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1
Wood, Vincent T., e Luther W. White. "A Parametric Wind–Pressure Relationship for Rankine versus Non-Rankine Cyclostrophic Vortices". Journal of Atmospheric and Oceanic Technology 30, n. 12 (1 dicembre 2013): 2850–67. http://dx.doi.org/10.1175/jtech-d-13-00041.1.
Testo completoAbstract (sommario):
Abstract A parametric tangential wind profile model is presented for depicting representative pressure deficit profiles corresponding to varying tangential wind profiles of a cyclostrophic, axisymmetric vortex. The model employs five key parameters per wind profile: tangential velocity maximum, radius of the maximum, and three shape parameters that control different portions of the profile. The model coupled with the cyclostrophic balance assumption offers a diagnostic tool for estimating and examining a radial profile of pressure deficit deduced from a theoretical superimposing tangential wind profile in the vortex. Analytical results show that the shape parameters for a given tangential wind maximum of a non-Rankine vortex have an important modulating influence on the behavior of realistic tangential wind and corresponding pressure deficit profiles. The first parameter designed for changing the wind profile from sharply to broadly peaked produces the corresponding central pressure fall. An increase in the second (third) parameter yields the pressure rise by lowering the inner (outer) wind profile inside (outside) the radius of the maximum. Compared to the Rankine vortex, the parametrically constructed non-Rankine vortices have a larger central pressure deficit. It is suggested that the parametric model of non-Rankine vortex tangential winds has good potential for diagnosing the pressure features arising in dust devils, waterspouts, tornadoes, tornado cyclones, and mesocyclones. Finally, presented are two examples in which the parametric model is fitted to a tangential velocity profile, one derived from an idealized numerical simulation and the other derived from high-resolution Doppler radar data collected in a real tornado.
2
ROSENFELD, MOSHE, EDMOND RAMBOD e MORTEZA GHARIB. "Circulation and formation number of laminar vortex rings". Journal of Fluid Mechanics 376 (10 dicembre 1998): 297–318. http://dx.doi.org/10.1017/s0022112098003115.
Testo completoAbstract (sommario):
The formation time scale of axisymmetric vortex rings is studied numerically for relatively long discharge times. Experimental findings on the existence and universality of a formation time scale, referred to as the ‘formation number’, are confirmed. The formation number is indicative of the time at which a vortex ring acquires its maximal circulation. For vortex rings generated by impulsive motion of a piston, the formation number was found to be approximately four, in very good agreement with experimental results. Numerical extensions of the experimental study to other cases, including cases with thick shear layers, show that the scaled circulation of the pinched-off vortex is relatively insensitive to the details of the formation process, such as the velocity programme, velocity profile, vortex generator geometry and the Reynolds number. This finding might also indicate that the properly scaled circulation of steady vortex rings varies very little. The formation number does depend on the velocity profile. Non-impulsive velocity programmes slightly increase the formation number, while non-uniform velocity profiles may decrease it significantly. In the case of a parabolic velocity profile of the discharged flow, for example, the formation number decreases by a factor as large as four. These findings indicate that a major source of the experimentally found small variations in the formation number is the different evolution of the velocity profile of the discharged flow.
3
Sutardi, S., e Agung E. Nurcahya. "Experimental Study on the Effect of Vortex Generator on the Aerodynamic Characteristics of NASA LS-0417 Airfoil". Applied Mechanics and Materials 758 (aprile 2015): 63–69. http://dx.doi.org/10.4028/www.scientific.net/amm.758.63.
Testo completoAbstract (sommario):
Boundary layer flow structure developing on an airfoil surfaces strongly affects drag and lift forces acting on the body. Many studies have been done to reduce drag, such as introducing surface roughness on the airfoil surface, gas injection, attachment of vortex generators, or moving surface on the airfoil. Previous results showed that the attachment of vortex generators has potentially been able to control boundary layer separation compared to other controlling devices. This study is focused on the evaluation of the effect of vortex generator attachment on the NASA LS-0417 airfoil profile as this profile is commonly used in wind turbine blade application. The models of this experimental study are NASA LS-0417 profiles, with and without vortex generator. The chord length of the profile is 110 mm, while the span is 210 mm. Profile of the vortex generator is a symmetrical profile of NACA 0012 configured in counter rotating and attached on the upper surface of the main profile. The chord length of the vortex generator is 7 mm with two different values of the height (h): 1 mm and 2 mm. The experiment was conducted in an open loop wind tunnel with maximum attainable freestream velocity of approximately 19 m/s and the turbulence intensity at the tunnel centerline is approximately 0.8%. The wind tunnel cross section is octagonal of 30 cm x 30 cm and of 45 cm to 60 cm adjustable length. The study was performed at two different freestream velocities of 12 m/s and 17 m/s corresponding with Reynolds numbers (Re) of 0.83 x 105 and 1.18 x 105 based on the airfoil chord length and the freestream velocity. Angle of attact (α) was varied from 0o to 24o. Drag and lift were measured using a force balance with measurement uncertainty of approximately 0.77% and 2.47% at measured drag of 0.65N and at measured lift of 0.202N, respectively. A flow visualization study using oil flow method was conducted to obtain qualitaive picture of flow structure on the airfoil surface. Results of this study showed that attachment of the vortex generator on the NASA LS-0417 profile has not been able to improve the profile performance compared to that of unmodified profile. There, however, seems Reynolds number effect on the airfoil performance flow conditions performed in this study. At lager Re, there is an increase in CL/CD of approximately 36% at angle of attack (α) 6o. Next, based on the flow visualization results, attachment of the 2mm vortex generator on the airfoil NASA LS-0417 surface results in an advancement of boundary layer separation at the two Re’s conducted in this study. Finally, the 2mm vortex generator accelerates airfoil stall at approximately 16o, while the 1mm vortex generator is relatively no effect on the airfoil stall angle.
4
Wood, Vincent T., e Luther W. White. "A New Parametric Model of Vortex Tangential-Wind Profiles: Development, Testing, and Verification". Journal of the Atmospheric Sciences 68, n. 5 (1 maggio 2011): 990–1006. http://dx.doi.org/10.1175/2011jas3588.1.
Testo completoAbstract (sommario):
Abstract A new parametric model of vortex tangential-wind profiles is presented that is primarily designed to depict realistic-looking tangential wind profiles such as those in intense atmospheric vortices arising in dust devils, waterspouts, tornadoes, mesocyclones, and tropical cyclones. The profile employs five key parameters: maximum tangential wind, radius of maximum tangential wind, and three power-law exponents that shape different portions of the velocity profile. In particular, a new parameter is included controlling the broadly or sharply peaked profile in the annular zone of tangential velocity maximum. Different combinations of varying the model parameters are considered to investigate and understand their effects on the physical behaviors of tangential wind and corresponding vertical vorticity profiles. Additionally, the parametric tangential velocity and vorticity profiles are favorably compared to those of an idealized Rankine model and also those of a theoretical stagnant core vortex model in which no tangential velocity exists within a core boundary and a potential flow occurs outside the core. Furthermore, the parametric profiles are evaluated against and compared to those of two other idealized vortex models (Burgers–Rott and Sullivan). The comparative profiles indicate very good agreements with low root-mean-square errors of a few tenths of a meter per second and high correlation coefficients of nearly one. Thus, the veracity of the parametric model is demonstrated.
5
ALBRECHT, TRENTON R., ALAN R. ELCRAT e KENNETH G. MILLER. "Steady vortex dipoles with general profile functions". Journal of Fluid Mechanics 670 (7 febbraio 2011): 85–95. http://dx.doi.org/10.1017/s0022112010005665.
