The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform
Patrice Meunier - One of the best experts on this subject based on the ideXlab platform.
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Laminar-turbulent transition in stratified bluff body wakes
2012Co-Authors: Patrice MeunierAbstract:This paper presents experimental and theoretical results on the transition from a laminar to a turbulent wake in a stratified fluid. The case of a cylinder is analysed in detail at low Reynolds number since it gives rise to the famous von Karman Vortex Street when the Reynolds number exceeds a critical value. This value highly depends on the stratification and on the tilt angle of the cylinder. A moderate stratification tends to suppress the von Karman Vortex Street, in agreement with the stabilisation of shear flows at high Richardson numbers. However, it is surprising to see that a strong stratification destabilises the flow when the cylinder is tilted. This new von Karman Vortex Street is allowed because the vortices exhibit horizontal streamlines although the vortices are tilted. The experimental stability diagram obtained by dye visualisations are compared to numerical results. At larger Reynolds numbers, the 2D von Karman Vortex Street leads to a 3D instability. Shadowgraph visualisations clearly reveal that the unstable mode is similar to the mode A well known in homogeneous cylinder wakes if the cylinder is vertical. This mode seems to be more unstable for moderate stratifications and more stable for strong stratifications. When the cylinder is tilted a new unstable mode appears at moderate Froude numbers, which exhibits thin undulated dark lines. This mode is due to a Kelvin-Helmholtz instability of the critical layer which appears in each tilted Vortex of the von Karman Street. Finally, at high Reynolds numbers, the wake becomes turbulent in the early stages for the case of a sphere. However, the late stages of the wake exhibit once again a von Karman Street of flat horizontal vortices. The size and the velocity of the wake vary algebraically with time. These scaling laws can be predicted by a simple model of turbulent diffusion in the horizontal direction and of viscous diffusion in the vertical direction.
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Stratified wake of a tilted cylinder. Part 1: suppression of von Karman Vortex Street
Journal of Fluid Mechanics, 2012Co-Authors: Patrice MeunierAbstract:This experimental and numerical study considers the two-dimensional stability of a circular cylinder wake, whose axis is tilted with respect to a stable density gradient. When the Reynolds number increases, the wake transitions from a steady flow to a periodic von Karman Vortex Street as in a homogeneous fluid. However, the presence of a moderate stratification delays the appearance of the von Karman Vortex Street, in agreement with the stabilisation of shear flows by a density gradient. This stabilisation, which does not occur for a vertical cylinder, increases with the tilt angle of the cylinder and is maximum for a horizontal cylinder. The critical Reynolds number increases when the stratification increases and diverges at a Froude number of order one for a horizontal cylinder. This critical Reynolds number can be predicted using the Richardson number based on the projection of the gravity and the density gradient in the direction of the shear, as was proposed by Candelier (2011) for a tilted stratified jet. This picture is completely different for a strongly stratified wake since a new unstable mode appears, creating a von Karman Vortex Street with a smaller Strouhal number. This surprising result is due to the presence of tilted vortices with no vertical velocity, i.e. with horizontal elliptic streamlines. This mode occurs in a band of Froude numbers which becomes smaller and smaller when the tilt angle increases, and eventually disappears for a horizontal cylinder. The presence of the tilt has thus a large impact on the structure of the wake at small Froude numbers and might need to be taken into account in geophysical flows.
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Stratified wake of a tilted cylinder. Part 1. Suppression of a von Kármán Vortex Street
Journal of Fluid Mechanics, 2012Co-Authors: Patrice MeunierAbstract:This experimental and numerical study considers the two-dimensional stability of a circular cylinder wake, whose axis is tilted with respect to a stable density gradient. When the Reynolds number increases, the wake transitions from a steady flow to a periodic von Karman Vortex Street as in a homogeneous fluid. However, the presence of a moderate stratification delays the appearance of the von Karman Vortex Street, in agreement with the stabilization of shear flows by a density gradient. This stabilization, which does not occur for a vertical cylinder, increases with the tilt angle of the cylinder and is maximum for a horizontal cylinder. The critical Reynolds number increases when the stratification increases and diverges at a Froude number of order one for a horizontal cylinder. This critical Reynolds number can be predicted using the Richardson number based on the projection of the gravity and the density gradient in the direction of the shear, as was proposed by Candelier ( J. Fluid Mech. , vol. 685, pp. 191–201) for a tilted stratified jet. This picture is completely different for a strongly stratified wake since a new unstable mode appears, creating a von Karman Vortex Street with a smaller Strouhal number. This surprising result is due to the presence of tilted vortices with no vertical velocity, i.e. with horizontal elliptic streamlines. This mode occurs in a band of Froude numbers which becomes smaller and smaller when the tilt angle increases, and eventually disappears for a horizontal cylinder. The presence of the tilt has thus a large impact on the structure of the wake at small Froude numbers and might need to be taken into account in geophysical flows.
