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1
Kimura, Y., and J. R. Herring. "Diffusion in stably stratified turbulence." Journal of Fluid Mechanics 328 (December 10, 1996): 253–69. http://dx.doi.org/10.1017/s0022112096008713.
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We examine results of direct numerical simulations (DNS) of homogeneous turbulence in the presence of stable stratification. We focus on the effects of stratification on eddy diffusion, and the distribution of pairs of particles released in the flow. DNS results are presented over a range of stratification, and at Reynolds numbers compatible with aliased free spectral results for a resolution of 128 mesh points. We compare results for particle dispersion to simple analytic theories such as that proposed by Csanady (1964) and Pearson et al. (1983) by adapting the basic Langevin model to decaying turbulence at low Reynolds numbers. Stable stratification is found to arrest both single particle displacements and pair separation in the direction of stratification, but it leaves these quantities nearly unaltered in the transverse direction. With respect to the dynamics of stratified flows, we find that regions of strong viscous dissipation are intermittently spaced, and are associated with large horizontal vorticity, consistent with recent experimental results by Fincham et al. (1994).
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Castell, A., M. Medrano, C. Solé, and L. F. Cabeza. "Dimensionless numbers used to characterize stratification in water tanks for discharging at low flow rates." Renewable Energy 35, no. 10 (October 2010): 2192–99. http://dx.doi.org/10.1016/j.renene.2010.03.020.
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BALMFORTH, NEIL J., and YUAN-NAN YOUNG. "Stratified Kolmogorov flow." Journal of Fluid Mechanics 450 (January 9, 2002): 131–67. http://dx.doi.org/10.1017/s0022111002006371.
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In this study we investigate the Kolmogorov flow (a shear flow with a sinusoidal velocity profile) in a weakly stratified, two-dimensional fluid. We derive amplitude equations for this system in the neighbourhood of the initial bifurcation to instability for both low and high Péclet numbers (strong and weak thermal diffusion, respectively). We solve amplitude equations numerically and find that, for low Péclet number, the stratification halts the cascade of energy from small to large scales at an intermediate wavenumber. For high Péclet number, we discover diffusively spreading, thermal boundary layers in which the stratification temporarily impedes, but does not saturate, the growth of the instability; the instability eventually mixes the temperature inside the boundary layers, so releasing itself from the stabilizing stratification there, and thereby grows more quickly. We solve the governing fluid equations numerically to compare with the asymptotic results, and to extend the exploration well beyond onset. We find that the arrest of the inverse cascade by stratification is a robust feature of the system, occurring at higher Reynolds, Richards and Péclet numbers – the flow patterns are invariably smaller than the domain size. At higher Péclet number, though the system creates slender regions in which the temperature gradient is concentrated within a more homogeneous background, there are no signs of the horizontally mixed layers separated by diffusive interfaces familiar from doubly diffusive systems.
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DIXIT, HARISH N., and RAMA GOVINDARAJAN. "Vortex-induced instabilities and accelerated collapse due to inertial effects of density stratification." Journal of Fluid Mechanics 646 (February 9, 2010): 415–39. http://dx.doi.org/10.1017/s0022112009992515.
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A vortex placed at a density interface winds it into an ever-tighter spiral. We show that this results in a combination of a centrifugal Rayleigh–Taylor (CRT) instability and a spiral Kelvin–Helmholtz (SKH) type of instability. The SKH instability arises because the density interface is not exactly circular, and dominates at large times. Our analytical study of an inviscid idealized problem illustrates the origin and nature of the instabilities. In particular, the SKH is shown to grow slightly faster than exponentially. The predicted form lends itself for checking by a large computation. From a viscous stability analysis using a finite-cored vortex, it is found that the dominant azimuthal wavenumber is smaller for lower Reynolds number. At higher Reynolds numbers, disturbances subject to the combined CRT and SKH instabilities grow rapidly, on the inertial time scale, while the flow stabilizes at low Reynolds numbers. Our direct numerical simulations are in good agreement with these studies in the initial stages, after which nonlinearities take over. At Atwood numbers of 0.1 or more, and a Reynolds number of 6000 or greater, both stability analysis and simulations show a rapid destabilization. The result is an erosion of the core, and breakdown into a turbulence-like state. In studies at low Atwood numbers, the effect of density on the inertial terms is often ignored, and the density field behaves like a passive scalar in the absence of gravity. The present study shows that such treatment is unjustified in the vicinity of a vortex, even for small changes in density when the density stratification is across a thin layer. The study would have relevance to any high-Péclet-number flow where a vortex is in the vicinity of a density-stratified interface.
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Dandekar, Rajat, Vaseem A. Shaik, and Arezoo M. Ardekani. "Swimming sheet in a density-stratified fluid." Journal of Fluid Mechanics 874 (July 4, 2019): 210–34. http://dx.doi.org/10.1017/jfm.2019.445.
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In this work, we theoretically investigate the swimming velocity of a Taylor swimming sheet immersed in a linearly density-stratified fluid. We use a regular perturbation expansion approach to estimate the swimming velocity up to second order in wave amplitude. We divide our analysis into two regimes of low ($\ll O(1)$) and finite Reynolds numbers. We use our solution to understand the effect of stratification on the swimming behaviour of organisms. We find that stratification significantly influences motility characteristics of the swimmer such as the swimming speed, hydrodynamic power expenditure, swimming efficiency and the induced mixing, quantified by mixing efficiency and diapycnal eddy diffusivity. We explore this dependence in detail for both low and finite Reynolds number and elucidate the fundamental insights obtained. We expect our work to shed some light on the importance of stratification in the locomotion of organisms living in density-stratified aquatic environments.
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Ezhova, Ekaterina, Claudia Cenedese, and Luca Brandt. "Interaction between a Vertical Turbulent Jet and a Thermocline." Journal of Physical Oceanography 46, no. 11 (November 2016): 3415–37. http://dx.doi.org/10.1175/jpo-d-16-0035.1.
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AbstractThe behavior of an axisymmetric vertical turbulent jet in an unconfined stratified environment is studied by means of well-resolved, large-eddy simulations. The stratification is two uniform layers separated by a thermocline. This study considers two cases: when the thermocline thickness is small and on the order of the jet diameter at the thermocline entrance. The Froude number of the jet at the thermocline varies from 0.6 to 1.9, corresponding to the class of weak fountains. The mean jet penetration, stratified turbulent entrainment, jet oscillations, and the generation of internal waves are examined. The mean jet penetration is predicted well by a simple model based on the conservation of the source energy in the thermocline. The entrainment coefficient for the thin thermocline is consistent with the theoretical model for a two-layer stratification with a sharp interface, while for the thick thermocline entrainment is larger at low Froude numbers. The data reveal the presence of a secondary horizontal flow in the upper part of the thick thermocline, resulting in the entrainment of fluid from the thermocline rather than from the upper stratification layer. The spectra of the jet oscillations in the thermocline display two peaks, at the same frequencies for both stratifications at fixed Froude number. For the thick thermocline, internal waves are generated only at the lower frequency, since the higher peak exceeds the maximal buoyancy frequency. For the thin thermocline, conversely, the spectra of the internal waves show the two peaks at low Froude numbers, whereas only one peak at the lower frequency is observed at higher Froude numbers.
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Gupta, Supriya, Phillip M. Ligrani, and J. Calvin Giddings. "Characteristics of flow instabilities from unstable stratification of density in channel shear layers at low Reynolds numbers." International Journal of Heat and Mass Transfer 42, no. 6 (March 1999): 1023–36. http://dx.doi.org/10.1016/s0017-9310(98)00169-0.
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Spurgin, J. M., and S. E. Allen. "Flow dynamics around downwelling submarine canyons." Ocean Science 10, no. 5 (October 14, 2014): 799–819. http://dx.doi.org/10.5194/os-10-799-2014.
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Abstract. Flow dynamics around a downwelling submarine canyon were analysed with the Massachusetts Institute of Technology general circulation model. Blanes Canyon (northwestern Mediterranean) was used for topographic and initial forcing conditions. Fourteen scenarios were modelled with varying forcing conditions. Rossby and Burger numbers were used to determine the significance of Coriolis acceleration and stratification (respectively) and their impacts on flow dynamics. A new non-dimensional parameter (χ) was introduced to determine the significance of vertical variations in stratification. Some simulations do see brief periods of upwards displacement of water during the 10-day model period; however, the presence of the submarine canyon is found to enhance downwards advection of density in all model scenarios. High Burger numbers lead to negative vorticity and a trapped anticyclonic eddy within the canyon, as well as an increased density anomaly. Low Burger numbers lead to positive vorticity, cyclonic circulation, and weaker density anomalies. Vertical variations in stratification affect zonal jet placement. Under the same forcing conditions, the zonal jet is pushed offshore in more uniformly stratified domains. The offshore jet location generates upwards density advection away from the canyon, while onshore jets generate downwards density advection everywhere within the model domain. Increasing Rossby values across the canyon axis, as well as decreasing Burger values, increase negative vertical flux at shelf break depth (150 m). Increasing Rossby numbers lead to stronger downwards advection of a passive tracer (nitrate), as well as stronger vorticity within the canyon. Results from previous studies are explained within this new dynamic framework.
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MacKinnon, J. A., and M. C. Gregg. "Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf." Journal of Physical Oceanography 35, no. 12 (December 1, 2005): 2425–43. http://dx.doi.org/10.1175/jpo2821.1.
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Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.
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Spurgin, J. M., and S. E. Allen. "Flow dynamics around downwelling submarine canyons." Ocean Science Discussions 11, no. 3 (May 23, 2014): 1301–56. http://dx.doi.org/10.5194/osd-11-1301-2014.