Testo completoAbstract (sommario):
Vortex dipoles in a two-dimensional, inviscid flow are obtained by prescribing the profile function relating the vorticity to the stream function. The profile functions used are smooth, and the solutions obtained have a smooth transition from the exterior flow to the interior of the vortex. The dipoles are nearly elliptical, and this relates this work to the ‘supersmooth’ dipoles obtained recently by Kizner & Khvoles (Regular Chaotic Dyn., vol. 9, 2004, pp. 509–518). The solutions found here are obtained by an iterative method for solving the nonlinear partial differential equation for the stream function. This iterative method is both robust and flexible. Solutions are also obtained in a β-plane, and they are shielded, as has also been found in previous work.
6
Martínez-Filgueira, P., U. Fernandez-Gamiz, E. Zulueta, I. Errasti e B. Fernandez-Gauna. "Parametric study of low-profile vortex generators". International Journal of Hydrogen Energy 42, n. 28 (luglio 2017): 17700–17712. http://dx.doi.org/10.1016/j.ijhydene.2017.03.102.
Testo completo
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Shen, Ya, Huimin Zhou, Jeffrey M. Coil, Bassim Aljazaeri, Rene Buttar, Zhejun Wang, Yu-feng Zheng e Markus Haapasalo. "ProFile Vortex and Vortex Blue Nickel-Titanium Rotary Instruments after Clinical Use". Journal of Endodontics 41, n. 6 (giugno 2015): 937–42. http://dx.doi.org/10.1016/j.joen.2015.02.003.
Testo completo
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Boldyrev, Aleksei V., Sergei V. Boldyrev e Dmitrii L. Karelin. "THE EFFECT OF BLADE PROFILE ON THE PERFORMANCE OF A SIDE CHANNEL PUMP". Tyumen State University Herald. Physical and Mathematical Modeling. Oil, Gas, Energy 6, n. 3 (2020): 23–37. http://dx.doi.org/10.21684/2411-7978-2020-6-3-23-37.
Testo completoAbstract (sommario):
This article presents the results of a numerical modeling of a steady turbulent flow of an incompressible fluid in an open-type vortex pump with an open side channel, comparing the generalized simulation results with the existing experimental data. The mathematical model is based on the Reynolds-averaged Navier — Stokes and continuity equations, as well as on the equations of the two-layer Realizable k-ε turbulence model that accounts for the curvature of streamlines. The authors have estimated the grid independence of the solution and studied the influence of 14 blade profiles on the head and efficiency of the vortex pump. The solution of the model equations was achieved by the finite volume method using a sequential algorithm in three calculation areas (“feeder channel”, “blade wheel”, “open hull side channel and diverter channel”) with the evaluation of grid independence of the solution. The result of the solution between the calculated areas was transmitted at the corresponding points of the interface surfaces. The authors have studied the influence of 14 profiles of a blade on pressure and efficiency of the vortex pump: the initial profile of the blade with the installation in the wheel coaxial shaft of the ring plate of different width, the initial profile of the blade with a bevel on the discharge side, a profile in the form of an isosceles triangle, a profile in the form of a quadrangle, the initial profile with a rounded blade on the suction side, and a profile in the form of a rectangular triangle with a rounded blade on the suction side, among others. The simulation results have aided in proposing the blade profiles: in the form of a rectangle with a convex rounding of the blade on the suction side with a 10 mm radius and a right-angled triangle with a concave rounding of the blade on the suction side with a 52 mm radius and without rounding, giving a significant increase in pressure — more than 20%. Nevertheless, none of the considered cases have revealed any significant increase in the vortex pump hydraulic efficiency.
9
Wang, Shuai, Fengbo Wen, Shibo Zhang, Shenzhan Zhang e Xun Zhou. "Influences of trailing boundary layer velocity profiles on wake vortex formation in a high-subsonic-turbine cascade". Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 233, n. 2 (6 giugno 2018): 186–98. http://dx.doi.org/10.1177/0957650918779935.
Testo completoAbstract (sommario):
In this paper, delayed detached eddy simulations are performed to study wake flows of a turbine blade at a high subsonic exit Mach number, [Formula: see text], and high Reynolds number, [Formula: see text], based on the chord length and outlet velocity. It is found that a slight change in the trailing suction profile would have a big influence on the formation of wake vortex street, which is believed to be caused by the change in the boundary layer state near the trailing edge, and suction boundary layer with a fuller velocity profile tends to destabilize the wake flow, promoting the generation of wake vortex and enhancing the unsteady effect. Local spatial-temporal stability analyses of the wake velocity profiles suggest that wake flows with asymmetric velocity profiles might have a stabilizing effect. It is suggested that the vortex formation and its strength can be controlled by making some slight modifications on the rear blade suction surface, and the mixing loss in the wake can be reduced due to a weaker unsteady effect.
10
Zierke, W. C., K. J. Farrell e W. A. Straka. "Measurements of the Tip Clearance Flow for a High-Reynolds-Number Axial-Flow Rotor". Journal of Turbomachinery 117, n. 4 (1 ottobre 1995): 522–32. http://dx.doi.org/10.1115/1.2836564.
Testo completoAbstract (sommario):
A high-Reynolds-number pump (HIREP) facility has been used to acquire flow measurements in the rotor blade tip clearance region, with blade chord Reynolds numbers of 3,900,000 and 5,500,000. The initial experiment involved rotor blades with varying tip clearances, while a second experiment involved a more detailed investigation of a rotor blade row with a single tip clearance. The flow visualization on the blade surface and within the flow field indicate the existence of a trailing-edge separation vortex, a vortex that migrates radially upward along the trailing edge and then turns in the circumferential direction near the casing, moving in the opposite direction of blade rotation. Flow visualization also helps in establishing the trajectory of the tip leakage vortex core and shows the unsteadiness of the vortex. Detailed measurements show the effects of tip clearance size and downstream distance on the structure of the rotor tip leakage vortex. The character of the velocity profile along the vortex core changes from a jetlike profile to a wakelike profile as the tip clearance becomes smaller. Also, for small clearances, the presence and proximity of the casing endwall affects the roll-up, shape, dissipation, and unsteadiness of the tip leakage vortex. Measurements also show how much circulation is retained by the blade tip and how much is shed into the vortex, a vortex associated with high losses.
11
Karlson, Matthew, Bogdan G. Nita e Ashwin Vaidya. "Numerical Computations of Vortex Formation Length in Flow Past an Elliptical Cylinder". Fluids 5, n. 3 (10 settembre 2020): 157. http://dx.doi.org/10.3390/fluids5030157.
Testo completoAbstract (sommario):
We examine two dimensional properties of vortex shedding past elliptical cylinders through numerical simulations. Specifically, we investigate the vortex formation length in the Reynolds number regime 10 to 100 for elliptical bodies of aspect ratio in the range 0.4 to 1.4. Our computations reveal that in the steady flow regime, the change in the vortex length follows a linear profile with respect to the Reynolds number, while in the unsteady regime, the time averaged vortex length decreases in an exponential manner with increasing Reynolds number. The transition in profile is used to identify the critical Reynolds number which marks the bifurcation of the Karman vortex from steady symmetric to the unsteady, asymmetric configuration. Additionally, relationships between the vortex length and aspect ratio are also explored. The work presented here is an example of a module that can be used in a project based learning course on computational fluid dynamics.
