt. „Towards a complete description of spectra and polarization of black hole accretion discs: albedo profiles and returning radiation“. Monthly Notices of the Royal Astronomical Society 493, Nr. 4 (28.02.2020): 4960–77. http://dx.doi.org/10.1093/mnras/staa598.
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ABSTRACT Accretion discs around stellarmass black holes (BHs) emit radiation peaking in the soft X-rays when the source is in the thermal state. The emerging photons are polarized and, for symmetry reasons, the polarization integrated over the source is expected to be either parallel or perpendicular to the (projected) disc symmetry axis, because of electron scattering in the disc. However, due to general relativity effects photon polarization vectors will rotate with respect to their original orientation, by an amount depending on both the BH spin and the observer’s inclination. Hence, X-ray polarization measurements may provide important information about strong gravity effects around these sources. Along with the spectral and polarization properties of radiation which reaches directly the observer once emitted from the disc, in this paper we also include the contribution of returning radiation, i.e. photons that are bent by the strong BH gravity to return again on the disc, where they scatter until eventually escaping to infinity. A comparison between our results and those obtained in previous works by different authors shows an overall good agreement, despite the use of different code architectures. We finally consider the effects of absorption in the disc material by including more realistic albedo profiles for the disc surface. Our findings in this respect show that considering also the ionization state of the disc may deeply modify the behaviour of polarization observables.
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Liu, Xiaobo, Miao Yang, Dekun Zhou und Yuguang Zhao. „Microstructure and Wear Resistance of Mg2Si–Al Composites Fabricated Using Semi-Solid Extrusion“. Metals 10, Nr. 5 (02.05.2020): 596. http://dx.doi.org/10.3390/met10050596.
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In situ Mg2Si–Al composites were prepared by using gravity casting and semi-solid extrusion. After P modification, the primary Mg2Si transformed to polygonal blocks. Extraction tests showed that the Mg2Si crystals had octahedral and tetrakaidekahedral morphologies. The semi-solid microstructure of the double-spheroidized α-Al matrix and reinforced-phase Mg2Si was successfully obtained by using semi-solid extrusion. Extraction tests showed that the Mg2Si crystals had a spherical morphology. Dry sliding wear behaviors of in situ Mg2Si–Al composites fabricated by using gravity casting and semi-solid extrusion with isothermal heat treatment holding times of 50, 60, and 160 min against 45 steel, under conditions of different sliding speeds and loads, were investigated. The worn surfaces were analyzed using SEM and EDS techniques. The results showed that Mg2Si–Al composites fabricated by using semi-solid extrusion were superior in terms of wear resistance to Mg2Si–Al composites fabricated by using gravity casting, because the former had uniformly distributed spherical reinforced phase particles of Mg2Si with weaker stress concentration around the particles, delaying the generation and expansion of cracks. The Mg2Si particles were not easily detached from the matrix, and once they fell off, the Mg2Si particles only served as spherical abrasive grains, with relatively small cutting and wear properties for the composite material. It was found that the Mg2Si/Al composite fabricated by using semi-solid extrusion with an isothermal heat treatment holding time of 60 min had the best wear resistance. The failure mechanisms of Mg2Si/Al composites were found to be mainly adhesive wear and abrasive wear.
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Gimenes, Evandro, und Gabriel Fernández. „Hydromechanical analysis of flow behavior in concrete gravity dam foundations“. Canadian Geotechnical Journal 43, Nr. 3 (01.03.2006): 244–59. http://dx.doi.org/10.1139/t05-095.
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A key requirement in the evaluation of sliding stability of new and existing concrete gravity dams is the prediction of the distribution of pore pressure and shear strength in foundation joints and discontinuities. This paper presents a methodology for evaluating the hydromechanical behavior of concrete gravity dams founded on jointed rock. The methodology consisted of creating a database of observed dam behavior throughout typical cycles of reservoir filling and simulating this behavior with a distinct element method (DEM) numerical model. Once the model is validated, variations of key parameters including lithology, in situ stress, joint geometry, and joint characteristics can be incorporated in the analysis. A site-specific simulation of a typical reservoir cycle was carried out for Albigna Dam, Switzerland, founded on granitic rock, to assess the nature of the flow regime in the rock foundations and to evaluate the potential for sliding surfaces other than the damrock interface to develop. The factor of safety against sliding of various rock wedges of differing geometry present within the dam foundations was also evaluated using the DEM model and conventional analytical procedures. Estimates of crack propagation patterns and corresponding uplift pressures and factors of safety against sliding along the damrock interface obtained with the DEM were also compared with those from simplified procedures currently used in engineering practice. It was found that in a jointed rock, foundation uplift estimates after crack development obtained from present design guidelines can be too conservative and result in factors of safety that are too low and do not correspond to the observed behavior.Key words: hydromechanical, jointed rock, flow, dam design.
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Hu, Liwen, und Chaowei Yuan. „Analysis of Splice Loss of Single-Mode Optical Fiber in the High Altitude Environment“. Coatings 11, Nr. 8 (22.07.2021): 876. http://dx.doi.org/10.3390/coatings11080876.