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Abstract. Flow dynamics around a downwelling submarine canyon were analysed with the Massachusetts Institute of Technology general circulation model. Blanes Canyon (Northwest Mediterranean) was used for topographic and initial forcing conditions. Fourteen scenarios were modelled with varying forcing conditions. Rossby number and Burger number were used to determine the significance of Coriolis acceleration and stratification (respectively) and their impacts on flow dynamics. A new non-dimensional parameter (χ) was introduced to determine the significance of vertical variations in stratification. Some simulations do see brief periods of upwards displacement of water during the 10 day model period, however, the presence of the submarine canyon is found to enhance downwards advection of density in all model scenarios. High Burger numbers lead to negative vorticity and a trapped anticyclonic eddy within the canyon, as well as an increased density anomaly. Low Burger numbers lead to positive vorticity, cyclonic circulation and weaker density anomalies. Vertical variations in stratification affect zonal jet placement. Under the same forcing conditions, the zonal jet is pushed offshore in more uniformly stratified domains. Offshore jet location generates upwards density advection away from the canyon, while onshore jets generate downwards density advection everywhere within the model domain. Increasing Rossby values across the canyon axis, as well as decreasing Burger values, increase negative vertical flux at shelf break depth (150 m). Increasing Rossby numbers lead to stronger downwards advection of a passive tracer (nitrate) as well as stronger vorticity within the canyon. Results from previous studies were explained within this new dynamic framework.
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Jarvise, Richard A., and Herbert E. Huppert. "Solidification of a binary alloy of variable viscosity from a vertical boundary." Journal of Fluid Mechanics 303 (November 25, 1995): 103–32. http://dx.doi.org/10.1017/s0022112095004198.
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We analyse the complete solidification from a side boundary of a finite volume of a binary alloy. Particular emphasis is placed upon the compositional stratification produced in the solid, the structure of which is determined by the competition between the rates of solidification and of laminar box filling by the fractionated fluid released at the solid/liquid interface. It is demonstrated by scaling arguments that numerical calculations performed at relatively low values of the Rayleigh and Lewis numbers may be used to describe equally well laboratory experiments previously performed at moderate Rayleigh and Lewis numbers and the high-Rayleigh-number, high-lewis-number convective regime expected during the solidification of a large magmatic body, provided that the balance between solidification and laminar box filling is maintained. This balance can be represented by a single dimensionless group of parameters. The boundary-layer analysis is extended to fluids whose viscosity is strongly dependent upon temperature and composition, and an effective viscosity is derived which may be used to describe both the magnitude and pattern of compositional stratification in the solid.
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Bartello, P., and S. M. Tobias. "Sensitivity of stratified turbulence to the buoyancy Reynolds number." Journal of Fluid Mechanics 725 (May 14, 2013): 1–22. http://dx.doi.org/10.1017/jfm.2013.170.
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AbstractIn this article we present direct numerical simulations of stratified flow at resolutions of up to $204{8}^{2} \times 513$, to explore scalings for the dynamics of stably stratified turbulence. Recent work suggests that for strong enough stratification, the vertical integral scale of the turbulence adjusts to yield a vertical Froude number, ${F}_{v} $, of order unity at high enough Reynolds number, whilst the horizontal Froude number, ${F}_{h} $, decreases as stratification is increased. Our numerical simulations are consistent with predictions by Lindborg (J. Fluid Mech., vol. 550, 2006, pp, 207–242), and with numerical simulations at lower resolution, in that the horizontal kinetic energy spectrum follows a Kolmogorov spectrum (after replacing the wavenumber with the horizontal wavenumber) and that the horizontal potential energy spectrum similarly follows the Corrsin–Obukhov spectrum for a passive scalar. Most importantly, we build upon these previous results by thoroughly exploring the dependence of the horizontal spectrum of horizontal kinetic energy on both the stratification and the relative size of the vertical dissipation terms, as quantified by the buoyancy Reynolds number. Our most important result is that variations in the power-law exponent scale entirely with the buoyancy Reynolds number and not with the stratification itself, lending considerable support to the Lindborg (2006) hypothesis that horizontal spectra are independent of stratification at large Reynolds numbers. We further demonstrate that even at the large numerical resolution of this study, the spectrum and hence the dynamics are affected by the buoyancy Reynolds number unless it is larger than $O(10)$, indicating that extreme care must be taken when assessing claims made from previous numerical simulations of stratified flow at low or moderate resolution and extrapolating the results to geophysical or astrophysical Reynolds numbers.
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Dorazio, Robert M. "Prerelease Stratification in Tag-Recovery Models with Time Dependence." Canadian Journal of Fisheries and Aquatic Sciences 50, no. 3 (March 1, 1993): 535–41. http://dx.doi.org/10.1139/f93-062.
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A statistical model is developed for estimating time-dependent survival and recovery rates of tagged populations that are stratified into groups at release. For populations of fish, the groups might include different sexes or stocks, or they might be distinguished by the location or method of capture. In the model developed here, survival and recovery rates are formulated as linear combinations of parameters that include the effects of time, group, and time–group interactions. Methods of testing the statistical significance of these potential sources of variation are described and illustrated with the recoveries of different stocks of anadromous striped bass (Morone saxatilis). For the striped bass data, elimination of time- and stock-specific sources of variation substantially improved the precision of survival estimates without introducing important bias. During 1988–89 the average survival of both stocks was 0.78 (SE = 0.04). Calculations of statistical power indicate that stock-specific differences in survival of ± 0.2 could have been detected about 50% of the time, given the low rates of recovery and numbers of fish released.
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Spalart, Philippe R. "On the motion of laminar wing wakes in a stratified fluid." Journal of Fluid Mechanics 327 (November 25, 1996): 139–60. http://dx.doi.org/10.1017/s002211209600849x.
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We present numerical solutions for two-dimensional laminar symmetric vortex systems descending in a stably stratified fluid, within the Boussinesq approximation. Three types of flows are considered: (I) tight vortices; (II) those deriving from an elliptical wing lift distribution; and (III) those deriving from a ‘high-lift’ distribution, with a part-span flap on the wing. The non-dimensional stratification ranges from zero to moderate, as it does for airliners. For Types I and II, with high Reynolds numbers and weak stratification, the solutions confirm the theory of Scorer & Davenport (1970) (their article lacks a crucial link which we provide, equivalent to one of Crow (1974)). Contrary to common conceptions and observations in small-scale experiments, the descent velocity increases exponentially with time, as the distance between vortices decreases and the circulation of the vortices proper is conserved. With moderate stratification, wakes with sufficient energy also attain the accelerating régime, until the vortex cores make contact. However, they first experience a rebound, which is both of practical importance and out of reach of simple formulas. Type III wakes produce two durable vortex pairs which tumble, and mitigate the buoyancy effect by exchanging fluid with the surroundings. These phenomena are obscured by low wing aspect ratios, Reynolds numbers below about 105, or appreciable surrounding turbulence; this may explain why neither a clear rebound nor an acceleration can be reconciled with experiments to date. We argue that airliner wakes have very little inherent diffusion, and that a rapid end to the wake's descent must reveal effects other than simple buoyancy. In particular, stratification promotes the Crow instability.
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Bolaños, Rodolfo, Jennifer M. Brown, Laurent O. Amoudry, and Alejandro J. Souza. "Tidal, Riverine, and Wind Influences on the Circulation of a Macrotidal Estuary." Journal of Physical Oceanography 43, no. 1 (January 1, 2013): 29–50. http://dx.doi.org/10.1175/jpo-d-11-0156.1.
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Abstract The effect of tides, river, wind and Earth’s rotation on the three-dimensional circulation in the Dee, a macrotidal estuary, are investigated using a fine-resolution model. The interactions of the large tidal amplitude, currents, river, and wind-generated circulation require baroclinic and unsteady studies to properly understand the estuarine dynamics. Assessment of the model skill has been carried out by model–observation comparisons for salinity, which is the main control for density, surface elevation, current, and turbulence. Stationary nondimensional numbers were only partially able to characterize the dynamics in this (real) complex macrotidal estuary. At low water, tidal straining and constrained river flow cause stratification. Large spatial variability occurs in the current and residual patterns, with flood-dominated maximum values occurring within the tidal channels. The tides control residual circulation by modulating stratification through tidal straining and bathymetric constraint on river flow. Tide–stratification–river interaction causes an unsteady pattern of residual circulation and tidal pulses. River-induced pulses are enhanced near low tide–inducing density-driven circulation. Wind effects are concentrated near the surface, mainly occurring at high tide because of increased fetch. Even though Coriolis has, overall, a small contribution it produces tidal pulses modifying the current and salinity distribution.
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Edelmann, P. V. F., L. Horst, J. P. Berberich, R. Andrassy, J. Higl, G. Leidi, C. Klingenberg, and F. K. Röpke. "Well-balanced treatment of gravity in astrophysical fluid dynamics simulations at low Mach numbers." Astronomy & Astrophysics 652 (August 2021): A53. http://dx.doi.org/10.1051/0004-6361/202140653.
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Context. Accurate simulations of flows in stellar interiors are crucial to improving our understanding of stellar structure and evolution. Because the typically slow flows are merely tiny perturbations on top of a close balance between gravity and the pressure gradient, such simulations place heavy demands on numerical hydrodynamics schemes. Aims. We demonstrate how discretization errors on grids of reasonable size can lead to spurious flows orders of magnitude faster than the physical flow. Well-balanced numerical schemes can deal with this problem. Methods. Three such schemes were applied in the implicit, finite-volume SEVEN-LEAGUE HYDRO code in combination with a low-Mach-number numerical flux function. We compare how the schemes perform in four numerical experiments addressing some of the challenges imposed by typical problems in stellar hydrodynamics. Results. We find that the α-β and deviation well-balancing methods can accurately maintain hydrostatic solutions provided that gravitational potential energy is included in the total energy balance. They accurately conserve minuscule entropy fluctuations advected in an isentropic stratification, which enables the methods to reproduce the expected scaling of convective flow speed with the heating rate. The deviation method also substantially increases accuracy of maintaining stationary orbital motions in a Keplerian disk on long timescales. The Cargo–LeRoux method fares substantially worse in our tests, although its simplicity may still offer some merits in certain situations. Conclusions. Overall, we find the well-balanced treatment of gravity in combination with low Mach number flux functions essential to reproducing correct physical solutions to challenging stellar slow-flow problems on affordable collocated grids.