12
Wei, Maoxing, Nian-Sheng Cheng, Yee-Meng Chiew e Fengguang Yang. "Vortex Evolution within Propeller Induced Scour Hole around a Vertical Quay Wall". Water 11, n. 8 (25 luglio 2019): 1538. http://dx.doi.org/10.3390/w11081538.
Testo completoAbstract (sommario):
This paper presents an experimental study on the characteristics of the propeller-induced flow field and its associated scour hole around a closed type quay (with a vertical quay wall). An “oblique particle image velocimetry” (OPIV) technique, which allows a concurrent measurement of the velocity field and scour profile, was employed in measuring the streamwise flow field (jet central plane) and the longitudinal centerline scour profile. The asymptotic scour profiles obtained in this study were compared with that induced by an unconfined propeller jet in the absence of any berthing structure, which demonstrates the critical role of the presence of the quay wall as an obstacle in shaping the scour profile under the condition of different wall clearances (i.e., longitudinal distance between propeller and wall). Moreover, by comparing the vortical structure within the asymptotic scour hole around the vertical quay wall with its counterpart in the case of an open quay (with a slope quay wall), the paper examines the effect of quay types on the formation of the vortex system and how it determines the geometrical characteristic of the final scour profile. Furthermore, the temporal development of the mean vorticity field and the vortex system are discussed in terms of their implications on the evolution of the scour hole. In particular, comparison of the circulation development of the observed vortices allows a better understanding of the vortex scouring mechanism. Energy spectra analysis reveals that at the vortex centers, their energy spectra distributions consistently follow the −5/3 law throughout the entire scouring process. As the scour process evolves, the turbulent energy associated with the near-bed vortex, which is responsible for scouring, is gradually reduced, especially for the small-scale eddies, indicating a contribution of the dissipated turbulent energy in excavating the scour hole. Finally, a comparison of the near-bed flow characteristics of the average kinetic energy (AKE), turbulent kinetic energy (TKE), and Reynolds shear stress (RSS) are also discussed in terms of their implications for the scour hole development.
13
Lee, Yeon Ui, Igor Ozerov, Frédéric Bedu, Ji Su Kim, Frédéric Fages e Jeong Weon Wu. "Optical spin-dependent beam separation in cyclic group symmetric metasurface". Nanophotonics 9, n. 10 (25 giugno 2020): 3459–71. http://dx.doi.org/10.1515/nanoph-2020-0160.
Testo completoAbstract (sommario):
AbstractCross-polarization scattering of a circularly polarized beam from nano-rod introduces a geometric phase to the outgoing beam with opposite circular polarization. By manipulating the spatial array of subwavelength nano-structure constituting metasurface, the geometric phase can be engineered to generate a variety of beam profiles, including vortex beam carrying orbital angular momentum via a process called spin-to-orbital angular momentum conversion. Here we introduce a cyclic group symmetric metasurface composed of tapered arc nano-rods and explore how azimuthal angular distribution of total phase determines the feature of spin-dependent beam separation. When scattered from a circular array of tapered arc nano-rods possessing varying width with a fixed length, a dynamical phase having non-constant azimuthal gradient is introduced to an incoming Gaussian beam. This leads to a spin-dependent beam separation in the outgoing vortex beam profile, which is attributed to an azimuthal angle dependent destructive interference between scatterings from two plasmonic excitations along the width and the length of tapered arc nano-rod. Relation of cyclic group symmetry property of metasurface and the generated vortex beam profile is examined in detail by experimental measurement and analysis in terms of partial-wave expansion and non-constant azimuthal gradient of total phase. Capability of spatial beam profiling by spin-dependent beam separation in vortex beam generation has an important implication for spatial demultiplexing in optical communication utilizing optical angular momentum mode division multiplexing as well as for optical vortex tweezers and optical signal processing employing vortex beams.
14
Barboza, R., U. Bortolozzo, M. G. Clerc, S. Residori e E. Vidal-Henriquez. "Light–matter interaction induces a single positive vortex with swirling arms". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 372, n. 2027 (28 ottobre 2014): 20140019. http://dx.doi.org/10.1098/rsta.2014.0019.
Testo completoAbstract (sommario):
Homeotropic nematic liquid crystal cells with a photosensitive wall and negative dielectric anisotropy exhibit, under the influence of local illumination, stable vortexes with swirling arms that are trapped at the illuminated area. Close to the Fréedericksz transition an amplitude equation is derived, which allows us to understand the origin of the induced vortex and the competition between the illuminating profile and the elastic anisotropy generating the swirling of the arms.
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Müller, M., R. Neuber, F. Fierli, A. Hauchecorne, H. Vömel e S. J. Oltmans. "Stratospheric water vapour as tracer for vortex filamentation in the Arctic winter 2002/2003". Atmospheric Chemistry and Physics Discussions 3, n. 4 (5 agosto 2003): 4393–410. http://dx.doi.org/10.5194/acpd-3-4393-2003.
Testo completoAbstract (sommario):
Abstract. During winter 2002/2003, three balloon-borne frost point hygrometers measured high-resolution profiles of stratospheric water vapour above Ny-Ålesund, Spitsbergen. All measurements reveal a high H2O mixing ratio of about 7 ppmv above 24 km, thus differing significantly from the 5 ppmv that are commonly assumed for the calculation of polar stratospheric cloud existence temperatures. The profiles obtained on 12 December 2002 and on 17 January 2003 provide an insight into the vertical distribution of water vapour in the core of the polar vortex. Unlike the earlier profiles, the water vapour sounding on 11 February 2003 detected the vortex edge region in the lower part of the stratosphere. Here, a striking diminuition in H2O mixing ratio stands out between 16 and 19 km. The according stratospheric temperatures clarify that this dehydration can not be caused by the presence of polar stratospheric clouds or earlier PSC particle sedimentation. On the same day, ozone observations by lidar indicate a large scale movement of the polar vortex, while an ozone sonde measurement even shows laminae in the same altitude range as in the water vapour profile. Tracer lamination in the vortex edge region is caused by filamentation of the vortex. The link between the observed water vapour diminuition and filaments in the vortex edge region is highlighted by results of the MIMOSA contour advection model. In the altitude of interest, adjoined filaments of polar and mid-latitudinal air can be identified above the Spitsbergen region. A vertical cross-section reveals that the water vapour sonde has flown through polar air in the lowest part of the stratosphere. Where the low water vapour mixing ratio was detected, the balloon passed through air from a mid-latitudinal filament from about 425 to 445 K, before it finally entered the polar vortex above 450 K. The MIMOSA model results elucidate the correlation that on 11 February 2003 the frost point hygrometer measured strongly variable water vapour concentrations as the sonde detected air with different origins, respectively. Instead of being linked to dehydration due to PSC particle sedimentation, the local diminuition in the stratospheric water vapour profile of 11 February 2003 has been found to be caused by dynamical processes in the polar stratosphere.
16
Zhao, Donge, Chaozheng Jia, Yayun Ma, Xuefeng Yang, Bin Zhang e Wenbo Chu. "High-Accuracy Surface Profile Measurement Based on the Vortex Phase-Shifting Interferometry". International Journal of Optics 2021 (2 settembre 2021): 1–8. http://dx.doi.org/10.1155/2021/6937072.