K. Noto - One of the best experts on this subject based on the ideXlab platform.
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Cooled Vortex Street Characteristics
Numerical Heat Transfer Part A: Applications, 2010Co-Authors: K. NotoAbstract:Common and different characteristics among the cooled Vortex Street, cooled Karman Vortex Street, and isothermal wavy wake in the cooled laminar wake above the isothermal or cooled circular cylinder exposing in the upward mainstream of mercury, air, or water at the Reynolds number Re = 44 are discussed. Employing the previous and new distributions, the cooled Vortex Street characteristics are elucidated, and the conclusions are obtained as follows. 1) In the cooled wake, i.e., the isothermal wavy wake, cooled Karman Vortex Street, and cooled Vortex Street, in any fluid, the negative buoyancy increases the amplitude of the meandering motion in the zero-streamlines, and decreases the distance between the neighboring streamlines near the zero-streamline. As a result, with increasing the cooling rate in any fluid, both the Vortex spiral size and the velocity near the zero-streamline become large. 2) In any fluid, the isothermal wavy wake, cooled Karman Vortex Street, and cooled Vortex Street are qualitatively...
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Numerical Computation of a New Vortex (A Cooled Vortex Street) and its Generation Mechanism in a Cooled Circular Cylinder Wake at Low Reynolds Number
Numerical Heat Transfer Part A: Applications, 2007Co-Authors: K. Noto, Satoshi SugimuraAbstract:A strongly cooled, circular cylinder wake with an upward main stream of air at low Reynolds number Re, i.e., 15 ≦ Re ≦ 44, is analyzed numerically, and is elucidated as follows. (1) A new Vortex Street, i.e., a “cooled Vortex Street,” is discovered, develops in the range of computed Re, i.e., 15 ≦ Re ≦ 44, has strong asymmetry, and is extremely different from the Karman Vortex. (2) The Vortex Street occurring in the cooled wake is either the Karman Vortex Street or the cooled Vortex Street. No Vortex Street except these Vortex Streets ever occurs in a wake. (3) The critical Reynolds number Rec, i.e., the minimum Re occurring in the Karman Vortex Street by cooling a cylinder, is nearly 24. When the isothermal wake is cooled weakly, the Karman Vortex Street certainly develops at Re > 24, but never occurs at any cooling rate at Re
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GENERATION OF THE Karman Vortex Street AT LOW REYNOLDS NUMBER DUE TO COOLING A CYLINDER: CAUSE AND FLUID TYPE EFFECT BY NUMERICAL COMPUTATION
Numerical Heat Transfer Part A: Applications, 2001Co-Authors: K. Noto, Toshiro Miyake, T. NakajimaAbstract:The aim is to clarify a main cause and a fluid-type effect of the generation of the Karman Vortex Street due to cooling a cylinder at a low Reynolds number where the isothermal wake is not a Karman Vortex Street but the wavy wake. The two-dimensional, laminar, time-dependent continuity equation; NavierStokes equations with the buoyancy term; and energy equation are solved numerically by finite difference methods in the wake from a cooled circular cylinder submerged in an upward freestream of mercury, air, or water. The main cause is clarified and is that one is the generation of the wake vorticity, which never occurs in any isothermal wake, and the other the stable arrangement with amplified asymmetry of the vorticity distribution in the wake. When the Prandtl number and / or the cylinder temperature are decreased, the Karman Vortex Street is generated easily and has a smaller wake frequency, smaller Vortex speed, and larger Vortex spiral than those in any isothermal wake. The characteristics of the Karm...
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generation of the Karman Vortex Street at low reynolds number due to cooling of a cylinder cause and fluid type effect
Transactions of the Japan Society of Mechanical Engineers. B, 1997Co-Authors: K. Noto, Toshiro Miyake, T. NakajimaAbstract:For clarification of the cause and effect of fluid type on the generation of the Karman Vortex Street due to cooling of a cylinder at a low Reynolds number where the Karman Vortex Street does not occur in an isothermal wake, the two-dimensional, laminar, time-dependent continuity equation, Navier-Stokes equations with the buoyant term of Boussinesq's approximation, and energy equation are solved numerically for a circular cylinder wake with an upward freestream of mercury, air, and water. The cause is the generation of the wake vorticity which does not occur in an isothermal wake, and a stable arrangement of vorticity. With a decrease in the Prandtl number of the fluid, the Karman Vortex Street is generated easily by cooling of a cylinder, and has lower frequency, lower velocity of the Vortex movement, and larger scale of Vortex spiral than the isothermal Karman Vortex.
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Generation of the Karman Vortex Street at Low Reynolds Number by Cooling a Circular Cylinder
Flow Visualization VI, 1992Co-Authors: K. Noto, T. NakajimaAbstract:This paper describes the generation of the Karman Vortex Street and the change of its Vortex shedding due to cooling a circular cylinder submerged in an upward free-stream of air at the low Reynolds number 40–50, by showing streaklines obtained by computations of the time-dependent Navier-Stokes equations with the buoyancy term and flow visualizations using the smokewire methods. The shedding frequency of the Vortex generated at the low Reynolds number is different from that in the isothermal wake.
François Gallaire - One of the best experts on this subject based on the ideXlab platform.