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Habib, Thomas J., David A. Moore, and Evelyn H. Merrill. "Detection and stratification approaches for aerial surveys of deer in prairie–parklands." Wildlife Research 39, no. 7 (2012): 593. http://dx.doi.org/10.1071/wr11175.
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Context For management decisions that require accurate and precise estimates of large mammal population numbers, aerial surveys are considered reliable despite their cost. However, aerial surveys may still suffer from underestimation because of undetected animals and low precision as a result of inefficient sampling designs. Aims We assess detection of two species of deer in prairie-parkland communities of western Canada and evaluate a suite of survey design features for improving the accuracy and precision of population estimates from aerial surveys. Methods Modelling detection of deer was based on 100 sightability trials involving 54 radio-collared white-tailed and 46 mule deer. We used empirical survey data to simulate surveys under three alternative stratification approaches, schemes for grouping strata, and allocations of survey effort and compared the precision and accuracy of the resulting population estimates. Key results We observed deer in 83 of the 100 trials, with detection decreasing with small group size, reduced deer activity, low snow cover, high forest cover and observer fatigue. Survey precision and accuracy were highest when stratification was based on natural breaks, calculated via Jenks optimisation, in the values of resource-selection function (RSF), although improvement was less pronounced for estimates of mule deer abundance. Optimal or equal allocation of sampling effort among strata outperformed proportional allocation of sampling effort. Use of RSF for stratification reduced the coefficient of variation (CV) in estimates of deer numbers from 38% to 23% for white-tailed deer and from 33% to 27% for mule deer compared with past approaches. Conclusions Stratification based on RSF values improved precision of deer surveys the most; however, using even simple measures related to habitat selection can improve population estimates. Where deer are highly aggregated, reliably recording all variables needed to implement sightability models can prove problematic; however, survey detection adjustments are nevertheless important to account for the relatively small, but still significant, proportion of missed animals in open prairie–parkland environments. Implications Field experiments to assess aerial survey design are impractical because of cost. We illustrate how simulated surveys using empirical data can be useful to evaluate alternative survey designs to improve population estimates in a region when high accuracy or precision are needed to address management questions or to calibrate more cost-effective approaches.
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Calkins, Michael A., Keith Julien, and Philippe Marti. "The breakdown of the anelastic approximation in rotating compressible convection: implications for astrophysical systems." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 471, no. 2175 (March 2015): 20140689. http://dx.doi.org/10.1098/rspa.2014.0689.
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The linear theory for rotating compressible convection in a plane layer geometry is presented for the astrophysically relevant case of low Prandtl number gases. When the rotation rate of the system is large, the flow remains geostrophically balanced for all stratification levels investigated and the classical (i.e. incompressible) asymptotic scaling laws for the critical parameters are recovered. For sufficiently small Prandtl numbers, increasing stratification tends to further destabilize the fluid layer, decrease the critical wavenumber and increase the oscillation frequency of the convective instability. In combination, these effects increase the relative magnitude of the time derivative of the density perturbation contained in the conservation of mass equation to non-negligible levels; the resulting convective instabilities occur in the form of compressional quasi-geostrophic oscillations. We find that the anelastic equations, which neglect this term, cannot capture these instabilities and possess spuriously growing eigenmodes in the rapidly rotating, low Prandtl number regime. It is shown that the Mach number for rapidly rotating compressible convection is intrinsically small for all background states, regardless of the departure from adiabaticity.
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KUHLMANN, H. C., and U. SCHOISSWOHL. "Flow instabilities in thermocapillary-buoyant liquid pools." Journal of Fluid Mechanics 644 (February 10, 2010): 509–35. http://dx.doi.org/10.1017/s0022112009992953.
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The linear stability of the incompressible axisymmetric flow in a buoyant-thermocapillary liquid pool is considered which is heated from above by a heat flux with a parabolic radial profile. Buoyancy forces and radial thermocapillary stresses due to the inhomogeneous surface temperature distribution drive a toroidal vortex. In the absence of buoyancy and for low Prandtl numbers the basic flow becomes unstable typically by a stationary centrifugal instability. At moderate Prandtl numbers the rotational symmetry is broken by hydrothermal waves. In the limit of vanishing Prandtl number two other critical modes are found if the pool is very shallow. One mode is a centrifugally destabilized rotating wave with high azimuthal wavenumber. The other mode is steady and it is driven by the deceleration of the radial inward return flow as it approaches the axis. The deceleration results from an entrainment of fluid into the thin layer of rapid radial outward surface flow. The centrifugal instability of the toroidal vortex flow is assisted by buoyancy in the low-Prandtl-number limit, because the cooling from the sidewall augments the thermocapillary driving. For moderately high Prandtl numbers a stable thermal stratification suppresses the hydrothermal-wave instabilities.
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Williams, Owen, Tristen Hohman, Tyler Van Buren, Elie Bou-Zeid, and Alexander J. Smits. "The effect of stable thermal stratification on turbulent boundary layer statistics." Journal of Fluid Mechanics 812 (January 11, 2017): 1039–75. http://dx.doi.org/10.1017/jfm.2016.781.
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The effects of stable thermal stratification on turbulent boundary layers are experimentally investigated for smooth and rough walls. For weak to moderate stability, the turbulent stresses are seen to scale with the wall shear stress, compensating for changes in fluid density in the same manner as done for compressible flows. This suggests little change in turbulent structure within this regime. At higher levels of stratification turbulence no longer scales with the wall shear stress and turbulent production by mean shear collapses, but without the preferential damping of near-wall motions observed in previous studies. We suggest that the weakly stable and strongly stable (collapsed) regimes are delineated by the point where the turbulence no longer scales with the local wall shear stress, a significant departure from previous definitions. The critical stratification separating these two regimes closely follows the linear stability analysis of Schlichting (Z. Angew. Math. Mech., vol. 15 (6), 1935, pp. 313–338) for both smooth and rough surfaces, indicating that a good predictor of critical stratification is the gradient Richardson number evaluated at the wall. Wall-normal and shear stresses follow atmospheric trends in the local gradient Richardson number scaling of Sorbjan (Q. J. R. Meteorol. Soc., vol. 136, 2010, pp. 1243–1254), suggesting that much can be learned about stratified atmospheric flows from the study of laboratory scale boundary layers at relatively low Reynolds numbers.
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Ligrani, Phillip M., Supriya Gupta, and J. Calvin Giddings. "Onset and effects of instabilities from unstable stratification of density on mass transfer in channel shear layers at low Reynolds numbers." International Journal of Heat and Mass Transfer 41, no. 12 (June 1998): 1667–79. http://dx.doi.org/10.1016/s0017-9310(97)00289-5.
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Jackson, P. Ryan, and Chris R. Rehmann. "Laboratory Measurements of Differential Diffusion in a Diffusively Stable, Turbulent Flow." Journal of Physical Oceanography 33, no. 8 (August 1, 2003): 1592–603. http://dx.doi.org/10.1175/2405.1.
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Abstract Laboratory experiments were performed to determine the conditions under which differential diffusion occurs and to evaluate its effect on the mixing efficiency. Diffusively stable profiles of temperature and salinity were stirred steadily by horizontally oscillating vertical rods. The two-component stratification ensures that both scalars experience the same stratification and forcing, or Richardson and Reynolds numbers. The eddy diffusivities KT and KS, for temperature and salinity, were estimated by fitting theoretical solutions of diffusion equations to measured profiles, and the mixing efficiency was computed as the ratio of the potential energy change during a stirring interval to the work done in that interval. Differential diffusion occurred for ɛa/νN2 < 300–500, where ɛa is an average dissipation rate computed from an integrated energy budget. The diffusivity ratio d = KS/KT varied between 0.5 and 1 in the range 50 < ɛa/νN2 < 500. The experiments also show that differential diffusion can significantly affect the mixing efficiency. An important dimensionless parameter is the density ratio Rρ, which is the ratio of the density change due to temperature to that due to salinity. Measurements in cases with low density ratio (Rρ ≈ 0.25) and high density ratio (Rρ ≈ 5) showed that the mixing efficiencies agreed well for weak stratification, or small Richardson number. For larger Richardson number, the efficiency for the high-density-ratio case exceeded that for the low-density-ratio case by as much as a factor of 1.5.
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Neuhaus, Jochen, and Bo Yang. "Liquid Biopsy Potential Biomarkers in Prostate Cancer." Diagnostics 8, no. 4 (September 21, 2018): 68. http://dx.doi.org/10.3390/diagnostics8040068.
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Prostate cancer (PCa) is the second most common cancer in men worldwide with an incidence of 14.8% and a mortality of 6.6%. Shortcomings in comprehensive medical check-ups in low- and middle-income countries lead to delayed detection of PCa and are causative of high numbers of advanced PCa cases at first diagnosis. The performance of available biomarkers is still insufficient and limited applicability, including logistical and financial burdens, impedes comprehensive implementation into health care systems. There is broad agreement on the need of new biomarkers to improve (i) early detection of PCa, (ii) risk stratification, (iii) prognosis, and (iv) treatment monitoring. This review focuses on liquid biopsy tests distinguishing high-grade significant (Gleason score (GS) ≥ 7) from low-grade indolent PCa. Available biomarkers still lack performance in risk stratification of biopsy naïve patients. However, biomarkers with highly negative predictive values may help to reduce unnecessary biopsies. Risk calculators using integrative scoring systems clearly improve decision-making for invasive prostate biopsy. Emerging biomarkers have the potential to substitute PSA and improve the overall performance of risk calculators. Until then, PSA should be used and may be replaced whenever enough evidence has accumulated for better performance of a new biomarker.