Testo completoAbstract (sommario):
According to the principle of phase-shifting interferometry and spiral phase characteristics of the vortex beam, this article proposes a method for detecting the surface profile of a transparent object, in which the +1 order vortex beam is generated by a spatial light modulator and is taken as the reference light. The influence of the nonlinear phase modulation characteristics of the spatial light modulator on the measurement precision is studied. The results show that nonlinear phase modulation has a great impact on the measurement. Then, the vortex lights with initial phases of 0, π/2, π, and 3π/2 are used to measure the H-type thin film sample based on the Twyman-Green interference system after correcting the nonlinear phase modulation characteristics. The experimental results show that the measurement error of the surface profile to an object with the theoretical value of 20 nm is 1.146 nm, and the feasibility of the optical vortex phase-shifting technique used to measure the surface profile of an object is verified.
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Machida, Masahiko, e Tomio Koyama. "Friedel oscillation in charge profile around superconducting vortex core". Physica C: Superconductivity 388-389 (maggio 2003): 659–60. http://dx.doi.org/10.1016/s0921-4534(02)02432-2.
Testo completo
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SAKAI, Seigo, Haruki MADARAME e Koji OKAMOTO. "Bathtub Vortex Profile and Flow Field Model with Circulation." Transactions of the Japan Society of Mechanical Engineers Series B 64, n. 620 (1998): 972–78. http://dx.doi.org/10.1299/kikaib.64.972.
Testo completo
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Kozłowski, Tomasz, e Henryk Kudela. "Transitions in the vortex wake behind the plunging profile". Fluid Dynamics Research 46, n. 6 (3 ottobre 2014): 061406. http://dx.doi.org/10.1088/0169-5983/46/6/061406.
Testo completo
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Wong, Jaime G., e David E. Rival. "Determining the relative stability of leading-edge vortices on nominally two-dimensional flapping profiles". Journal of Fluid Mechanics 766 (9 febbraio 2015): 611–25. http://dx.doi.org/10.1017/jfm.2015.39.
Testo completoAbstract (sommario):
AbstractIt is hypothesized that the relative stability of leading-edge vortices (LEVs) on flapping profiles can be improved by moderating LEV growth through spanwise vorticity convection and vortex stretching. Moreover, it is hypothesized that the reduced frequency $k$ and profile sweep ${\it\Lambda}$ are critical in predicting relative LEV stability as determined by the aforementioned effects. These hypotheses are then confirmed experimentally with phase-averaged particle image velocimetry (PIV) and three-dimensional particle tracking velocimetry. In particular, more stable LEVs are observed at higher reduced frequencies, which is argued to represent the ratio between the limiting vortex size and the rate of vorticity feeding. The introduction of profile sweep increased both relative LEV stability and spanwise vorticity transport. It is thought that spanwise vorticity transport improved LEV stability by acting as a sink for vorticity generated in the leading-edge shear layer.
21
Müller, M., R. Neuber, F. Fierli, A. Hauchecorne, H. Vömel e S. J. Oltmans. "Stratospheric water vapour as tracer for Vortex filamentation in the Arctic winter 2002/2003". Atmospheric Chemistry and Physics 3, n. 6 (13 novembre 2003): 1991–97. http://dx.doi.org/10.5194/acp-3-1991-2003.
Testo completoAbstract (sommario):
Abstract. Balloon-borne frost point hygrometers measured three high-resolution profiles of stratospheric water vapour above Ny-Ålesund, Spitsbergen during winter 2002/2003. The profiles obtained on 12 December 2002 and on 17 January 2003 provide an insight into the vertical distribution of water vapour in the core of the polar vortex. The water vapour sounding on 11 February 2003 was obtained within the vortex edge region of the lower stratosphere. Here, a significant reduction of water vapour mixing ratio was observed between 16 and 19 km. The stratospheric temperatures indicate that this dehydration was not caused by the presence of polar stratospheric clouds or earlier PSC particle sedimentation. Ozone observations on this day indicate a large scale movement of the polar vortex and show laminae in the same altitude range as the water vapour profile. The link between the observed water vapour reduction and filaments in the vortex edge region is indicated in the results of the semi-lagrangian advection model MIMOSA, which show that adjacent filaments of polar and mid latitude air can be identified above the Spitsbergen region. A vertical cross-section produced by the MIMOSA model reveals that the water vapour sonde flew through polar air in the lowest part of the stratosphere below 425 K, then passed through filaments of mid latitude air with lower water vapour concentrations, before it finally entered the polar vortex above 450 K. These results indicate that on 11 February 2003 the frost point hygrometer measured different water vapour concentrations as the sonde detected air with different origins. Instead of being linked to dehydration due to PSC particle sedimentation, the local reduction in the stratospheric water vapour profile was in this case caused by dynamical processes in the polar stratosphere.
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Hsiao, Chao-Tsung, e Laura L. Pauley. "Numerical Study of the Steady-State Tip Vortex Flow Over a Finite-Span Hydrofoil". Journal of Fluids Engineering 120, n. 2 (1 giugno 1998): 345–53. http://dx.doi.org/10.1115/1.2820654.
Testo completoAbstract (sommario):
The flow over a finite-span hydrofoil creating a tip vortex was numerically studied by computing the full Navier-Stokes equations. A good agreement in pressure distribution and oil flow pattern was achieved between the numerical solution and available experimental data. The steady-state roll-up process of the tip vortex was described in detail from the numerical results. The effect of the angle of attack, the Reynolds number, and the hydrofoil planform on the tip vortex was investigated. The axial and tangential velocities within the tip-vortex core in the near-field wake region were greatly influenced by the angle of attack. A jet-like profile in the axial velocity was found within the tip-vortex core at high angle of attack, while a wake-like profile in the axial velocity was found at low angle of attack. Increasing the Reynolds number was found to increase the maximum axial velocity, but only had a slight impact on the tangential velocity. Finally, a swept hydrofoil planform was found to attenuate the strength of the tip vortex due to the low-momentum boundary layer traveling into the tip vortex on the suction side.
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Zhang, Qiang, e Phillip M. Ligrani. "Wake Turbulence Structure Downstream of a Cambered Airfoil in Transonic Flow: Effects of Surface Roughness and Freestream Turbulence Intensity". International Journal of Rotating Machinery 2006 (2006): 1–12. http://dx.doi.org/10.1155/ijrm/2006/60234.
Testo completoAbstract (sommario):
The wake turbulence structure of a cambered airfoil is studied experimentally, including the effects of surface roughness, at different freestream turbulence levels in a transonic flow. As the level of surface roughness increases, all wake profile quantities broaden significantly and nondimensional vortex shedding frequencies decrease. Freestream turbulence has little effect on the wake velocity profiles, turbulence structure, and vortex shedding frequency, especially downstream of airfoils with rough surfaces. Compared with data from a symmetric airfoil, wake profiles produced by the cambered airfoils also have significant dependence on surface roughness, but are less sensitive to variations of freestream turbulence intensity. The cambered airfoil also produces larger streamwise velocity deficits, and broader wakes compared to the symmetric airfoil.
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Blondeaux, P., e G. Vittori. "Vorticity dynamics in an oscillatory flow over a rippled bed". Journal of Fluid Mechanics 226 (maggio 1991): 257–89. http://dx.doi.org/10.1017/s0022112091002380.