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Up to now, there have been no complete theoretical researches and field experiment reports on the fiber fusion loss at high altitude. Therefore, we have conducted an exploratory study on the fiber splicing loss at high altitude, and firstly analyze the influence of mode field diameter mismatch, axial offset, angle tilt or end face gap affected by high altitude on splice loss, and then discuss the influence of fusion-splicing parameters on splice loss. Besides, a mathematical model for reducing the splicing loss of single-mode fiber at high altitude is established by combining the effects of temperature, humidity, oxygen content, atmospheric pressure, gale and gravity. We have conducted repeated field fusion experiments in different altitude areas (53, 2980, 4000, 4200, 4300, 5020, and 5200 m) more than once, hence obtaining a large number of field experimental data, making a deep comparison between typical “plain” area and typical “high altitude” area. The splice loss of most fusion points achieved successfully has been reduced by at least 0.07 dB. The simulation results are basically consistent with the theoretical analysis. Ultimately, the method proposed has been directly applied to on-site splicing engineering in high altitude environment and achieves good results.
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Whitehouse, Pippa L. „Glacial isostatic adjustment modelling: historical perspectives, recent advances, and future directions“. Earth Surface Dynamics 6, Nr. 2 (29.05.2018): 401–29. http://dx.doi.org/10.5194/esurf-6-401-2018.
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Abstract. Glacial isostatic adjustment (GIA) describes the response of the solid Earth, the gravitational field, and the oceans to the growth and decay of the global ice sheets. A commonly studied component of GIA is “postglacial rebound”, which specifically relates to uplift of the land surface following ice melt. GIA is a relatively rapid process, triggering 100 m scale changes in sea level and solid Earth deformation over just a few tens of thousands of years. Indeed, the first-order effects of GIA could already be quantified several hundred years ago without reliance on precise measurement techniques and scientists have been developing a unifying theory for the observations for over 200 years. Progress towards this goal required a number of significant breakthroughs to be made, including the recognition that ice sheets were once more extensive, the solid Earth changes shape over time, and gravity plays a central role in determining the pattern of sea-level change. This article describes the historical development of the field of GIA and provides an overview of the processes involved. Significant recent progress has been made as concepts associated with GIA have begun to be incorporated into parallel fields of research; these advances are discussed, along with the role that GIA is likely to play in addressing outstanding research questions within the field of Earth system modelling.
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Zhong, Wei, und Cong Yu. „The Critical Core Mass of Rotating Planets“. Astrophysical Journal 926, Nr. 1 (01.02.2022): 43. http://dx.doi.org/10.3847/1538-4357/ac4261.
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Abstract The gravitational harmonics measured from the Juno and Cassini spacecraft help us specify the internal structure and chemical elements of Jupiter and Saturn, respectively. However, we still do not know much about the impact of rotation on the planetary internal structure as well as on their formation. The centrifugal force induced by the rotation deforms the planetary shape and partially counteracts the gravitational force. Thus, rotation will affect the critical core mass of the exoplanet. Once the atmospheric mass becomes comparable to the critical core mass, the planet will enter the runaway accretion phase and become a gas giant. We have confirmed that the critical core masses of rotating planets depend on the stiffness of the polytrope, the outer boundary conditions, and the thickness of the isothermal layer. The critical core mass with the Bondi boundary condition is determined by the surface properties. The critical core mass of a rotating planet will increase with the core gravity (i.e., the innermost density). For the Hill boundary condition, the soft polytrope shares the same properties as planets with the Bondi boundary condition. Because the total mass for planets with the Hill boundary condition increases with the decrease of the polytropic index, a higher core gravity is required for rotating planets. As a result, the critical core mass in the stiff Hill model sharply increases. The rotational effects become more important when the radiative and convective regions coexist. Further, the critical core mass of planets with the Hill (Bondi) boundary increases noticeably as the radiative layer becomes thinner (thicker).
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Bernasconi, Diana, und Giorgio Guariso. „Rooftop PV: Potential and Impacts in a Complex Territory“. Energies 14, Nr. 12 (21.06.2021): 3687. http://dx.doi.org/10.3390/en14123687.
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When developing a sustainability plan in a complex and heavily urbanized territory, one of the most relevant options available is installing rooftop photovoltaic (PV) panels. Thus, it is essential to determine the amount of available surface and the potential impact of such installations on the energy and emission budget of the area. Instead of processing remotely sensed imagery, which is a long process and does not allow considering the buildings’ ownership, this study develops an approach based on a cluster analysis of the urban/morphological characteristics of the municipalities. Once a clear group diversification is obtained, the roof surface of the center of gravity of each cluster is extrapolated to all similar settlements. This, together with the information of local solar irradiation, allowed us to compute each cluster’s potential solar energy production and its capability to respond to the local energy demand, a key parameter to decide about the possibility of a local smart electricity network. Finally, the emissions avoided thanks to solar PV development are computed in terms of carbon dioxide and other relevant pollutants. This approach is applied to the residential rooftop of Lombardy, a Northern Italy region with a wide variety of urban morphologies and landscapes. The potential production of rooftop PV exceeds the estimated electricity consumption of residential buildings and would allow sparing almost 4 M ton of CO2 equivalent or 5% of the overall regional emissions.
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Shinder, Michael E., und Jeffrey S. Taube. „Three-dimensional tuning of head direction cells in rats“. Journal of Neurophysiology 121, Nr. 1 (01.01.2019): 4–37. http://dx.doi.org/10.1152/jn.00880.2017.