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Sahu, Kirti Chandra, and Rama Govindarajan. "Linear stability analysis and direct numerical simulation of two-layer channel flow." Journal of Fluid Mechanics 798 (June 13, 2016): 889–909. http://dx.doi.org/10.1017/jfm.2016.346.
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We study the stability of two-fluid flow through a plane channel at Reynolds numbers of 100–1000 in the linear and nonlinear regimes. The two fluids have the same density but different viscosities. The fluids, when miscible, are separated from each other by a mixed layer of small but finite thickness, across which the viscosity changes from that of one fluid to that of the other. When immiscible, the interface is sharp. Our study spans a range of Schmidt numbers, viscosity ratios and locations and thicknesses of the mixed layer. A region of instability distinct from that of the Tollmien–Schlichting mode is obtained at moderate Reynolds numbers. We show that the overlap of the layer of viscosity-stratification with the critical layer of the dominant disturbance provides a mechanism for this instability. At very low values of diffusivity, the miscible flow behaves exactly like the immiscible one in terms of stability characteristics. High levels of miscibility make the flow more stable. At intermediate levels of diffusivity however, in both linear and nonlinear regimes, miscible flow can be more unstable than the corresponding immiscible flow without surface tension. This difference is greater when the thickness of the mixed layer is decreased, since the thinner the layer of viscosity stratification, the more unstable the miscible flow. In direct numerical simulations, disturbance growth occurs at much earlier times in the miscible flow, and also the miscible flow breaks spanwise symmetry more readily to go into three-dimensionality. The following observations hold for both miscible and immiscible flows without surface tension. The stability of the flow is moderately sensitive to the location of the interface between the two fluids. The response is non-monotonic, with the least stable location of the layer being mid-way between the wall and the centreline. As expected, flow at higher Reynolds numbers is more unstable.
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Li, Ming, John Trowbridge, and Rocky Geyer. "Asymmetric Tidal Mixing due to the Horizontal Density Gradient*." Journal of Physical Oceanography 38, no. 2 (February 1, 2008): 418–34. http://dx.doi.org/10.1175/2007jpo3372.1.
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Abstract Stratification and turbulent mixing exhibit a flood–ebb tidal asymmetry in estuaries and continental shelf regions affected by horizontal density gradients. The authors use a large-eddy simulation (LES) model to investigate the penetration of a tidally driven bottom boundary layer into stratified water in the presence of a horizontal density gradient. Turbulence in the bottom boundary layer is driven by bottom stress during flood tides, with low-gradient (Ri) and flux (Rf) Richardson numbers, but by localized shear during ebb tides, with Ri = ¼ and Rf = 0.2 in the upper half of the boundary layer. If the water column is unstratified initially, the LES model reproduces periodic stratification associated with tidal straining. The model results show that the energetics criterion based on the competition between tidal straining and tidal stirring provides a good prediction for the onset of periodic stratification, but the tidally averaged horizontal Richardson number Rix has a threshold value of about 0.2, which is lower than the 3 suggested in a recent study. Although the tidal straining leads to negative buoyancy flux on flood tides, the authors find that for typical values of the horizontal density gradient and tidal currents in estuaries and shelf regions, buoyancy production is much smaller than shear production in generating turbulent kinetic energy.
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Grisouard, Nicolas. "Extraction of Potential Energy from Geostrophic Fronts by Inertial–Symmetric Instabilities." Journal of Physical Oceanography 48, no. 5 (May 2018): 1033–51. http://dx.doi.org/10.1175/jpo-d-17-0160.1.
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AbstractSubmesoscale oceanic density fronts are structures in geostrophic and hydrostatic balance, which are prone to inertial and/or symmetric instabilities. We argue in this article that drainage of potential energy from the geostrophic flow is a significant source of their growth. We illustrate our point with two-dimensional Boussinesq numerical simulations of oceanic density fronts on the f plane. A set of two-dimensional initial conditions covers the submesoscale portion of a three-dimensional parameter space consisting of the Richardson and Rossby numbers and a measure of stratification or latitude. Because we let the lateral density gradient decay with depth, the parameter space map is nontrivial, excluding low-Rossby, low-Richardson combinations. Dissipation and the presence of boundaries select a growing mode of inertial–symmetric instability consisting of flow cells that disturb isopycnal contours. Systematically, these isopycnal displacements correspond to a drainage of potential energy from the geostrophic fronts to the ageostrophic perturbations. In the majority of our experiments, this energy drainage is at least as important as the drainage of kinetic energy from the front. Various constraints, some physical, some numerical, make the energetics in our experiments more related to inertial rather than symmetric instabilities. Our results depend very weakly on the Richardson number and more on the Rossby number and relative stratification.
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Holt, Steven E., Jeffrey R. Koseff, and Joel H. Ferziger. "A numerical study of the evolution and structure of homogeneous stably stratified sheared turbulence." Journal of Fluid Mechanics 237 (April 1992): 499–539. http://dx.doi.org/10.1017/s0022112092003513.
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The structure and evolution of homogeneous stably stratified sheared turbulence have been investigated through direct numerical simulation. In these simulations the primary dimensionless parameter is the Richardson number which measures the relative importance of stratification and mean shear.For Richardson numbers less than the transition value the Reynolds stress and vertical density flux are down-gradient. Some of the vertical kinetic energy gained indirectly through production is expended in creating potential energy. Included in this shear-dominated regime is the stationary Richardson number at which the turbulent kinetic energy is constant in time although the spectra are evolving. At low dimensionless shear rate the stationary Richardson number increases with increasing Reynolds number.At the transition Richardson number the maximum anisotropy and energy partition are achieved. For larger Richardson numbers potential energy is released into vertical kinetic energy and the vertical density flux becomes counter-gradient. The associated production reversal enhances the decay rate of the turbulent kinetic energy.The effects of other dimensionless parameters have been investigated. After initial transients the developed flow is rather insensitive to the presence of significant initial potential energy. An increase in the Schmidt number increases the effect of stable stratification, e.g. the counter-gradient vertical density flux occurs earlier.In the shear dominated case the down-gradient fluxes are produced by the pumping of fluid through coherent hairpin-shaped vorticity. In the buoyancy dominated flow the counter-gradient fluid parcels induce helical vorticity structures as they move toward a position of neutral buoyancy.
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Schowalter, David G., Charles W. Van Van Atta, and Juan C. Lasheras. "A study of streamwise vortex structure in a stratified shear layer." Journal of Fluid Mechanics 281 (December 25, 1994): 247–91. http://dx.doi.org/10.1017/s0022112094003101.
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The existence of an organized streamwise vortical structure, which is superimposed on the well known coherent spanwise vorticity in nominally two-dimensional free shear layers, has been studied extensively. In the presence of stratification, however, buoyancy forces contribute to an additional mechanism for the generation of streamwise vorticity. As the spanwise vorticity layer rolls up and pulls high-density fluid above low-density fluid, a local instability results. The purpose of the current investigation is to force the three-dimensional instability in the stratified shear layer. In this manner, we experimentally observe the effect of buoyancy on the streamwise vortex tube evolution, the evolution of the buoyancy-induced instability, and the interaction between these two vortical structures. A simple numerical model is proposed which captures the relevant physics of the flow evolution. It is found that, depending on the location, streamwise vortices resulting from vortex stretching may be weakened or enhanced by the stratification. Buoyancy-induced vortex structures are shown to form where the unstable part of the interface is tilted by the streamwise vortex tubes. These vortices strengthen initially, then weaken downstream, the timescale for this process depending upon the degree of stratification. For initial Richardson numbers larger than about 0.03, the baroclinically weakened vortex tubes eventually disappear as the flow evolves downstream and the baroclinically generated vortices dominate the three-dimensional flow structure.
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SAMEEN, A., and RAMA GOVINDARAJAN. "The effect of wall heating on instability of channel flow." Journal of Fluid Mechanics 577 (April 19, 2007): 417–42. http://dx.doi.org/10.1017/s0022112007004636.
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A comprehensive study of the effect of wall heating or cooling on the linear, transient and secondary growth of instability in channel flow is conducted. The effect of viscosity stratification, heat diffusivity and of buoyancy are estimated separately, with some unexpected results. From linear stability results, it has been accepted that heat diffusivity does not affect stability. However, we show that realistic Prandtl numbers cause a transient growth of disturbances that is an order of magnitude higher than at zero Prandtl number. Buoyancy, even at fairly low levels, gives rise to high levels of subcritical energy growth. Unusually for transient growth, both of these are spanwise-independent and not in the form of streamwise vortices. At moderate Grashof numbers, exponential growth dominates, with distinct Poiseuille–Rayleigh–Bénard and Tollmien–Schlichting modes for Grashof numbers up to ∼ 25 000, which merge thereafter. Wall heating has a converse effect on the secondary instability compared to the primary instability, destabilizing significantly when viscosity decreases towards the wall. It is hoped that the work will motivate experimental and numerical efforts to understand the role of wall heating in the control of channel and pipe flows.
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Clarisse, J. M., J. L. Pfister, S. Gauthier, and C. Boudesocque-Dubois. "A hydrodynamic analysis of self-similar radiative ablation flows." Journal of Fluid Mechanics 848 (June 5, 2018): 219–55. http://dx.doi.org/10.1017/jfm.2018.343.