Testo completoAbstract (sommario):
In the present paper we determine the oscillatory flow generated by surface gravity waves near a sea bottom covered with large-amplitude ripples. The vorticity equation and Poisson equation for the stream function are solved by means of a numerical approach based on spectral methods and finite-difference approximations. In order to test the numerical algorithm and in particular the numerical scheme used to generate vorticity along the ripple profile, we also perform an asymptotic analysis, which holds as the timettends to zero. The main features of the time development of vorticity are analysed and particular attention is paid to the dynamics of the large vortices generated by flow separation at the ripple crests and along the ripple profile. Some of the results obtained by Longuet-Higgins (1981) are recovered; in particular, the present results show a vortex pair shed from the ripple crest every half-cycle. The determination of flow separation along the ripple profile induced by the pressure gradient and the inclusion of viscous effects allows us to obtain accurate quantitative results and detect some important phenomena never observed before.In particular it is shown that: (i) Whenever a vortex structure moves towards the bottom, a secondary vortex is generated near the ripple profile, which interacts with the primary vortex and causes it to move away from the bottom, (ii) Depending on the values of the parameters, the time development of the free shear layer shed from the ripple crest may produce two or even more vortex structures, (iii) Occasionally vortices generated previously may coalesce with the free shear layer shed from the ripple crest, generating a unique vortex structure.
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Tao, Shi-bo, Ai-ping Tang, Da-bo Xin, Ke-tong Liu e Hong-fu Zhang. "Vortex-Induced Vibration Suppression of a Circular Cylinder with Vortex Generators". Shock and Vibration 2016 (2016): 1–10. http://dx.doi.org/10.1155/2016/5298687.
Testo completoAbstract (sommario):
The vortex-induced vibration is one of the most important factors to make the engineering failure in wind engineering. This paper focuses on the suppression method of vortex-induced vibration that occurs on a circular cylinder fitted with vortex generators, based on the wind tunnel experiment. The effect of the vortex generators is presented with comparisons including the bare cylinder. The experimental results reveal that the vortex generators can efficiently suppress vortex-induced vibration of the circular cylinder. Vortex generator control can make the boundary layer profile fuller and hence more resistant to separation. The selections of skew angles and the angular position have a significant influence on the vortex generator control effect. By correlation analysis, it can be concluded that the vortex generators can inhibit the communication between the two shear layers and produce streamwise vortices to generate a disturbance in the spanwise direction.
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FLÓR, J. B., e I. EAMES. "Dynamics of monopolar vortices on a topographic beta-plane". Journal of Fluid Mechanics 456 (9 aprile 2002): 353–76. http://dx.doi.org/10.1017/s0022112001007728.
Testo completoAbstract (sommario):
The dynamics of a cyclonic monopolar vortex on a topographic beta-plane are studied experimentally and theoretically. Detailed measurements of the vortex structure are conducted using high-resolution quantitative velocity measurements. The initial velocity profiles were described in terms of a radius Rvm, maximum azimuthal velocity vθm, and a dimensionless parameter α which characterizes the steepness of the velocity profile. The initial direction of motion of the monopolar vortex is critically dependent on α and weakly dependent of the initial strength and size of the vortex: isolated vortices (α ∼ 3) move north, whereas non-isolated vortices characterized by α ∼ 1 move northwest. When the azimuthal velocity decays slowly with radial distance (α < 1.4), Rossby wave generation dominates the vortex dynamics and the translational speed of the vortex correlates with the Rossby wave speed. When the azimuthal velocity decays rapidly with radial distance (α > 1.4) the vortex is isolated and the translational speed is much slower than the Rossby wave speed. To interpret the effect of the vortex structure on the direction of motion, a mechanistic model is developed which includes the Rossby force and a lift force arising from circulation around the vortex, but does not include the effect of Rossby waves. The Rossby force results from the integrated effect of the Coriolis force on the vortex and drives the vortex north; the lift force is determined from the circulation around the vortex and drives the vortex west. Comparison with the experimental data reveals two regimes: α < 1.4, where the vortex dynamics are dominated by Rossby waves whereas for α > 1.4 Rossby waves are weak and favourable agreement is found with the mechanistic model.
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TURNER, M. R., e A. D. GILBERT. "Thresholds for the formation of satellites in two-dimensional vortices". Journal of Fluid Mechanics 614 (16 ottobre 2008): 381–405. http://dx.doi.org/10.1017/s0022112008003558.
Testo completoAbstract (sommario):
This paper examines the evolution of a two-dimensional vortex which initially consists of an axisymmetric monopole vortex with a perturbation of azimuthal wavenumber m = 2 added to it. If the perturbation is weak, then the vortex returns to an axisymmetric state and the non-zero Fourier harmonics generated by the perturbation decay to zero. However, if a finite perturbation threshold is exceeded, then a persistent nonlinear vortex structure is formed. This structure consists of a coherent vortex core with two satellites rotating around it.The paper considers the formation of these satellites by taking an asymptotic limit in which a compact vortex is surrounded by a weak skirt of vorticity. The resulting equations match the behaviour of a normal mode riding on the vortex with the evolution of fine-scale vorticity in a critical layer inside the skirt. Three estimates of inviscid thresholds for the formation of satellites are computed and compared: two estimates use qualitative diagnostics, the appearance of an inflection point or neutral mode in the mean profile. The other is determined quantitatively by solving the normal mode/critical-layer equations numerically. These calculations are supported by simulations of the full Navier–Stokes equations using a family of profiles based on the tanh function.
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LE DIZÈS, STÉPHANE, e ALBERTO VERGA. "Viscous interactions of two co-rotating vortices before merging". Journal of Fluid Mechanics 467 (24 settembre 2002): 389–410. http://dx.doi.org/10.1017/s0022112002001532.
Testo completoAbstract (sommario):
The viscous evolution of two co-rotating vortices is analysed using direct two-dimensional numerical simulations of the Navier–Stokes equations. The article focuses on vortex interaction regimes before merging. Two parameters are varied: a steepness parameter n which measures the steepness of the initial vorticity profiles in a given family of profiles, and the Reynolds number Re (between 500 and 16 000). Two distinct relaxation processes are identified. The first one is non-viscous and corresponds to a rapid adaptation of each vortex to the external (strain) field generated by the other vortex. This adaptation process, which is profile dependent, is described and explained using the damped Kelvin modes of each vortex. The second relaxation process is a slow diffusion phenomenon. It is similar to the relaxation of any non-Gaussian axisymmetrical vortex towards the Gaussian. The quasi-stationary solution evolves on a viscous-time scale toward a single attractive solution which corresponds to the evolution from two initially Gaussian vortices. The attractive solution is analysed in detail up to the merging threshold a/b ≈ 0.22 where a and b are the vortex radius and the separation distance respectively. The vortex core deformations are quantified and compared to those induced by a single vortex in a rotating strain field. A good agreement with the asymptotic predictions is demonstrated for the eccentricity of vortex core streamlines. A weak anomalous Reynolds number dependence of the solution is also identified. This dependence is attributed to the advection–diffusion of vorticity towards the hyperbolic points of the system and across the separatrix connecting these points. A Re1/3 scaling for the vorticity at the central hyperbolic point is obtained. These findings are discussed in the context of a vortex merging criterion.
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TURNER, M. R., e A. D. GILBERT. "Spreading of two-dimensional axisymmetric vortices exposed to a rotating strain field". Journal of Fluid Mechanics 630 (10 luglio 2009): 155–77. http://dx.doi.org/10.1017/s0022112009006855.