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Head direction (HD) cells fire when the animal faces that cell’s preferred firing direction (PFD) in the horizontal plane. The PFD response when the animal is oriented outside the earth-horizontal plane could result from cells representing direction in the plane of locomotion or as a three-dimensional (3D), global-referenced direction anchored to gravity. To investigate these possibilities, anterodorsal thalamic HD cells were recorded from restrained rats while they were passively positioned in various 3D orientations. Cell responses were unaffected by pitch or roll up to ~90° from the horizontal plane. Firing was disrupted once the animal was oriented >90° away from the horizontal plane and during inversion. When rolling the animal around the earth-vertical axis, cells were active when the animal’s ventral surface faced the cell’s PFD. However, with the rat rolled 90° in an ear-down orientation, pitching the rat and rotating it around the vertical axis did not produce directionally tuned responses. Complex movements involving combinations of yaw-roll, but usually not yaw-pitch, resulted in reduced directional tuning even at the final upright orientation when the rat had full visual view of its environment and was pointing in the cell’s PFD. Directional firing was restored when the rat’s head was moved back-and-forth. There was limited evidence indicating that cells contained conjunctive firing with pitch or roll positions. These findings suggest that the brain’s representation of directional heading is derived primarily from horizontal canal information and that the HD signal is a 3D gravity-referenced signal anchored to a direction in the horizontal plane. NEW & NOTEWORTHY This study monitored head direction cell responses from rats in three dimensions using a series of manipulations that involved yaw, pitch, roll, or a combination of these rotations. Results showed that head direction responses are consistent with the use of two reference frames simultaneously: one defined by the surrounding environment using primarily visual landmarks and a second defined by the earth’s gravity vector.
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Ahmadi, Saeb, Mostafa Hosseini, Ebrahim Tangestani, Seyyed Ebrahim Mousavi und Mohammad Niazi. „Wettability alteration and oil recovery by spontaneous imbibition of smart water and surfactants into carbonates“. Petroleum Science 17, Nr. 3 (02.01.2020): 712–21. http://dx.doi.org/10.1007/s12182-019-00412-1.
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AbstractNaturally fractured carbonate reservoirs have very low oil recovery efficiency owing to their wettability and tightness of matrix. However, smart water can enhance oil recovery by changing the wettability of the carbonate rock surface from oil-wet to water-wet, and the addition of surfactants can also change surface wettability. In the present study, the effects of a solution of modified seawater with some surfactants, namely C12TAB, SDS, and TritonX-100 (TX-100), on the wettability of carbonate rock were investigated through contact angle measurements. Oil recovery was studied using spontaneous imbibition tests at 25, 70, and 90 °C, followed by thermal gravity analysis to measure the amount of adsorbed material on the carbonate surface. The results indicated that Ca2+, Mg2+, and SO42− ions may alter the carbonate rock wettability from oil-wet to water-wet, with further water wettability obtained at higher concentrations of the ions in modified seawater. Removal of NaCl from the imbibing fluid resulted in a reduced contact angle and significantly enhanced oil recovery. Low oil recoveries were obtained with modified seawater at 25 and 70 °C, but once the temperature was increased to 90 °C, the oil recovery in the spontaneous imbibition experiment increased dramatically. Application of smart water with C12TAB surfactant at 0.1 wt% changed the contact angle from 161° to 52° and enhanced oil recovery to 72%, while the presence of the anionic surfactant SDS at 0.1 wt% in the smart water increased oil recovery to 64.5%. The TGA analysis results indicated that the adsorbed materials on the carbonate surface were minimal for the solution containing seawater with C12TAB at 0.1 wt% (SW + CTAB (0.1 wt%)). Based on the experimental results, a mechanism was proposed for wettability alteration of carbonate rocks using smart water with SDS and C12TAB surfactants.
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Edwards, Matthew R. „Indications from space geodesy, gravimetry and seismology for slow Earth expansion at present – comment on “The Earth expansion theory and its transition from scientific hypothesis to pseudoscientific belief” by Sudiro (2014)“. History of Geo- and Space Sciences 7, Nr. 2 (22.12.2016): 125–33. http://dx.doi.org/10.5194/hgss-7-125-2016.
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Abstract. In a recent article in this journal, Paolo Sudiro (2014) considered the long history of the expanding Earth theory and its recent descent into what he termed “pseudoscientific belief”. The expanding Earth theory contends that the radius of the Earth was once one-half to two-thirds of its current value, with the Earth's continents forming a continuous sialic cover over the Earth. The theory has had two main variants: slow expansion at about 0.5 mm yr−1 radial increase since the time of Earth's formation and fast expansion at about 5 mm yr−1 since the Triassic. Focusing on Maxlow's model, Sudiro thoroughly addresses the possibly insurmountable difficulties of the fast version, such as an improbably high density and surface gravity prior to 200 Ma. He omits, however, any discussion of the slow expansion model, which has a longer history and far fewer theoretical difficulties. Moreover, recent evidence from space geodesy, gravimetry and seismology indicates that the Earth at present may be slowly expanding at 0.1–0.4 mm yr−1. It is concluded that Sudiro's obituary of the expanding Earth theory as a whole must be considered premature at this time.
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Adamaref, Solmaz, Weizhu An, Maria Ophelia Jarligo, Tetyana Kuznicki und Steven M. Kuznicki. „Natural clinoptilolite composite membranes on tubular stainless steel supports for water softening“. Water Science and Technology 70, Nr. 8 (13.09.2014): 1412–18. http://dx.doi.org/10.2166/wst.2014.385.