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Self-similar solutions to the compressible Euler equations with nonlinear conduction are considered as particular instances of unsteady radiative deflagration – or ‘ablation’ – waves with the goal of characterizing the actual hydrodynamic properties that such flows may present. The chosen family of solutions, corresponding to the ablation of an initially quiescent perfectly cold and homogeneous semi-infinite slab of inviscid compressible gas under the action of increasing external pressures and radiation fluxes, is well suited to the description of the early ablation of a target by gas-filled cavity X-rays in experiments of high energy density physics. These solutions are presently computed by means of a highly accurate numerical method for the radiative conduction model of a fully ionized plasma under the approximation of a non-isothermal leading shock wave. The resulting set of solutions is unique for its high fidelity description of the flows down to their finest scales and its extensive exploration of external pressure and radiative flux ranges. Two different dimensionless formulations of the equations of motion are put forth, yielding two classifications of these solutions which are used for carrying out a quantitative hydrodynamic analysis of the corresponding flows. Based on the main flow characteristic lengths and on standard characteristic numbers (Mach, Péclet, stratification and Froude numbers), this analysis points out the compressibility and inhomogeneity of the present ablative waves. This compressibility is further analysed to be too high, whether in terms of flow speed or stratification, for the low Mach number approximation, often used in hydrodynamic stability analyses of ablation fronts in inertial confinement fusion (ICF), to be relevant for describing these waves, and more specifically those with fast expansions which are of interest in ICF. Temperature stratification is also shown to induce, through the nonlinear conductivity, supersonic upstream propagation of heat-flux waves, besides a modified propagation of quasi-isothermal acoustic waves, in the flow conduction regions. This description significantly departs from the commonly admitted depiction of a quasi-isothermal conduction region where wave propagation is exclusively ascribed to isothermal acoustics and temperature fluctuations are only diffused.
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Weigert, Oliver, and David M. Weinstock. "The promises and challenges of using gene mutations for patient stratification in follicular lymphoma." Blood 130, no. 13 (September 28, 2017): 1491–98. http://dx.doi.org/10.1182/blood-2017-07-737353.
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Abstract Follicular lymphoma (FL) is a clinically and molecularly highly heterogeneous disease. Most patients achieve long-lasting remissions and have excellent overall survival (OS) with current treatment. However, ∼20% of patients have early progression of disease and short OS. At present, therapies are not guided by individual risk or disease biology. Reliable tools for patient stratification are urgently needed to avoid overtreatment of low-risk patients and to prioritize alternative approaches in high-risk patients. A rapidly expanding repertoire of promising therapeutic options is available for clinical evaluation; however, the numbers of patients with FL and the resources to conduct adequately powered trials are limited. Recent studies have shown that gene mutations can serve as prognostic and/or predictive biomarkers, in particular when integrated into composite risk models. Before translating these findings into routine clinical practice, however, several challenges loom. We review aspects of “clinicogenetic” risk model development and validation that apply to FL and more generally to other cancers. Finally, we propose a crowdsourcing effort that could expedite the development, validation, refinement, and selection of risk models. A new era of collaboration and harmonization is required if we hope to transition from empiric selection of therapeutics to risk-based, biology-guided treatment of patients with FL.
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Gelfgat, A. Yu. "Destabilization of free convection by weak rotation." Journal of Fluid Mechanics 685 (September 21, 2011): 377–412. http://dx.doi.org/10.1017/jfm.2011.323.
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AbstractThis study offers an explanation of the recently observed effect of destabilization of free convective flows by weak rotation. After studying several models where flows are driven by the simultaneous action of convection and rotation, it is concluded that destabilization is observed in cases where the centrifugal force acts against the main convective circulation. At relatively low Prandtl numbers, this counter-action can split the main vortex into two counter-rotating vortices, where the interaction leads to instability. At larger Prandtl numbers, the counter-action of the centrifugal force steepens an unstable thermal stratification, which triggers the Rayleigh–Bénard instability mechanism. Both cases can be enhanced by advection of azimuthal velocity disturbances towards the axis, where they grow and excite perturbations of the radial velocity. The effect was studied by considering a combined convective and rotating flow in a cylinder with a rotating lid and a parabolic temperature profile at the sidewall. Next, explanations of the destabilization effect for rotating-magnetic-field-driven flow and melt flow in a Czochralski crystal growth model were derived.
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Mkhinini, Nadia, Thomas Dubos, and Philippe Drobinski. "Secondary instability of the stably stratified Ekman layer." Journal of Fluid Mechanics 728 (July 1, 2013): 29–57. http://dx.doi.org/10.1017/jfm.2013.250.
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AbstractThe Ekman flow, an exact solution of the Boussinesq equations with rotation, is a prototype flow for both atmospheric and oceanic boundary layers. The effect of stratification on the finite-amplitude longitudinal rolls developing in the Ekman flow and their three-dimensional stability is studied by means of linearized and nonlinear numerical simulations. Similarities and differences with respect to billows developing in the Kelvin–Helmholtz (KH) unidirectional stratified shear flow are discussed. Prandtl number effects are investigated as well as the role played by the buoyant-convective instability. For low Prandtl number, the amplitude of the saturated rolls vanishes at the critical bulk Richardson number, while at high Prandtl number, finite-amplitude rolls are found. The Prandtl number also affects how the growth rate of the secondary instability evolves as the Richardson number is increased. For low Prandtl number, the growth rate decreases as the Richardson number increases while it remains significant for large Prandtl number over the range of stratification studied. This behaviour is likely a result of the differing amplitudes of the roll vortices. Furthermore, the most unstable wave vector is much lower than for the secondary instability of KH billows. Examination of the energetics of the secondary instability shows that buoyant-convective instability is present locally at high Reynolds and Prandtl numbers but plays an overall minor role despite the presence in the base flow of statically unstable regions characterized by a high Richardson number.
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WINGATE, BETH A., PEDRO EMBID, MIRANDA HOLMES-CERFON, and MARK A. TAYLOR. "Low Rossby limiting dynamics for stably stratified flow with finite Froude number." Journal of Fluid Mechanics 676 (April 27, 2011): 546–71. http://dx.doi.org/10.1017/jfm.2011.69.
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In this paper, we explore the strong rotation limit of the rotating and stratified Boussinesq equations with periodic boundary conditions when the stratification is order 1 ([Rossby number] Ro = ε, [Froude number] Fr = O(1), as ε → 0). Using the same framework of Embid & Majda (Geophys. Astrophys. Fluid Dyn., vol. 87, 1998, p. 1), we show that the slow dynamics decouples from the fast. Furthermore, we derive equations for the slow dynamics and their conservation laws. The horizontal momentum equations reduce to the two-dimensional Navier–Stokes equations. The equation for the vertically averaged vertical velocity includes a term due to the vertical average of the buoyancy. The buoyancy equation, the only variable to retain its three-dimensionality, is advected by all three two-dimensional slow velocity components. The conservation laws for the slow dynamics include those for the two-dimensional Navier–Stokes equations and a new conserved quantity that describes dynamics between the vertical kinetic energy and the buoyancy. The leading order potential enstrophy is slow while the leading order total energy retains both fast and slow dynamics. We also perform forced numerical simulations of the rotating Boussinesq equations to demonstrate support for three aspects of the theory in the limit Ro → 0: (i) we find the formation and persistence of large-scale columnar Taylor–Proudman flows in the presence of O(1) Froude number; after a spin-up time, (ii) the ratio of the slow total energy to the total energy approaches a constant and that at the smallest Rossby numbers that constant approaches 1 and (iii) the ratio of the slow potential enstrophy to the total potential enstrophy also approaches a constant and that at the lowest Rossby numbers that constant is 1. The results of the numerical simulations indicate that even in the presence of the low wavenumber white noise forcing the dynamics exhibit characteristics of the theory.
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Djebali, Ridha, Mohamed Ammar Abbassi, and Ahlem Rouahi. "Conjugate Effects of Buoyancy and Magnetic Field on Heat and Fluid Flow Pattern at Low-to-Moderate Prandtl Numbers." International Letters of Chemistry, Physics and Astronomy 66 (May 2016): 79–95. http://dx.doi.org/10.18052/www.scipress.com/ilcpa.66.79.
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This study aims to present a numerical investigation of unsteady two-dimensional natural convection of an electrically conducting fluid in a square medium under externally imposed magnetic field. A temperature gradient is applied between the two opposing side walls parallel to y-direction, while the floor and ceiling parallel to x-direction are kept adiabatic. The coupled momentum and energy equations associated with the Lorentz ‘decelerating’ force as well as the buoyancy force terms are solved using the single relaxation lattice Boltzmann (LB) approach. The flow is characterized by the Rayleigh number Ra (103-106), the Prandtl number Pr (0.01-10), the Hartman number Ha (0-100) determined by the strength of the imposed magnetic field and its tilt angle from x-axis ranging from 0° to 90°. The changes in the buoyant flow patterns and temperature contours due to the effects of varying the controlling parameters and associated heat transfer are examined. It was found that the developed thermal LB model gives excellent results by comparison with former experimental and numerical findings. Starting from the values 105 of the Rayleigh number Ra and Ha=0, the flow is unsteady multicellular for low Prandtl number typical of liquid metal. Increasing gradually Pr, the flow undergoes transition to steady bicellular. The transition occurs at a threshold value between Pr=0.01 and 0.1. Increasing more the Prandtl number, the flow structure is distorted due to the viscous forces which outweigh the buoyancy forces and a thermal stratification is clearly established. For high Hartman number, the damping effects suppress the unsteady behaviour and results in steady state with extended unicellular pattern in the direction of Lorentz force and the heat transfer rate is reduced considerably.
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KING, STUART E., MAGDA CARR, and DAVID G. DRITSCHEL. "The steady-state form of large-amplitude internal solitary waves." Journal of Fluid Mechanics 666 (November 10, 2010): 477–505. http://dx.doi.org/10.1017/s0022112010004301.