Testo completoAbstract (sommario):
This paper examines the evolution of an axisymmetric two-dimensional vortex in a steadily rotating strain field and the dynamical interactions that can enhance vortex spreading through resonant behaviour. Starting with a point vortex localized at the origin, the applied strain field generates a cat's eye topology in the co-rotating streamfunction, localized around a radius rext. Now the vortex is allowed to spread viscously: initially rext lies outside the vortex, but as it spreads, vorticity is advected into the cat's eyes, leading to a local flattening of the mean profile of the vortex and so to enhanced mixing and spreading of the vortex. Together with this is a feedback: the response of the vortex to the external strain depends on the modified profile. The feedback is particularly strong when rext coincides with the radius rcat at which the vortex can support cat's eyes of infinitesimal width. There is a particular time at which this occurs, as these radii change with the viscous spread of the vortex: rext moves inwards and rcat outwards. This resonance behaviour leads to increased mixing of vorticity, along with a rapid stretching of vorticity contours and a sharp increase in the amplitude of the non-axisymmetric components. The dynamical feedback and enhanced diffusion are studied for viscously spreading vortices by means of numerical simulations of their time evolution, parameterized only by the Reynolds number R and the dimensionless strength A of the external strain field.
30
Jeans, Gus, Colin Grant, e Graham Feld. "Improved Current Profile Criteria for Deepwater Riser Design". Journal of Offshore Mechanics and Arctic Engineering 125, n. 4 (1 ottobre 2003): 221–24. http://dx.doi.org/10.1115/1.1596235.
Testo completoAbstract (sommario):
New types of current profile criteria are described that are designed to take into account the vertical coherence of deepwater currents in offshore engineering. Reasoning is given why traditional types of current profile criteria can be inappropriate for deepwater locations. The paper describes current profile occurrence criteria that consist of a discrete number of characteristic profiles, each of which has an associated percentage frequency of occurrence. These criteria are appropriate for riser fatigue calculations, including Vortex Induced Vibration (VIV) modelling, and operability assessments. Use of these criteria can reduce the level of overconservatism associated with the traditional criteria, potentially leading to significant cost savings. Two possible methods of deriving these criteria are described, one of which uses Empirical Orthogonal Function (EOF) analysis to simplify the vertical structure of current profiles. The relative merits of each analysis technique are assessed.
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Griffin, Debora, Kaley A. Walker, Ingo Wohltmann, Sandip S. Dhomse, Markus Rex, Martyn P. Chipperfield, Wuhu Feng, Gloria L. Manney, Jane Liu e David Tarasick. "Stratospheric ozone loss in the Arctic winters between 2005 and 2013 derived with ACE-FTS measurements". Atmospheric Chemistry and Physics 19, n. 1 (16 gennaio 2019): 577–601. http://dx.doi.org/10.5194/acp-19-577-2019.
Testo completoAbstract (sommario):
Abstract. Stratospheric ozone loss inside the Arctic polar vortex for the winters between 2004–2005 and 2012–2013 has been quantified using measurements from the space-borne Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). For the first time, an evaluation has been performed of six different ozone loss estimation methods based on the same single observational dataset to determine the Arctic ozone loss (mixing ratio loss profiles and the partial-column ozone losses between 380 and 550 K). The methods used are the tracer-tracer correlation, the artificial tracer correlation, the average vortex profile descent, and the passive subtraction with model output from both Lagrangian and Eulerian chemical transport models (CTMs). For the tracer-tracer, the artificial tracer, and the average vortex profile descent approaches, various tracers have been used that are also measured by ACE-FTS. From these seven tracers investigated (CH4, N2O, HF, OCS, CFC-11, CFC-12, and CFC-113), we found that CH4, N2O, HF, and CFC-12 are the most suitable tracers for investigating polar stratospheric ozone depletion with ACE-FTS v3.5. The ozone loss estimates (in terms of the mixing ratio as well as total column ozone) are generally in good agreement between the different methods and among the different tracers. However, using the average vortex profile descent technique typically leads to smaller maximum losses (by approximately 15–30 DU) compared to all other methods. The passive subtraction method using output from CTMs generally results in slightly larger losses compared to the techniques that use ACE-FTS measurements only. The ozone loss computed, using both measurements and models, shows the greatest loss during the 2010–2011 Arctic winter. For that year, our results show that maximum ozone loss (2.1–2.7 ppmv) occurred at 460 K. The estimated partial-column ozone loss inside the polar vortex (between 380 and 550 K) using the different methods is 66–103, 61–95, 59–96, 41–89, and 85–122 DU for March 2005, 2007, 2008, 2010, and 2011, respectively. Ozone loss is difficult to diagnose for the Arctic winters during 2005–2006, 2008–2009, 2011–2012, and 2012–2013, because strong polar vortex disturbance or major sudden stratospheric warming events significantly perturbed the polar vortex, thereby limiting the number of measurements available for the analysis of ozone loss.
32
Duke, Frederic, Ya Shen, Huimin Zhou, N. Dorin Ruse, Zhe-jun Wang, Ahmed Hieawy e Markus Haapasalo. "Cyclic Fatigue of ProFile Vortex and Vortex Blue Nickel-Titanium Files in Single and Double Curvatures". Journal of Endodontics 41, n. 10 (ottobre 2015): 1686–90. http://dx.doi.org/10.1016/j.joen.2015.06.012.
Testo completo
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Monkewitz, Peter A. "A note on vortex shedding from axisymmetric bluff bodies". Journal of Fluid Mechanics 192 (luglio 1988): 561–75. http://dx.doi.org/10.1017/s0022112088001983.
Testo completoAbstract (sommario):
The linear parallel and incompressible stability of a family of axisymmetric wake profiles is studied in the range of Reynolds numbers where helical vortex shedding from bluff bodies of revolution is observed. The family of mean flow profiles allows for the variation of the wake depth as well as for a variable ratio of wake width to mixing-layer thickness. It is found that, even without reverse flow, the first helical mode is absolutely unstable in the near wake for Reynolds numbers, based on wake diameter and free-stream velocity, in excess of 3.3 × 103. A survey of the region of local absolute instability as a function of profile parameters and Reynolds number suggests that the large-scale helical vortex shedding, which is observed between Reynolds numbers of 6000 and 3 × 105for spheres, may be ‘driven’ by a self-excited oscillation in the near wake.
34
Melamed, L. E., e G. A. Filippov. "The concept of turbulent «vortex backfill» - models and methods. Power engineering: research, equipment, technology". Power engineering: research, equipment, technology 21, n. 5 (17 dicembre 2019): 97–109. http://dx.doi.org/10.30724/1998-9903-2019-21-5-97-109.
Testo completoAbstract (sommario):
Models and methods for studying turbulence based on the concept of turbulent "vortex backfill" are presented. The essence of this concept is that the turbulent flow is considered as laminar, flowing through a "vortex backfill ", which creates internal resistance. This resistance can be considered either as distributed, or as locally concentrated. Based on the first representation, a modified Navier-Stokes equation, its approximate analytical and numerical solutions are obtained. Based on the second concept and the local fluctuation method developed for these purposes, a computer model of the turbulent flow in the pipes is obtained. Using simulation, it is shown that, when a certain system of local viscosity fluctuations is specified, the calculated flow profile corresponds to the profile of the turbulent flow velocity. The magnitude and profile of the turbulent viscosity of the flow are completely determined by the structure and properties of the "vortex backfill ". The results of the work confirm the possibility and efficiency of considering turbulence based on this concept.
35
Dai, Zhi Ping, Zhen Feng Yang, Zhen Jun Yang, Zhao Guang Pang e Shu Min Zhang. "Numerical Simulation of Anomalous Vortex Beams on Different Fractional Fourier Transform Planes". Applied Mechanics and Materials 556-562 (maggio 2014): 3745–48. http://dx.doi.org/10.4028/www.scientific.net/amm.556-562.3745.