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Disk membranes generated from high-purity natural clinoptilolite mineral rock have shown promising water desalination and de-oiling performance. In order to scale up production of these types of membranes for industrial wastewater treatment applications, a coating strategy was devised. A composite mixture of natural clinoptilolite from St. Cloud (Winston, NM, USA) and aluminum phosphate was deposited on the inner surface of porous stainless steel tubes by the slip casting technique. The commercial porous stainless steel tubes were pre-coated with a TiO2 layer of about 10 μm. Phase composition and morphology of the coating materials were investigated using X-ray diffraction and scanning electron microscopy. Water softening performance of the fabricated membranes was evaluated using Edmonton (Alberta, Canada) municipal tap water as feed source. Preliminary experimental results show a high water flux of 7.7 kg/(m2 h) and 75% reduction of hardness and conductivity in a once-through membrane process at 95 °C and feed pressure of 780 kPa. These results show that natural zeolite coated, stainless steel tubular membranes have high potential for large-scale purification of oil sands steam-assisted gravity drainage water at high temperature and pressure requirements.
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KAY, ANTHONY. „Warm discharges in cold fresh water. Part 1. Line plumes in a uniform ambient“. Journal of Fluid Mechanics 574 (15.02.2007): 239–71. http://dx.doi.org/10.1017/s0022112006004101.
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Turbulent buoyant plumes in cold fresh water are analysed, assuming a quadratic dependence of density on temperature. The model is based on the assumption that entrainment velocity is proportional to vertical velocity in the plume. Numerical and asymptotic solutions are obtained for both rising and descending plumes from virtual sources with all possible combinations of buoyancy, volume and momentum fluxes. Physical sources can be identified as points on trajectories of plumes from virtual sources.The zero-buoyancy condition, at which the plume and the ambient have equal densities but their temperatures are on opposite sides of the temperature of maximum density, is of particular importance. If an upwardly buoyant plume rising through a body of water reaches the surface before passing through its zero-buoyancy level, it will form a surface gravity current; otherwise, the plume water will return to the source as a fountain. The height at which zero buoyancy is attained generally decreases as the source momentum flux increases: greater plume velocity produces greater entrainment and hence more rapid temperature change. Descending plumes, if ejected downwards against upward buoyancy, may be classified as strongly or weakly forced according to whether they reach the zero-buoyancy condition before being brought to rest. If they do, they continue to descend with favourable buoyancy; otherwise, they may form an inverted fountain. Once a descending plume has attained downward buoyancy, it can continue to descend indefinitely, ultimately behaving like a plume in a fluid with a linear equation of state. In contrast, a rising plume will eventually come to rest, however large its initial upward buoyancy and momentum fluxes are.
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Barbosa, Valéria C. F., João B. C. Silva und Walter E. Medeiros. „Stable inversion of gravity anomalies of sedimentary basins with nonsmooth basement reliefs and arbitrary density contrast variations“. GEOPHYSICS 64, Nr. 3 (Mai 1999): 754–64. http://dx.doi.org/10.1190/1.1444585.
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We present a new, stable method for interpreting the basement relief of a sedimentary basin which delineates sharp discontinuities in the basement relief and incorporates any law known a priori for the spatial variation of the density contrast. The subsurface region containing the basin is discretized into a grid of juxtaposed elementary prisms whose density contrasts are the parameters to be estimated. Any vertical line must intersect the basement relief only once, and the mass deficiency must be concentrated near the earth’s surface, subject to the observed gravity anomaly being fitted within the experimental errors. In addition, upper and lower bounds on the density contrast of each prism are introduced a priori (one of the bounds being zero), and the method assigns to each elementary prism a density contrast which is close to either bound. The basement relief is therefore delineated by the contact between the prisms with null and nonnull estimated density contrasts, the latter occupying the upper part of the discretized region. The method is stabilized by introducing constraints favoring solutions having the attributes (shared by most sedimentary basins) of being an isolated compact source with lateral borders dipping either vertically or toward the basin center and having horizontal dimensions much greater than its largest vertical dimension. Arbitrary laws of spatial variations of the density contrast, if known a priori, may be incorporated into the problem by assigning suitable values to the nonnull bound of each prism. The proposed method differs from previous stable methods by using no smoothness constraint on the interface to be estimated. As a result, it may be applied not only to intracratonic sag basins where the basement relief is essentially smooth but also to rift basins whose basements present discontinuities caused by faults. The method’s utility in mapping such basements was demonstrated in tests using synthetic data produced by simulated rift basins. The method mapped with good precision a sequence of step faults which are close to each other and present small vertical slips, a feature particularly difficult to detect from gravity data only. The method was also able to map isolated discontinuities with large vertical throw. The method was applied to the gravity data from Reco⁁ncavo basin, Brazil. The results showed close agreement with known geological structures of the basin. It also demonstrated the method’s ability to map a sequence of alternating terraces and structural lows that could not be detected just by inspecting the gravity anomaly. To demostrate the method’s flexibility in incorporating any a priori knowledge about the density contrast variation, it was applied to the Bouguer anomaly over the San Jacinto Graben, California. Two different exponential laws for the decrease of density contrast with depth were used, leading to estimated maximum depths between 2.2 and 2.4 km.
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Sterne, Edward J. „Structure and genesis of the Boulder-Weld allochthon, Denver Basin, Colorado - Gravity slide or Laramide thrust sheet?“ Mountain Geologist 57, Nr. 3 (01.07.2020): 271–304. http://dx.doi.org/10.31582/rmag.mg.57.3.271.