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A new numerical scheme for obtaining the steady-state form of an internal solitary wave of large amplitude is presented. A stratified inviscid two-dimensional fluid under the Boussinesq approximation flowing between horizontal rigid boundaries is considered. The stratification is stable, and buoyancy is continuously differentiable throughout the domain of the flow. Solutions are obtained by tracing the buoyancy frequency along streamlines from the undisturbed far field. From this the vorticity field can be constructed and the streamfunction may then be obtained by inversion of Laplace's operator. The scheme is presented as an iterative solver, where the inversion of Laplace's operator is performed spectrally. The solutions agree well with previous results for stratification in which the buoyancy frequency is a discontinuous function. The new numerical scheme allows significantly larger amplitude waves to be computed than have been presented before and it is shown that waves with Richardson numbers as low as 0.062 can be computed straightforwardly. The method is also extended to deal in a novel way with closed streamlines when they occur in the domain. The new solutions are tested in independent fully nonlinear time-dependent simulations and are verified to be steady. Waves with regions of recirculation are also discussed.
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Peter, Jochen, and Siamak Pourhassan. "Evidence of Foam Sclerotherapy in the Treatment of Recurrent Varicose Veins." Phlebologie 48, no. 02 (April 2019): 95–101. http://dx.doi.org/10.1055/a-0843-4324.
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Abstract Purpose Efficacy and safety of ultrasound-guided foam sclerotherapy (UGFS) for the treatment of primary venous disease is proven, but the main application field of UGFS remains the treatment of recurrent varicose veins (RVV). The aim of this study is to determine the extent to which this treatment indication is evidence-based. Methods In PubMed and other publicly accessible databases, literature was reviewed until June 2018 and checked for relevance. The focus was on results with regard to anatomical success as well as clinical success and perception by patients. Results Several observational studies comprised a subgroup of RVV patients, but systematic stratification for RVV patients were only rarely performed. In most of these studies including the few reports specifically investigating RVV patients anatomical success as the primary outcome measure revealed similar results compared to primary interventions and favourable results in patients with neovascularization. Data on clinical outcome and patient perception were inconclusive due to low study numbers and insufficient stratification. Complications in RVV patients were rare. Conclusion Although the advantages inherent to the UGFS principle provide a strong rationale that this method best meets the requirements of the specific RVV characteristics, concept and previous observations should be supported by more evidence. Systematic studies in RVV patients with sufficient sample size are required.
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Pieri, Alexandre B., F. S. Godeferd, C. Cambon, and A. Salhi. "Non-geostrophic instabilities of an equilibrium baroclinic state." Journal of Fluid Mechanics 734 (October 14, 2013): 535–66. http://dx.doi.org/10.1017/jfm.2013.478.
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AbstractWe consider non-geostrophic homogeneous baroclinic turbulence without solid boundaries, and we focus on its energetics and dynamics. The homogeneous turbulent flow is therefore submitted to both uniform vertical shear $S$ and stable vertical stratification, parametrized by the Brunt–Väisälä frequency $N$, and placed in a rotating frame with Coriolis frequency $f$. Direct numerical simulations show that the threshold of baroclinic instability growth depends mostly on two dimensionless numbers, the gradient Richardson number $\mathit{Ri}= {N}^{2} / {S}^{2} $ and the Rossby number $\mathit{Ro}= S/ f$, whereas linear theory predicts a threshold that depends only on $\mathit{Ri}$. At high Rossby numbers the nonlinear limit is found to be $\mathit{Ri}= 0. 2$, while in the limit of low $\mathit{Ro}$ the linear stability bound $\mathit{Ri}= 1$ is recovered. We also express the stability results in terms of background potential vorticity, which is an important quantity in baroclinic flows. We show that the linear symmetric instability occurs from the presence of negative background potential vorticity. The possibility of simultaneous existence of symmetric and baroclinic instabilities is also investigated. The dominance of symmetric instability over baroclinic instability for $\mathit{Ri}\ll 1$ is confirmed by our direct numerical simulations, and we provide an improved understanding of the dynamics of the flow by exploring the details of energy transfers for moderate Richardson numbers.
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Cox, Grace A., Christopher J. Davies, Philip W. Livermore, and James Singleton. "Penetration of boundary-driven flows into a rotating spherical thermally stratified fluid." Journal of Fluid Mechanics 864 (February 11, 2019): 519–53. http://dx.doi.org/10.1017/jfm.2018.999.
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Motivated by the dynamics within terrestrial bodies, we consider a rotating, strongly thermally stratified fluid within a spherical shell subject to a prescribed laterally inhomogeneous heat-flux condition at the outer boundary. Using a numerical model, we explore a broad range of three key dimensionless numbers: a thermal stratification parameter (the relative size of boundary temperature gradients to imposed vertical temperature gradients), $10^{-3}\leqslant S\leqslant 10^{4}$, a buoyancy parameter (the strength of applied boundary heat-flux anomalies), $10^{-2}\leqslant B\leqslant 10^{6}$, and the Ekman number (ratio of viscous to Coriolis forces), $10^{-6}\leqslant E\leqslant 10^{-4}$. We find both steady and time-dependent solutions and delineate the regime boundaries. We focus on steady-state solutions, for which a clear transition is found between a low $S$ regime, in which buoyancy dominates the dynamics, and a high $S$ regime, in which stratification dominates. For the low-$S$ regime, we find that the characteristic flow speed scales as $B^{2/3}$, whereas for high-$S$, the radial and horizontal velocities scale respectively as $u_{r}\sim S^{-1}$, $u_{h}\sim S^{-3/4}B^{1/4}$ and are confined within a thin layer of depth $(SB)^{-1/4}$ at the outer edge of the domain. For the Earth, if lower mantle heterogeneous structure is due principally to chemical anomalies, we estimate that the core is in the high-$S$ regime and steady flows arising from strong outer boundary thermal anomalies cannot penetrate the stable layer. However, if the mantle heterogeneities are due to thermal anomalies and the heat-flux variation is large, the core will be in a low-$S$ regime in which the stable layer is likely penetrated by boundary-driven flows.
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Mussolin, Lara, Christine Damm-Welk, Marta Pillon, and Wilhelm Woessmann. "Minimal Disease Monitoring in Pediatric Non-Hodgkin’s Lymphoma: Current Clinical Application and Future Challenges." Cancers 13, no. 8 (April 15, 2021): 1907. http://dx.doi.org/10.3390/cancers13081907.
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Minimal residual disease (MRD) detection is established routine practice for treatment stratification in leukemia and used for treatment optimization in adult lymphomas. Minimal disease studies in childhood non-Hodgkin lymphomas are challenged by stratified treatment in different subtypes, high cure rates, low patient numbers, limited initial tumor material, and early progression. Current clinical applications differ between the subtypes. A prognostic value of minimal disseminated disease (MDD) could not yet be clearly established for lymphoblastic lymphoma using flow cytometry and PCR-based methods for T-cell receptor (TCR) or immunoglobulin (IG) rearrangements. MYC–IGH fusion sequences or IG rearrangements enable minimal disease detection in Burkitt lymphoma and -leukemia. An additional prognostic value of MDD in Burkitt lymphoma and early MRD in Burkitt leukemia is implicated by single studies with risk-adapted therapy. MDD and MRD determined by PCR for ALK-fusion transcripts are independent prognostic parameters for patients with ALK-positive anaplastic large cell lymphoma (ALCL). They are introduced in routine clinical practice and used for patient stratification in clinical studies. Early MRD might serve as an endpoint for clinical trials and for guiding individual therapy. Validation of MDD and MRD as prognostic parameters is required for all subtypes but ALCL. Next-generation sequencing-based methods may provide new options and applications for minimal disease evaluation in childhood lymphomas.
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Kazuma, Yasuhiro, Kazunari Aoki, Yotaro Ochi, Yusuke Koba, Yoshimitsu Shimomura, Yosuke Nagahata, Nobuhiko Yamauchi, et al. "Risk Stratification of DLBCL Patients According to NCCN-IPI in Our Hospital." Blood 124, no. 21 (December 6, 2014): 3030. http://dx.doi.org/10.1182/blood.v124.21.3030.3030.