Testo completoAbstract (sommario):
The properties of fractional Fourier transform of anomalous vortex beams are studied. A new type of analytical expression of fractional Fourier transform for anomolous vortex beams is obtained. The properties of anomolous vortex beams on different fractional Fourier transform planes with different parameters are illustrated. The results show that the anomolous vortex beams always has a doughnut profile, the distribution of intensity on different fractional Fourier transform planes highly depends on the fractional order and the beam parameters, such as the beam order and the topological charge.
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Chen, Y. N., U. Haupt e M. Rautenberg. "The Vortex-Filament Nature of Reverse Flow on the Verge of Rotating Stall". Journal of Turbomachinery 111, n. 4 (1 ottobre 1989): 450–61. http://dx.doi.org/10.1115/1.3262293.
Testo completoAbstract (sommario):
On the verge of rotating stall, very orderly reverse flow forms from the outlet of the rotor/impeller along the casing/shroud toward the inlet in axial/centrifugal compressors (Koch, 1970; Haupt, et al., 1987). The experiment on a centrifugal compressor reveals furthermore that the reverse flow is composed of stable spiral vortex filaments. Their vorticity can be transferred to the inlet tip vortex, known as prerotation. The behavior of these vortex filaments is examined based on the fundamental research work on rotating bodies available in the literature. This result shows that the vortex filaments are composed of Taylor’s vortex pairs, but with unequal vortex strengths within the pair. They form the transition range from a laminar to a turbulent three-dimensional boundary layer with a very steep tangential velocity profile. This profile is associated with the appearance of a toroidal ring vortex in the rotor/impeller, acting as a recirculatig secondary flow. It can be further shown from the analysis of the extensive literature that the orderly path of the reverse flow is enabled by the cessation of the leakage flow of the rotor tip clearance. The reason for this is that the growing tangential flow field extends beyond the rotor tip up to the close proximity of the endwall, so that the tip clearance is blocked.
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Wood, Vincent T., e Rodger A. Brown. "Simulated Tornadic Vortex Signatures of Tornado-Like Vortices Having One- and Two-Celled Structures". Journal of Applied Meteorology and Climatology 50, n. 11 (novembre 2011): 2338–42. http://dx.doi.org/10.1175/jamc-d-11-0118.1.
Testo completoAbstract (sommario):
AbstractA tornadic vortex signature (TVS) is a degraded Doppler velocity signature that occurs when the tangential velocity core region of a tornado is smaller than the effective beamwidth of a sampling Doppler radar. Early Doppler radar simulations, which used a uniform reflectivity distribution across an idealized Rankine vortex, showed that the extreme Doppler velocity peaks of a TVS profile are separated by approximately one beamwidth. The simulations also indicated that neither the size nor the strength of the tornado is recoverable from a TVS. The current study was undertaken to investigate how the TVS might change if vortices having more realistic tangential velocity profiles were considered. The one-celled (axial updraft only) Burgers–Rott vortex model and the two-celled (annular updraft with axial downdraft) Sullivan vortex model were selected. Results of the simulations show that the TVS peaks still are separated by approximately one beamwidth—signifying that the TVS not only is unaffected by the size or strength of a tornado but also is unaffected by whether the tornado structure consists of one or two cells.
38
Sauer, H., R. Mu¨ller e K. Vogeler. "Reduction of Secondary Flow Losses in Turbine Cascades by Leading Edge Modifications at the Endwall". Journal of Turbomachinery 123, n. 2 (1 febbraio 2000): 207–13. http://dx.doi.org/10.1115/1.1354142.
Testo completoAbstract (sommario):
Experimental results are presented which show the influence on the secondary flow and its losses by a profile modification of the leading edge very close to the endwall. The investigation was carried out with a well-known turbine profile that originally was developed for highly loaded low pressure turbines. The tests were done in a low speed cascade wind tunnel. The geometrical modification was achieved by a local thickness increase; a leading edge endwall bulb. It was expected that this would intensify the suction side branch of the horse-shoe (hs-) vortex with a desirable weakening effect on the passage vortex. The investigated configuration shows a reduction of secondary losses by 2.1 percent points that represents approximately 50 percent of these losses compared to the reference profile. Detailed measurements of the total pressure field behind the cascade are presented for both the reference and the modified profile. The influence of the modified hs-vortex on the overall passage vortex can be clearly seen. The results of a numerical analysis are compared with the experimental findings. A numerical analysis shows that the important details of the experimental findings can be reproduced. Quantitative values are locally different. The theoretical approach taken cannot yet be used for an exact prediction of the loss reduction. However, the analysis of the interaction and the resulting tendencies are considered to be valid. Hence, theoretical investigations as a guideline for the design of a leading edge bulb at the endwall are a valuable tool.
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Jawarneh, Ali M., e Georgios H. Vatistas. "Reynolds Stress Model in the Prediction of Confined Turbulent Swirling Flows". Journal of Fluids Engineering 128, n. 6 (22 marzo 2006): 1377–82. http://dx.doi.org/10.1115/1.2354530.
Testo completoAbstract (sommario):
Strongly swirling vortex chamber flows are examined experimentally and numerically using the Reynolds stress model (RSM). The predictions are compared against the experimental data in terms of the pressure drop across the chamber, the axial and tangential velocity components, and the radial pressure profiles. The overall agreement between the measurements and the predictions is reasonable. The predictions provided by the numerical model show clearly the forced and free vortex modes of the tangential velocity profile. The reverse flow (or back flow) inside the core and near the outlet, known from experiments, is captured by the numerical simulations. The swirl number has been found to have a measurable impact on the flow features. The vortex core size is shown to contract with the swirl number which leads to higher pressure drop, higher peak tangential velocity, and deeper radial pressure profiles near the axis of rotation. The adequate agreement between the experimental data and the simulations using RSM turbulence model provides a valid tool to study further these industrially important swirling flows.
40
ANH, Tran Ngoc, e Takashi HOSODA. "Free Surface Profile Analysis of Flows with Air-core Vortex". Journal of applied mechanics 7 (2004): 1061–68. http://dx.doi.org/10.2208/journalam.7.1061.
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Aboelkassem, Yasser, Georgios H. Vatistas e Nabil Esmail. "Viscous dissipation of Rankine vortex profile in zero meridional flow". Acta Mechanica Sinica 21, n. 6 (21 novembre 2005): 550–56. http://dx.doi.org/10.1007/s10409-005-0073-3.
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Shen, Ya, Jeffrey M. Coil, Hui-min Zhou, Esther Tam, Yu-feng Zheng e Markus Haapasalo. "ProFile Vortex Instruments after Clinical Use: A Metallurgical Properties Study". Journal of Endodontics 38, n. 12 (dicembre 2012): 1613–17. http://dx.doi.org/10.1016/j.joen.2012.09.018.
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Martinez Suarez, J., P. Flaszyński e P. Doerffer. "Streamwise vortex generator for separation reduction on wind turbine profile". Journal of Physics: Conference Series 760 (ottobre 2016): 012018. http://dx.doi.org/10.1088/1742-6596/760/1/012018.
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Szumowski, A., J. Piechna, W. Selerowicz e G. Sobieraj. "Modified Formula for the Flow Velocity Profile in a Vortex". Journal of Fluids Engineering 121, n. 1 (1 marzo 1999): 208–10. http://dx.doi.org/10.1115/1.2822005.