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This study was undertaken to determine the structure and genesis of the Boulder-Weld allochthon (BWA), the 216 mi2 (559 km2) remnant of a once larger feature, that moved east from the flank of the Front Range into the western part of the Denver Basin. This review of surface and subsurface data revealed new aspects of the BWA, especially in its western part. There, the decollement of the BWA ramps 900 feet up-section to the east from a near bedding-parallel detachment low in the upper transition member of the Pierre Shale to a bedding-parallel detachment near the base of the Fox Hills Formation. Repeated sections found in wells east of the decollement ramp demonstrate up to two miles of translation in the system. Secondary faults in the hanging wall of the allochthon include antithetic thrusts bounding pop-up structures and occasional normal faults that almost exclusively overprint the decollement ramp. The hanging wall is also cut by a postulated tear fault separating areas exhibiting different amounts of translation. The western, trailing edge of the decollement shows attenuation in its hanging wall that increases to the west. This part of the decollement either represents a very low-angle breakaway normal fault or a thrust fault cutting slightly down-section in the direction of transport. Past studies perceived a southeast transport direction for the BWA in contrast to the northeast slip directions on nearby Laramide thrusts, a difference used to interpret the allochthon as a gravity slide. However, similar east-west oriented slickenlines on thrusts across the western part of the allochthon and into the neighboring Front Range leave open the possibility the BWA originated as a Laramide thrust sheet. Furthermore, both the BWA and Laramide thrusts in the neighboring Front Range utilized detachments near the top of the Pierre Shale, suggesting a possible common genesis. Given the available data, both the gravity slide and Laramide thrust models provide viable explanations for the BWA.
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Sayahi, Emborg, Hedlund und Cwirzen. „Plastic Shrinkage Cracking in Concrete“. Proceedings 34, Nr. 1 (18.11.2019): 2. http://dx.doi.org/10.3390/proceedings2019034002.
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Plastic shrinkage cracking in concrete is mainly a physical process, in which chemical reactions between cement and water do not play a decisive role. It is commonly believed that rapid and excessive moisture loss, due to evaporation is the primary cause of the phenomenon. Once the concrete is cast, its solid particles start to settle due to gravity, causing an upward water-flow from the concrete interior and through its pore system to the surface, i.e., bleeding regime. When the amount of the evaporated water exceeds the amount of the water accumulated at the concrete surface, i.e., bleed water, concrete enters the so called drying regime, during which water menisci form inside the pores causing a build-up of a negative pore pressure, also known as capillary pressure. The progressive evaporation gradually decreases the radii of the menisci, which causes a further increase of the pore pressure and solid particles consolidation. Eventually, the skeleton of the concrete becomes stiff enough to resist the gravitational forces, which means that the vertical deformation of the concrete either completely stops or continues at a much lower rate. At this point, the capillary pressure is no longer able to further consolidate the concrete and move the pore water towards the surface. Instead, the developed tensile forces reduce the inter particle distances and the horizontal deformation continues. If the concrete member is restrained (e.g., due to reinforcement, variation in sectional depth, the friction of the form, etc.), the shrinkage can lead to tensile stresses accumulation. Once the tensile stresses exceed the early age tensile strength of the concrete, cracks start to form, preparing passageways for ingress of harmful materials into the concrete interior, which eventually may impair the durability and serviceability of the structure. This abstract reports the findings of a PhD research, carried out at Luleå University of Technology (LTU) to investigate the impact of parameters such as, admixtures, water-cement ratio (w/c), cement type, dosage of superplasticizer (SP), and steel fibers, on concrete’s cracking tendency while in plastic state. The results show that presence of accelerators, retarders, coarser cement particles, high w/c, and more SP increases the cracking risk, while stabilizers, air entraining agents (AEA), shrinkage reducing admixtures (SRA), and steel fibers notably decrease the cracking potential. Based on the findings of the above mentioned investigation a new model is proposed to estimate the severity of plastic shrinkage cracking, based on the initial setting time and the amount of the evaporated water from within the concrete bulk. The experimental results of the PhD research, alongside those reported by other researchers, were utilized to check the validity of the proposed model. According to the outcomes, the model could predict the cracking severity of the tested concretes with a good precision.
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Zhang, Xiaoguang, und Osman A. Basaran. „Dynamics of drop formation from a capillary in the presence of an electric field“. Journal of Fluid Mechanics 326 (10.11.1996): 239–63. http://dx.doi.org/10.1017/s0022112096008300.
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This paper reports an experimental study of the effects of an externally applied electric field on the dynamics of drop formation in the dripping mode from a vertical metal capillary. The fluid issuing out of the capillary is a viscous liquid, the surrounding ambient fluid is air, and the electric field is generated by establishing a potential difference between the capillary and a horizontal, circular electrode of large radius placed downstream of the capillary outlet. By means of an ultra-high-speed video system that is capable of recording up to 12000 frames per second, special attention is paid to the dynamics of the liquid thread that connects the primary drop that is about to detach and fall from the capillary to the rest of the conical liquid mass that is hanging from it. The experiments show that as the strength of the electric field increases, the volume of the primary drop decreases whereas the maximum length attained by the thread increases. The reduction in the volume of primary drops and the increase in the length of threads occur because the effective electromechanical surface tension of the fluid interface falls as the field strength rises. For the highly conducting drops of aqueous NaCl solutions studied in this work, the increase in thread length is due solely to the rising importance of normal electric stress relative to the falling importance of surface tension. However, as the conductivity of the drop liquid decreases, the thread length is further increased on account of the stabilizing influence exerted by the increasing electric shear stress that acts on the charged liquid–gas interface. Two new phenomena are also reported that have profound implications for electrohydrodynamics and practical applications. First, it is shown that whereas the liquid thread always ruptures at its downstream end in the absence of an applied electric field or when the field strength is low, it ruptures at its upstream end when the field strength is sufficiently high. Since satellite drops are produced directly from the thread once both of its ends have ruptured, the change in the mechanism of breakup with field strength influences the dynamics and fate of satellite drops. Second, it is demonstrated that the generation of satellites, which are often undesirable in applications, can be suppressed by the judicious application of an electric field. This is accomplished by using a field of moderate strength to induce charges of the opposite sign on the nearby surfaces of the satellite drop and the liquid that remains pendant from the tube following thread rupture. At high field strengths, induced charge effects are too weak to compete with net charge effects: the satellite is repelled by the pendant drop and falls under gravity as a distinct entity.