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Abstract Title Risk stratification of DLBCL patients according to NCCN-IPI in our hospital Background Recently, a robust prognostic tool termed enhanced International Prognostic Index (NCCN-IPI) for the rituximab era was reported. The aim of this study was to assess the usefulness of NCCN-IPI in risk discrimination and its suitability in clinical applications. Patients and Methods We retrospectively analyzed consecutive patients with de novo diffuse large B cell lymphoma (DLBCL) who were diagnosed and treated with R-CHOP or CHOP-like regimens between January 2004 and December 2013. Patients were required to be cancer-free for 5 years before diagnosis and had to have no prior documented history of indolent lymphoma. We stratified DLBCL patients using IPI and NCCN-IPI, and estimated overall survival (OS) in each risk group. The unadjusted probabilities of OS were estimated using the Kaplan-Meier(K-M) method. The log-lank test and multivariate Cox regression analysis were used to assess the prognostic values of each clinical variable. Results Three-hundred and seventy one patients were identified. The median age was 69 (20–93) years. The median follow-up time was 41 months. The numbers of patients with NCCN-IPI-defined low (L), low-intermediate (L-I), high-intermediate (H-I), and high (H) risk were 35 (9.4%), 125 (33%), 139 (37%), and 72(19%), respectively. The 3-year OS in each risk group was 93%, 89%, 70%, and 52%, respectively. NCCN-IPI was better for the discrimination of low- and high-risk groups (3-year OS, 93% vs 52%) than IPI (3-year OS, 87% vs 56%), and NCCN-IPI gave a better concordance index than IPI (c-Harrel = 0.678 vs 0.665). However, the discriminating power of NCCN-IPI was not as good as previously reported. In the multivariate analysis using the five independent variables of NCCN-IPI as covariates, age was not a significant factor in the <40, 40-60, and 61-75 age groups, and LDH ratio was not a significant factor between≤1 and >1-3 groups. Ann Arbor stage was not a significant factor either. Next, we examined extranodal involvement, which was a significant prognostic factor (HR, 2.3; p<0.001). Multivariate analysis, using age, LDH, performance status, stage, and extranodal involvement in major organs as covariates, lung (HR, 3.5; p<0.001) and bone marrow (HR, 1.8; p=0.048) involvement were significantly poor prognostic factors, and CNS (HR, 2.5; p=0.066) or GI tract (HR, 1.4; p=0.13) involvement adversely affected OS, although they were not significant factors. In contrast, liver involvement did not affect OS (HR, 0.87; p=0.70). We simplified the NCCN-IPI method by using a maximum of four scoring points for age (>75, 1pt), LDH ratio (>3, 1pt), extranodal disease (bone marrow, lung, CNS, or GI tract, but not liver, 1pt), and ECOG PS (≥2, 1pt) and not using Ann Arbor stage as a prognostic factor. Four distinct groups were formed based on K-M curves for OS: low (L, 0 pt), low-intermediate (L-I, 1pt), high-intermediate (H-I, 2pt), and high (H, 3-4 pt). Simplified NCCN-IPI better discriminated low- and high-risk groups (3-year OS, 93% vs 34%; c-Harrel = 0.705) than NCCN-IPI. When patient age was compared in each scoring system, high-risk patients were older in the NCCN-IPI (median age, 79 years old) and its simplified version (median age, 79 years old) than in IPI (median age, 72 years old). Conclusion NCCN-IPI showed better discrimination in our cohort than IPI. However, we found that the enhancement in predicting outcomes by including age and LDH was not as useful as previously reported, and stage and liver involvement was not an independent prognostic factor in our cohort. In the rituximab era, Ann Arbor stage is not a useful prognostic factor. Further validation and optimization of cut-off in each variable could improve the NCCN-IPI. The majority of the high risk patients evaluated by both NCCN-IPI and simplified NCCN-IPI were elderly, so innovative therapeutic approaches adjusted for age are required to improve the outcome of the high-risk group. To identify high-risk groups especially among younger patients, a refined scoring system including not only conventional clinical factors but also other factors such as biological markers will be required. Figure 1 Figure 1. Figure 2 Figure 2. Figure 3 Figure 3. Disclosures No relevant conflicts of interest to declare.
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PAWLAK, G., and L. ARMI. "Mixing and entrainment in developing stratified currents." Journal of Fluid Mechanics 424 (November 16, 2000): 45–73. http://dx.doi.org/10.1017/s0022112000001877.
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The spatially developing region of a steady downslope current in transition from a hydraulically controlled, arrested wedge flow is examined through a set of laboratory experiments. The mixing and entrainment mechanisms at the shear interface are explored with the use of imaging techniques. The initial, unstable accelerating region, characterized by rapid development, low shear layer Richardson numbers and high entrainment rates, is limited by the effects of stratification which suppress the large-scale overturning. A subsequent high Richardson number region is characterized by weak entrainment and the collapse of turbulence from the initial region. The flow approaches a quasi-stable state as the mixed layer draws energy from the accelerating density current. Observed large-scale structure is attributed to an instability of the marginally unstable shear layer. Entrainment rates are calculated within each region for the various slopes considered and estimates for the extent of the transition regions are obtained.
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Schmid, Bernhard H., Michael A. Hengl, and Ursula Stephan. "Density effects on salt tracer breakthrough curves from constructed wetland ponds." Hydrology Research 35, no. 3 (June 1, 2004): 237–50. http://dx.doi.org/10.2166/nh.2004.0017.
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Salt tracer experiments are a convenient method to determine travel time distributions in constructed wetland ponds. Typically, these flows are characterized by low Reynolds numbers at times even within the laminar flow regime. In this environment the injection of salt may cause strong density effects, thereby jeopardizing the usefulness of the recorded breakthrough curves. After a tracer experiment has been completed, an indication of potential density stratification in the field may be noticed in the form of surprisingly small recovery rates of a tracer considered as nearly conservative. To provide a tool that permits the intended experiment to be judged at the planning stage already, criteria have been developed that yield approximate maximum concentrations, not to be exceeded if density effects shall be avoided. Laboratory experiments were carried out and the newly derived relationships applied with success. The criteria may in future be useful, too, in the planning of tracer experiments in slowly flowing rivers and streams.
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Banakh, Viktor A., Igor N. Smalikho, and Andrey V. Falits. "Wind–Temperature Regime and Wind Turbulence in a Stable Boundary Layer of the Atmosphere: Case Study." Remote Sensing 12, no. 6 (March 16, 2020): 955. http://dx.doi.org/10.3390/rs12060955.
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The paper presents the results of probing the stable atmospheric boundary layer in the coastal zone of Lake Baikal with a coherent Doppler wind lidar and a microwave temperature profiler. Two-dimensional height–temporal distributions of the wind velocity vector components, temperature, and parameters characterizing atmospheric stability and wind turbulence were obtained. The parameters of the low-level jets and the atmospheric waves arising in the stable boundary layer were determined. It was shown that the stable atmospheric boundary layer has an inhomogeneous fine scale layered structure characterized by strong variations of the Richardson number Ri. Layers with large Richardson numbers alternate with layers where Ri is less than the critical value of the Richardson number Ricr = 0.25. The channels of decreased stability, where the conditions are close to neutral stratification 0 < Ri < 0.25, arise in the zone of the low-level jets. The wind turbulence in the central part of the observed jets, where Ri > Ricr, is weak, increases considerably to the periphery of jets, at heights where Ri < Ricr. The turbulence may intensify at the appearance of internal atmospheric waves.
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Norrish, Gabrielle, Tao Ding, Ella Field, Karen McLeod, Maria Ilina, Graham Stuart, Vinay Bhole, et al. "A validation study of the European Society of Cardiology guidelines for risk stratification of sudden cardiac death in childhood hypertrophic cardiomyopathy." EP Europace 21, no. 10 (June 1, 2019): 1559–65. http://dx.doi.org/10.1093/europace/euz118.
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Abstract Aims Sudden cardiac death (SCD) is the most common cause of death in children with hypertrophic cardiomyopathy (HCM). The European Society of Cardiology (ESC) recommends consideration of an implantable cardioverter-defibrillator (ICD) if two or more clinical risk factors (RFs) are present, but this approach to risk stratification has not been formally validated. Methods and results Four hundred and eleven paediatric HCM patients were assessed for four clinical RFs in accordance with current ESC recommendations: severe left ventricular hypertrophy, unexplained syncope, non-sustained ventricular tachycardia, and family history of SCD. The primary endpoint was a composite outcome of SCD or an equivalent event (aborted cardiac arrest, appropriate ICD therapy, or sustained ventricular tachycardia), defined as a major arrhythmic cardiac event (MACE). Over a follow-up period of 2890 patient years (median 5.5 years), MACE occurred in 21 patients (7.5%) with 0 RFs, 19 (16.8%) with 1 RFs, and 3 (18.8%) with 2 or more RFs. Corresponding incidence rates were 1.13 [95% confidence interval (CI) 0.7–1.73], 2.07 (95% CI 1.25–3.23), and 2.52 (95% CI 0.53–7.35) per 100 patient years at risk. Patients with two or more RFs did not have a higher incidence of MACE (log-rank test P = 0.34), with a positive and negative predictive value of 19% and 90%, respectively. The C-statistic was 0.62 (95% CI 0.52–0.72) at 5 years. Conclusions The incidence of MACE is higher for patients with increasing numbers of clinical RFs. However, the current ESC guidelines have a low ability to discriminate between high- and low-risk individuals.
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TORRES, C. R., H. HANAZAKI, J. OCHOA, J. CASTILLO, and M. VAN WOERT. "Flow past a sphere moving vertically in a stratified diffusive fluid." Journal of Fluid Mechanics 417 (August 25, 2000): 211–36. http://dx.doi.org/10.1017/s0022112000001002.
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Numerical studies are described of the flows generated by a sphere moving vertically in a uniformly stratified fluid. It is found that the axisymmetric standing vortex usually found in homogeneous fluids at moderate Reynolds numbers (25 [les ] Re [les ] 200) is completely collapsed by stable stratification, generating a strong vertical jet. This is consistent with our experimental visualizations. For Re = 200 the complete collapse of the vortex occurs at Froude number F ≃ 19, and the critical Froude number decreases slowly as Re increases. The Froude number and the Reynolds number are here defined by F = W/Na and Re = 2Wa/v, with W being the descent velocity of the sphere, N the Brunt–Väisälä frequency, a the radius of the sphere and v the kinematic viscosity coefficient. The inviscid processes, including the generation of the vertical jet, have been investigated by Eames & Hunt (1997) in the context of weak stratification without buoyancy effects. They showed the existence of a singularity of vorticity and density gradient on the rear axis of the flow and also the impossibility of realizing a steady state. When there is no density diffusion, all the isopycnal surfaces which existed initially in front of the sphere accumulate very near the front surface because of density conservation and the fluid in those thin layers generates a rear jet when returning to its original position. In the present study, however, the fluid has diffusivity and the buoyancy effects also exist. The density diffusion prevents the extreme piling up of the isopycnal surfaces and allows the existence of a steady solution, preventing the generation of a singularity or a jet. On the other hand, the buoyancy effect works to increase the vertical velocity to the rear of the sphere by converting the potential energy to vertical kinetic energy, leading to the formation of a strong jet. We found that the collapse of the vortex and the generation of the jet occurs at much weaker stratifications than those necessary for the generation of strong lee waves, showing that jet formation is independent of the internal waves. At low Froude numbers (F [les ] 2) the lee wave patterns showed good agreement with the linear wave theory and the previous experiments by Mowbray & Rarity (1967). At very low Froude numbers (F [les ] 1) the drag on a sphere increases rapidly, partly due to the lee wave drag but mainly due to the large velocity of the jet. The jet causes a reduction of the pressure on the rear surface of the sphere, which leads to the increase of pressure drag. High velocity is induced also just outside the boundary layer of the sphere so that the frictional drag increases even more significantly than the pressure drag.