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Vatistas, G. H., S. Lin e P. M. Li. "A similar profile for the tangential velocity in vortex chambers". Experiments in Fluids 6, n. 2 (gennaio 2004): 135–37. http://dx.doi.org/10.1007/bf00196465.
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TURNER, M. R., e ANDREW D. GILBERT. "Linear and nonlinear decay of cat's eyes in two-dimensional vortices, and the link to Landau poles". Journal of Fluid Mechanics 593 (23 novembre 2007): 255–79. http://dx.doi.org/10.1017/s0022112007008944.
Testo completoAbstract (sommario):
This paper considers the evolution of smooth, two-dimensional vortices subject to a rotating external strain field, which generates regions of recirculating, cat's eye stream line topology within a vortex. When the external strain field is smoothly switched off, the cat's eyes may persist, or they may disappear as the vortex relaxes back to axisymmetry. A numerical study obtains criteria for the persistence of cat's eyes as a function of the strength and time scale of the imposed strain field, for a Gaussian vortex profile.In the limit of a weak external strain field and high Reynolds number, the disturbance decays exponentially, with a rate that is linked to a Landau pole of the linear inviscid problem. For stronger strain fields, but not strong enough to give persistent cat's eyes, the exponential decay of the disturbance varies: as time increases the decay slows down, because of the nonlinear feedback on the mean profile of the vortex. This is confirmed by determining the decay rate given by the Landau pole for these modified profiles. For strain fields strong enough to generate persistent cat's eyes, their location and rotation rate are determined for a range of angular velocities of the external strain field, and are again linked to Landau poles of the mean profiles, modified through nonlinear effects.
47
TURNER, MATTHEW R., ANDREW P. BASSOM e ANDREW D. GILBERT. "Diffusion and the formation of vorticity staircases in randomly strained two-dimensional vortices". Journal of Fluid Mechanics 638 (21 settembre 2009): 49–72. http://dx.doi.org/10.1017/s0022112009990875.
Testo completoAbstract (sommario):
The spreading and diffusion of two-dimensional vortices subject to weak external random strain fields is examined. The response to such a field of given angular frequency depends on the profile of the vortex and can be calculated numerically. An effective diffusivity can be determined as a function of radius and may be used to evolve the profile over a long time scale, using a diffusion equation that is both nonlinear and non-local. This equation, containing an additional smoothing parameter, is simulated starting with a Gaussian vortex. Fine scale steps in the vorticity profile develop at the periphery of the vortex and these form a vorticity staircase. The effective diffusivity is high in the steps where the vorticity gradient is low: between the steps are barriers characterized by low effective diffusivity and high vorticity gradient. The steps then merge before the vorticity is finally swept out and this leaves a vortex with a compact core and a sharp edge. There is also an increase in the effective diffusion within an encircling surf zone.In order to understand the properties of the evolution of the Gaussian vortex, an asymptotic model first proposed by Balmforth, Llewellyn Smith & Young (J. Fluid Mech., vol. 426, 2001, p. 95) is employed. The model is based on a vorticity distribution that consists of a compact vortex core surrounded by a skirt of relatively weak vorticity. Again simulations show the formation of fine scale vorticity steps within the skirt, followed by merger. The diffusion equation we develop has a tendency to generate vorticity steps on arbitrarily fine scales; these are limited in our numerical simulations by smoothing the effective diffusivity over small spatial scales.
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Jamshidi, Sean, e Edward Johnson. "Vortex competition in coastal outflows". Journal of Marine Research 77, n. 3 (1 maggio 2019): 325–49. http://dx.doi.org/10.1357/002224019828410566.
Testo completoAbstract (sommario):
Experiments and field observations have shown that there are at least two modes of behavior for river plumes. In many cases, the plume turns to the right (in the Northern Hemisphere) on leaving the river mouth and follows the direction of Kelvin-wave propagation. Alternatively, a “bulge” can form in the plume and a fraction of the outflow volume becomes trapped near the mouth. This paper discusses how bulge formation can be affected by the vorticity profile at the river mouth. Due to the image effect, regions of cyclonic vorticity tend to propagate rightwards, whereas regions of anticyclonic vorticity propagate leftward upon exit from the source. If an outflow consists of regions of cyclonic vorticity to the left of regions of anticyclonic vorticity, the two image effects are in competition. We explore this phenomenon using a quasi-geostrophic model with piecewise-constant potential vorticity, which allows the vorticity profile at the source to be set as part of the problem. We present analytic solutions valid in the source region and at the head of the plume and show that all of the outflow travels rightwards if and only if the region of cyclonic vorticity is dominant. The initial-value problem for the model is integrated numerically using the method of contour dynamics, and the full parameter space is explored. We find that if the cyclonic and anticyclonic contributions cancel, as in the experiments of Avicola and Huq (2003), then steady solutions are unstable and a bulge can form downstream of the river mouth.
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BECKERS, M., R. VERZICCO, H. J. H. CLERCX e G. J. F. VAN HEIJST. "Dynamics of pancake-like vortices in a stratified fluid: experiments, model and numerical simulations". Journal of Fluid Mechanics 433 (25 aprile 2001): 1–27. http://dx.doi.org/10.1017/s0022112001003482.
Testo completoAbstract (sommario):
The dynamics and the three-dimensional structure of vortices in a linearly stratified, non-rotating fluid are investigated by means of laboratory experiments, an analytical model and through numerical simulations. The laboratory experiments show that such vortices have a thin pancake-like appearance. Due to vertical diffusion of momentum the strength of these vortices decreases rapidly and their thickness increases in time. Also it is found that inside a vortex the linear ambient density profile becomes perturbed, resulting in a local steepening of the density gradient. Based on the assumption of a quasi-two-dimensional axisymmetric flow (i.e. with zero vertical velocity) a model is derived from the Boussinesq equations that illustrates that the velocity field of the vortex decays due to diffusion and that the vortex is in so-called cyclostrophic balance. This means that the centrifugal force inside the vortex is balanced by a pressure gradient force that is provided by a perturbation of the density profile in a way that is observed in the experiments. Numerical simulations are performed, using a finite difference method in a cylindrical coordinate system. As an initial condition the three-dimensional vorticity and density structure of the vortex, found with the diffusion model, are used. The influence of the Froude number, Schmidt number and Reynolds number, as well as the initial thickness of the vortex, on the evolution of the flow are investigated. For a specific combination of flow parameters it is found that during the decay of the vortex the relaxation of the isopycnals back to their undisturbed positions can result in a stretching of the vortex. Potential energy of the perturbed isopycnals is then converted into kinetic energy of the vortex. However, when the stratification is strong enough (i.e. for small Froude numbers), the evolution of the vortex can be described almost perfectly by the diffusion model alone.
50
Humphries, J. A., e D. H. Walker. "Vortex-Excited Response of Large-Scale Cylinders in Sheared Flow". Journal of Offshore Mechanics and Arctic Engineering 110, n. 3 (1 agosto 1988): 272–77. http://dx.doi.org/10.1115/1.3257061.
Testo completoAbstract (sommario):
A series of experiments were performed to measure the vortex-excited response of a 0.168-m-dia slender circular cylinder in a range of linear shear velocity profiles. Reynolds numbers of up to 2.5 × 105 were achieved. The results clearly showed that regular large-amplitude cylinder vibrations occurred well within the critical drag transition region. It was found that increasing the linear shear profile decreased the peak amplitude response but broadened the range of lock-on over which large oscillations occurred. The flow-induced vibration of the cylinder caused amplification of the mean hydrodynamic drag forces acting on the cylinder when compared with those expected for a similar rigid cylinder.