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JACOBS, P., und G. N. IVEY. „The influence of rotation on shelf convection“. Journal of Fluid Mechanics 369 (25.08.1998): 23–48. http://dx.doi.org/10.1017/s0022112098001827.
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A series of laboratory experiments was conducted to study the flows and exchange processes generated by turbulent convection in a shallow fluid with a combination of a shelf and slope topography in the presence of rotation. For convenience, heat loss at the ocean surface was modelled by heating from below with a buoyancy flux B0 applied to a circular portion (of radius R) of the base of a cylindrical tank, rotating with angular frequency f. The working volume was closed by an inverted model of a shelf and slope topography (with slope angle ϕ), creating a fluid height H between the forced surface and the shelf. After the initiation of the buoyancy forcing, the average temperature in the actively convecting region initially increases linearly with time but slows down once a lateral heat flux is generated by baroclinic instability at the edge of the convecting region. The wavelength of this instability is described by λ=(5.9±0.3) RD, with RD the Rossby radius of deformation, defined by (g′H)1/2/f, where g′ is the reduced gravity based on the density difference between the convecting and ambient fluids. A steady state is eventually reached when the lateral heat flux balances the (vertical) heat flux due to the forcing. The results differ from previous work in either unbounded or in constant-depth environments. It is shown that the steady-state density anomaly between the convecting and ambient regions is given by g′f=(1.6±0.2) (B0f)1/2 (R/H), while the time to reach this steady state is τ=(3.1±0.5) (f/B0)1/2R. The eddy velocity, characterizing the lateral exchange process, is given by vflux≈1.2 (B0/f)1/2. These results are consistent with the description of the lateral exchange process by eddy diffusion (rather than advection). Comparisons are made between the experimental results and field observations of convection events.
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Bøggild, Carl Egede. „Preferential Flow and Melt Water Retention in Cold Snow Packs in West-Greenland“. Hydrology Research 31, Nr. 4-5 (01.08.2000): 287–300. http://dx.doi.org/10.2166/nh.2000.0017.
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Runoff from seasonal snow covers in Greenland provides the major source of stream flow in the non-glacierized catchments, and has gained interest in relation to hydropower potentials and water supply. During a two-month period the seasonal snow cover is melted and the discharge generated constitutes a major part of the annual runoff. Motivated by discrepancies between modelled and observed hydrographs during the early melt period in the Tasersuaq basin, we have performed tracer experiments in several pits and at different levels below the snow surface, to improve our understanding of water flow and storage in cold snow. It is observed that vertical percolation more readily occurs the deeper in the snow pack the tracer is sprinkled. At places where preferential vertical flow is established, we believe the water will eventually reach the bottom of the snow pack once it has passed the wind crusts on the top of the snow pack. From observations a parameterization of melt-water retention in the cold snow is suggested. We propose a time dependent term for the thermal and the gravity component, respectively, of storage in the snow pack. This time dependent term describes the fraction of snow which is exposed to melt water at the time when the water reaches the base of the snow. Incorporating such a term in hydrological models would improve them by providing an explanation of the observed fast hydrological response in the early melt period and by limiting the magnitude of the modeled peak flow.
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Grimley, Edward C., Jon Roussey, Nadia Petlakh-Co, Casey Wegner und Brandon McNaughton. „A superior dead cell removal platform using Akadeum’s BACS microbubbles“. Journal of Immunology 206, Nr. 1_Supplement (01.05.2021): 26.05. http://dx.doi.org/10.4049/jimmunol.206.supp.26.05.
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Abstract Single cell sequencing (SCS) is one of the most commonly used analytical techniques for cellular characterization and has become a mainstay in immune profiling. As SCS has gained in popularity and improved in sensitivity, it has become apparent that ambient nucleic acids can have a major impact on the quality and reproducibility of obtained SCS results. Since the bulk of ambient nucleic acids in a sample come from dead or dying cells that have lost membrane integrity, one way to dramatically reduce ambient nucleic acids and decrease background noise in SCS is to remove dead cells from the sample prior to sequencing. To address the challenge presented by contaminating dead cells, Akadeum has developed a Buoyancy Activated Cell Sorting (BACS™) microbubble approach for depleting dead cells from biological samples. Leveraging the power of gravity to drive self-isolation, Akadeum’s BACS microbubbles represent some of the gentlest isolations available; therefore, they are ideally suited for applications where cell viability is integral to success of the experiment. Additionally, BACS microbubbles from Akadeum are quick and easy to use since they require only minimal manipulation and do not rely on any extra equipment or consumables. Depletion of dead cells is achieved through the selective capture of cells with exposed phosphatidylserine with Annexin V conjugated BACS microbubbles. Once mixed with the sample, the Annexin V BACS microbubbles capture dead cells and float them to the sample surface for removal leaving behind healthy and untouched cells. Samples exposed to the Annexin V BACS microbubbles demonstrated significantly reduced dead cell populations and ultimately yielded more reliable results.