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Rosendaal, Frits, Roberta Palla, Isabella Garagiola, Piermannuccio Mannucci, and Flora Peyvandi. "Genetic Risk Stratification to Minimize Inhibitor Risk with the Use of Recombinant Factor VIII Concentrates: A Sippet Analysis." Blood 128, no. 22 (December 2, 2016): 325. http://dx.doi.org/10.1182/blood.v128.22.325.325.
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Abstract Background The development of neutralizing antibodies against factor VIII is a common and serious complication of replacement therapy, occurring mainly in the early stages of treatment. Meta-analyses of observational studies have suggested a higher risk of inhibitor development with concentrates produced by recombinant technologies (rFVIII) than with those derived from human plasma (pdFVIII) containing von Willebrand factor, which was recently confirmed in a randomized trial. In this trial cumulative incidences of inhibitor development were 44.5% for rFVIII and 26.8% for pdFVIII, for a hazard ratio (HR) of 1.87 (95% confidence interval (CI95) 1.17-2.96). Given the particularly high risk with rFVIII , it has been suggested to restrict the use of rFVIII to low risk patients, and treat high-risk patients with pdFVIII. We investigated such a strategy in a post-hoc analysis of the SIPPET study, in which we used the FVIII genotype (F8 gene mutation) to classify patients by prior risk. Methods SIPPET is an open label international randomized trial on which 251 previously untreated (n=142) or minimally treated (less than five exposure to blood components other than concentrate or cryoprecipitate, n=109) in 42 centers to be treated exclusively with a concentrate from the class of rFVIII or pdFVIII. Patients were tested for inhibitors before entry and at regular intervals during 50 exposure days, 3 years or the development of an inhibitor of at least 0.4 Bethesda units (BU). The trial ran from 2010 to 2014 and was terminated when the prespecified number of patients was included. Patients who had not reached 50ED by that time were censored. Patients were classified at high risk when they carried a null mutation (inversion, large deletion, frameshift, nonsense mutation) in the F8 gene and as low risk when they carried another causative variant (missense, splice site, polymorphisms, no mutation). We estimated cumulative incidences, hazard ratios and numbers needed to harm (NNH) for rFVIII vs pdFVIII for high- and low risk patients. Results Among 251 patients, 76 developed an inhibitor (all > 0.7 BU) of which 50 were high- titer (> 5 BU). Among 197 patients classified as high risk, 65 developed an inhibitor (cumulative incidence 38.2%, CI95 30.8-45.6), whereas among the 38 patients classified as low risk 7 developed an inhibitor (cumulative incidence 23.9%, CI95 8.2-39.6). High and low risk patients were equally distributed over the two arms of the trial, i.e., 96 out of 126 treated with rFVIII were high risk, and 101 out of 125 treated with pdFVIII. Among high risk patients, cumulative incidence was 30.7% when treated with pdFVIII , and 46.5% when treated with rFVIII (risk difference 15.8%). Among low risk patients, no inhibitors developed with pdFVIII, whereas the cumulative incidence was 43.2% with rFVIII (risk difference 43.2%). This implies that the Number Needed to Harm was 5.6 overall, 6.3 for high-risk patients, and 2.3 in low risk patients. Conclusion Risk stratification by the type of F8 mutation does not identify previously untreated patients with hemophilia A who have a low inhibitor risk when exposed to rFVIII. Other means need to be found to reduce the occurrence of inhibitors with rFVIII. Disclosures Palla: Pfizer: Other: travel support . Mannucci:NovoNordisk: Speakers Bureau; Kedrion: Speakers Bureau; Grifols: Speakers Bureau; Bayer: Speakers Bureau. Peyvandi:Bayer: Speakers Bureau; SOBI: Speakers Bureau; Kedrion Biopharma: Consultancy, Other: research funding paid to Luigi Villa Foundation, Research Funding; Alexion: Other: research funding paid to Luigi Villa Foundation, Research Funding; Octapharma: Consultancy; Ablynx: Membership on an entity's Board of Directors or advisory committees, Other: research funding paid to Luigi Villa Foundation, Research Funding; Biotest: Other: research funding paid to Luigi Villa Foundation, Research Funding, Speakers Bureau; CSL Behring: Speakers Bureau; LFB: Consultancy; Grifols: Speakers Bureau; Novo Nordisk: Other: research funding paid to Luigi Villa Foundation, Research Funding, Speakers Bureau.
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Ovink, Sarah M. "“In Today’s Society, It’s a Necessity”." Social Currents 4, no. 2 (August 20, 2016): 128–45. http://dx.doi.org/10.1177/2329496516663220.
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Latino/a enrollments at U.S. colleges are rapidly increasing. However, Latinos/as remain underrepresented at four-year universities, and college completion rates and household earnings lag other groups’. Yet, little theoretical attention has been paid to the processes that drive these trends, or to what happens when students not traditionally expected to attend college begin to enroll in large numbers. Longitudinal interviews with 50 Latino/a college aspirants in the San Francisco East Bay Area reveal near-universal college enrollment among these mostly low-income youth, despite significant barriers. East Bay Latino/a youth draw on a set of interrelated logics (economic, regional, family/group, college-for-all) supporting their enrollment, because they conclude that higher education is necessary for socioeconomic mobility. In contrast to the predictions of status attainment and rational choice models, these rationally optimistic college aspirants largely ignore known risks, instead focusing on anticipated gains. Given a postrecession environment featuring increasing costs and uncertain employment, this approach led many to enroll in low-cost, less supportive two-year institutions, resulting in long and winding pathways for some. Results suggest that without structural supports, access to college fails to meaningfully redress stratification processes in higher education and the postrecession economy that significantly shape possibilities for mobility.
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Anamthawat-Jónsson, Kesara, Audrey Pace, and Sigurður H. Árnason. "Sea sandwort from Surtsey: chromosomal evidence of active evolution via wide-hybridization." Surtsey research 14 (June 2020): 99–114. http://dx.doi.org/10.33112/surtsey.14.9.
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Sea sandwort (Honckenya peploides) was among the first species of vascular plants colonizing Surtsey. It is a member of the carnation family, Caryophyllaceae, a coastal plant with circumpolar distribution. The species is dioecious comprising separate female and hermaphrodite (male) plants. Our previous study of this plant revealed high molecular polymorphism, indicating rapid expansion and multiple origins, but low genetic differentiation, suggesting gene flow on Surtsey. The maintenance and/or expansion of populations with high gene diversity on the island are most likely fostered by several factors, one of them being the polyploid nature of the study species providing fixed heterozygosity. We therefore investigated chromosome number diversity of H. peploides from Surtsey, in comparison with accessions from Heimaey and other locations within and outside Iceland. Seeds were germinated with and without cold stratification. Chromosomes were isolated from root tips using the cellulase-pectinase enzymatic squash method. DAPIstained chromosomes were counted from microscopic images that were taken at 1000x magnification. The results show that the most common 2n somatic chromosome number of this species is 68 (2n=4x=68), but a tetraploid cytotype with 66 chromosomes also exists. The karyotype analysis shows that the species is an autotetraploid, most likely originating via chromosome doubling (whole genome duplication) in a diploid ancestor. Numerous other 2n numbers were discovered, from the diploid number 2n=2x=34 in Heimaey to several different numbers between 40 and 64. The triploid hybrid numbers 2n=3x=51 (50-52) were discovered from both Surtsey and Heimaey, as well as from other regions. Triploid hybrids serve as a bridge promoting gene flow within populations, promoting heterozygosity in the tetraploid species. All other numbers are aneuploids, most likely deriving from back-crossing of triploid hybrids and the euploid parents. The presence of aneuploids across the species distribution range is due to its ability to propagate asexually by clonal expansion. The presence of the lower ploidy levels within species, together with the extensive aneuploidy, may be an evolutionary characteristic of a pioneering plant, with great dispersal ability and genetic diversity, such as sea sandwort.
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BOUDESOCQUE-DUBOIS, C., S. GAUTHIER, and J. M. CLARISSE. "Self-similar solutions of unsteady ablation flows in inertial confinement fusion." Journal of Fluid Mechanics 603 (April 30, 2008): 151–78. http://dx.doi.org/10.1017/s0022112008001043.
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We exhibit and detail the properties of self-similar solutions for inviscid compressible ablative flows in slab symmetry with nonlinear heat conduction which are relevant to inertial confinement fusion (ICF). These solutions have been found after several contributions over the last four decades. We first derive the set of ODEs – a nonlinear eigenvalue problem – which governs the self-similar solutions by using the invariance of the Euler equations with nonlinear heat conduction under the two-parameter Lie group symmetry. A sub-family which leaves the density invariant is detailed since these solutions may be used to model the ‘early-time’ period of an ICF implosion where a shock wave travels from the front to the rear surface of a target. A chart allowing us to determine the starting point of a numerical solution, knowing the physical boundary conditions, has been built. A physical analysis of these unsteady ablation flows is then provided, the associated dimensionless numbers (Mach, Froude and Péclet numbers) being calculated. Finally, we show that self-similar ablation fronts generated within the framework of the above hypotheses (electron heat conduction, growing heat flux at the boundary, etc.) and for large heat fluxes and not too large pressures at the boundary do not satisfy the low-Mach-number criteria. Indeed both the compressibility and the stratification of the hot-flow region are too large. This is, in particular, the case for self-similar solutions obtained for energies in the range of the future Laser MegaJoule laser facility. Two particular solutions of this latter sub-family have been recently used for studying stability properties of ablation fronts.