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Shamekh, Sara, Caroline Muller, Jean-Philippe Duvel und Fabio D’Andrea. „How Do Ocean Warm Anomalies Favor the Aggregation of Deep Convective Clouds?“ Journal of the Atmospheric Sciences 77, Nr. 11 (01.11.2020): 3733–45. http://dx.doi.org/10.1175/jas-d-18-0369.1.
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AbstractWe investigate the role of a warm sea surface temperature (SST) anomaly (hot spot of typically 3 to 5 K) on the aggregation of convection using cloud-resolving simulations in a nonrotating framework. It is well known that SST gradients can spatially organize convection. Even with uniform SST, the spontaneous self-aggregation of convection is possible above a critical SST (here 295 K), arising mainly from radiative feedbacks. We investigate how a circular hot spot helps organize convection, and how self-aggregation feedbacks modulate this organization. The hot spot significantly accelerates aggregation, particularly for warmer/larger hot spots, and extends the range of SSTs for which aggregation occurs; however, at cold SST (290 K) the aggregated cluster disaggregates if we remove the hot spot. A large convective instability over the hot spot leads to stronger convection and generates a large-scale circulation which forces the subsidence drying outside the hot spot. Indeed, convection over the hot spot brings the atmosphere toward a warmer temperature. The warmer temperatures are imprinted over the whole domain by gravity waves and subsidence warming. The initial transient warming and concomitant subsidence drying suppress convection outside the hot spot, thus driving the aggregation. The hot-spot-induced large-scale circulation can enforce the aggregation even without radiative feedbacks for hot spots sufficiently large/warm. The strength of the large-scale circulation, which defines the speed of aggregation, is a function of the hot spot fractional area. At equilibrium, once the aggregation is well established, the moist convective region with upward midtropospheric motion, centered over the hot spot, has an area surprisingly independent of the hot spot size.
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Rojas Castillo, Wilson Alexander, und Jose Robel Arenas Salazar. „A Conceptual Model for the Origin of the Cutoff Parameter in Exotic Compact Objects“. Symmetry 12, Nr. 12 (14.12.2020): 2072. http://dx.doi.org/10.3390/sym12122072.
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A Black Hole (BH) is a spacetime region with a horizon and where geodesics converge to a singularity. At such a point, the gravitational field equations fail. As an alternative to the problem of the singularity arises the existence of Exotic Compact Objects (ECOs) that prevent the problem of the singularity through a transition phase of matter once it has crossed the horizon. ECOs are characterized by a closeness parameter or cutoff, ϵ, which measures the degree of compactness of the object. This parameter is established as the difference between the radius of the ECO’s surface and the gravitational radius. Thus, different values of ϵ correspond to different types of ECOs. If ϵ is very big, the ECO behaves more like a star than a black hole. On the contrary, if ϵ tends to a very small value, the ECO behaves like a black hole. It is considered a conceptual model of the origin of the cutoff for ECOs, when a dust shell contracts gravitationally from an initial position to near the Schwarzschild radius. This allowed us to find that the cutoff makes two types of contributions: a classical one governed by General Relativity and one of a quantum nature, if the ECO is very close to the horizon, when estimating that the maximum entropy is contained within the material that composes the shell. Such entropy coincides with the Bekenstein–Hawking entropy. The established cutoff corresponds to a dynamic quantity dependent on coordinate time that is measured by a Fiducial Observer (FIDO). Without knowing the details about quantum gravity, parameter ϵ is calculated, which, in general, allows distinguishing the ECOs from BHs. Specifically, a black shell (ECO) is undistinguishable from a BH.
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Haemmerlé, L., R. S. Klessen, L. Mayer und L. Zwick. „Maximum accretion rate of supermassive stars“. Astronomy & Astrophysics 652 (August 2021): L7. http://dx.doi.org/10.1051/0004-6361/202141376.
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Context. The formation of the most massive quasars observed at high redshifts requires extreme inflows of gas down to the length scales of the central compact object. Aims. Here we estimate the maximum inflow rate allowed by gravity down to the surface of supermassive stars, the possible progenitors of these supermassive black holes. Methods. We use the continuity equation and the assumption of spherical symmetry and free fall to derive the maximum allowed inflow rates for various density profiles. We apply our approach to the mass–radius relation of rapidly accreting supermassive stars to estimate an upper limit to the accretion rates allowed during the formation of these objects. Results. We find that, as long as the density of the accreted gas is smaller than or equal to the average density of the accretor, the maximum allowed rate, Ṁmax, is given uniquely by the compactness of the accretor. We argue that a density inversion between accreting matter and the accretor is inconsistent with gravitational collapse. For the compactness of rapidly accreting supermassive stars, Ṁmax is related to the stellar mass, M, by a power law, Ṁmax ∝ M3/4. The rates of atomically cooled halos (0.1−10 M⊙ yr−1) are allowed as soon as M ≳ 1 M⊙. The largest rates expected in galaxy mergers (104 − 105 M⊙ yr−1) become accessible once the accretor is supermassive (M ≳ 104 M⊙). Conclusions. These results suggest that supermassive stars can accrete up to masses > 106 M⊙ before they collapse via the general-relativistic instability. At such masses, the collapse is expected to lead to the direct formation of a supermassive black hole, even within metal-rich gas, resulting in a black hole seed that is significantly heavier than in conventional direct collapse models for atomic cooling halos.
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Hicks, J. D., A. W. Troesch und C. Jiang. „Simulation and Nonlinear Dynamics Analysis of Planing Hulls“. Journal of Offshore Mechanics and Arctic Engineering 117, Nr. 1 (01.02.1995): 38–45. http://dx.doi.org/10.1115/1.2826989.
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The high speeds, small trim angles, and shallow drafts of planing hulls produce large changes in vessel wetted surface which, in turn, lead to significant hydrodynamic and dynamic nonlinearities. Due to the complex nonlinearities of this type of craft, naval architects and planing boat designers tend to rely upon experimental tests or simulation for guidance. In order for simulation to be an effective design tool, a fundamental understanding of the system’s dynamic characteristics is required. This paper describes a developing methodology by which the necessary insight may be obtained. A demonstration of the combined use of modern methods of dynamical system analysis with simulation is given in the evaluation of the vertical motions of a typical planing hull. Extending the work of Troesch and Hicks (1992) and Troesch and Falzarano (1993), the complete nonlinear hydrodynamic force and moment equations of Zarnick (1978) are expanded in a multi-variable Taylor series. As a result, the nonlinear integro-differential equations of motion are replaced by a set of highly coupled, ordinary differential equations with constant coefficients, valid through third order. Closed-form, analytic expressions are available for the coefficients (Hicks, 1993). Numerical examples for all first-order and some second-order terms are presented. Once completely determined, the coefficient matrices will serve as input to path following or continuation methods (e.g., Seydel, 1988) where heave and pitch magnification curves can be generated, allowing the entire system response to be viewed. The branching behavior of the solutions resulting from a variation of the center of gravity is examined in detail. These studies of the second-order accurate model show the potential of the method to identify areas of critical dynamic response, which in turn can be verified and explored further through the use of the simulator.
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Kieweg, Sarah L., und David F. Katz. „Squeezing Flows of Vaginal Gel Formulations Relevant to Microbicide Drug Delivery“. Journal of Biomechanical Engineering 128, Nr. 4 (01.02.2006): 540–53. http://dx.doi.org/10.1115/1.2206198.
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Efficacy of topical microbicidal drug delivery formulations against HIV depends in part on their ability to coat, distribute, and be retained on epithelium. Once applied to the vagina, a formulation is distributed by physical forces including: gravity, surface tension, shearing, and normal forces from surrounding tissues, i.e., squeezing forces. The present study focused on vaginal microbicide distribution due to squeezing forces. Mathematical simulations of squeezing flows were compared with squeezing experiments, using model vaginal gel formulations. Our objectives were: (1) to determine if mathematical simulations can accurately describe squeezing flows of vaginal gel formulations; (2) to find the best model and optimized parameter sets to describe these gels; and (3) to examine vaginal coating due to squeezing using the best models and summary parameters for each gel. Squeezing flow experiments revealed large differences in spreadability between formulations, suggesting different coating distributions in vivo. We determined the best squeezing flow models and summary parameters for six test gels of two compositions, cellulose and polyacrylic acid (PAA). We found that for some gels it was preferable to deduce model input parameters directly from squeezing flow experiments. For the cellulose gels, slip conditions in squeezing flow experiments needed to be evaluated. For PAA gels, we found that in the absence of squeezing experiments, rotational viscometry measurements (to determine Herschel-Bulkley parameters) led to reasonably accurate predictions of squeezing flows. Results indicated that yield stresses may be a strong determinant of squeezing flow mechanics. This study serves as a template for further investigations of other gels and determination of which sources of rheological data best characterize potential microbicidal formulations. These mathematical simulations can serve as useful tools for exploring drug delivery parameters, and optimizing formulations, prior to costly clinical trials.
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Karthik Krishna Ramakrishnan, Ashwini Govisetty, Naveen Nagendran, Meyyappan Meenakshisomasundaram, Paarthipan Natarajan und Seena Cheppala Rajan. „Prenatal Ultrasonographic and Magnetic Resonance Imaging Diagnosis of Occipital Meningio-Encephalocele- An Interesting Case Report“. International Journal of Research in Pharmaceutical Sciences 11, SPL4 (21.12.2020): 2332–35. http://dx.doi.org/10.26452/ijrps.v11ispl4.4462.
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Meningo encephalocele is a congenital anomaly and is a neural tube defect with occipital meningo encephalocele being the most common and is a result of a failure of the surface ectoderm to separate from the neuroectoderm. This condition can be identified in 1st trimester in 80% of cases and almost all by 2nd trimester. A 20-year-old third gravida was referred for antenatal Ultrasonography at five months of amenorrhoea to rule out fetal anomalies. On targeted imaging, for fetal anomalies, a defect was seen in occipital bone with herniation of posterior fossa contents with overlying meningeal covering. No other fetal anomalies were noted. A diagnosis of isolated occipital meningoencephalocele was made with additional fetal MRI correlation. The mother underwent termination of her pregnancy by Department of Obstetrics and Gynecology because of the grim fetal prognosis. The mother was advised to plan the subsequent pregnancies and was advised pre-conceptional folic acid supplementation. We present a case of isolated occipital meningoencephalocele- a rare congenital anomaly which was diagnosed prenatally in our hospital. This case provides an opportunity for identifying such neurological defects early and prompt termination of pregnancy to prevent comorbidity to mother. This study also helps to establish occipital meninigioencephalocele as an isolated clinicoradiological diagnosis and to distinguish it from syndrome associated occipital meninigioencephalocele or those associated with other neural tube defects like Chiari III malformations. It also allows us to stress once again the role of periconceptional folic acid in preventing the occurrence of neural tube defects.