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Spatio-temporal stability of the Kármán Vortex Street and the effect of confinement
Journal of Fluid Mechanics, 2016Co-Authors: Saviz Mowlavi, Cristóbal Arratia, François GallaireAbstract:The instability of the Karman Vortex Street is revisited under a spatio-temporal perspective that allows the taking into account of the advection of the vortices by the external flow. We analyse a simplified point Vortex model and show through numerical simulations of its linear impulse response that the system becomes convectively unstable above a certain critical advection velocity. This critical velocity decreases as the aspect ratio approaches its specific value for temporal stability, and increases with the confinement induced by lateral walls. In the limiting unconfined case, direct application of the Briggs–Bers criterion to the dispersion relation gives results in excellent agreement with the numerical simulations. Finally, a direct numerical simulation of the Re=100 flow past a confined cylinder is performed, and the actual advection velocity of the resulting Vortex Street is found to be much larger than the critical advection velocity for convective instability given by our model. The Karman Vortex Street is therefore strongly convectively unstable.
T. Nakajima - One of the best experts on this subject based on the ideXlab platform.
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GENERATION OF THE Karman Vortex Street AT LOW REYNOLDS NUMBER DUE TO COOLING A CYLINDER: CAUSE AND FLUID TYPE EFFECT BY NUMERICAL COMPUTATION
Numerical Heat Transfer Part A: Applications, 2001Co-Authors: K. Noto, Toshiro Miyake, T. NakajimaAbstract:The aim is to clarify a main cause and a fluid-type effect of the generation of the Karman Vortex Street due to cooling a cylinder at a low Reynolds number where the isothermal wake is not a Karman Vortex Street but the wavy wake. The two-dimensional, laminar, time-dependent continuity equation; NavierStokes equations with the buoyancy term; and energy equation are solved numerically by finite difference methods in the wake from a cooled circular cylinder submerged in an upward freestream of mercury, air, or water. The main cause is clarified and is that one is the generation of the wake vorticity, which never occurs in any isothermal wake, and the other the stable arrangement with amplified asymmetry of the vorticity distribution in the wake. When the Prandtl number and / or the cylinder temperature are decreased, the Karman Vortex Street is generated easily and has a smaller wake frequency, smaller Vortex speed, and larger Vortex spiral than those in any isothermal wake. The characteristics of the Karm...
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generation of the Karman Vortex Street at low reynolds number due to cooling of a cylinder cause and fluid type effect
Transactions of the Japan Society of Mechanical Engineers. B, 1997Co-Authors: K. Noto, Toshiro Miyake, T. NakajimaAbstract:For clarification of the cause and effect of fluid type on the generation of the Karman Vortex Street due to cooling of a cylinder at a low Reynolds number where the Karman Vortex Street does not occur in an isothermal wake, the two-dimensional, laminar, time-dependent continuity equation, Navier-Stokes equations with the buoyant term of Boussinesq's approximation, and energy equation are solved numerically for a circular cylinder wake with an upward freestream of mercury, air, and water. The cause is the generation of the wake vorticity which does not occur in an isothermal wake, and a stable arrangement of vorticity. With a decrease in the Prandtl number of the fluid, the Karman Vortex Street is generated easily by cooling of a cylinder, and has lower frequency, lower velocity of the Vortex movement, and larger scale of Vortex spiral than the isothermal Karman Vortex.
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Generation of the Karman Vortex Street at Low Reynolds Number by Cooling a Circular Cylinder
Flow Visualization VI, 1992Co-Authors: K. Noto, T. NakajimaAbstract:This paper describes the generation of the Karman Vortex Street and the change of its Vortex shedding due to cooling a circular cylinder submerged in an upward free-stream of air at the low Reynolds number 40–50, by showing streaklines obtained by computations of the time-dependent Navier-Stokes equations with the buoyancy term and flow visualizations using the smokewire methods. The shedding frequency of the Vortex generated at the low Reynolds number is different from that in the isothermal wake.
Helmut Eckelmann - One of the best experts on this subject based on the ideXlab platform.
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discrete shedding modes in the von Karman Vortex Street
Physics of Fluids, 1993Co-Authors: Michael König, Bernd R. Noack, Helmut EckelmannAbstract:In the laminar Vortex Street behind a circular cylinder, only few distinct shedding modes for a large range of aspect ratios and various end conditions are observed. The nth mode is characterized by a Strouhal number Stn and a shedding angle, θn, both of which vary continuously with the Reynolds number. The experimentally observed discontinuities in the Strouhal–Reynolds number relationship are transitions between these shedding modes.
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Three‐dimensional stability analysis of the periodic flow around a circular cylinder
Physics of Fluids A: Fluid Dynamics, 1993Co-Authors: Bernd R. Noack, Michael König, Helmut EckelmannAbstract:The onset of three‐dimensionality in the von Karman Vortex Street behind a circular cylinder is investigated by carrying out the first global, nonparallel, three‐dimensional stability analysis of the periodic flow. This flow is found to become unstable at a Reynolds number of 170 by a critical, three‐dimensional Floquet mode with a spanwise wavelength of 1.8 diam. The spatial structure of this mode indicates that the onset of three‐dimensionality is due to a near‐wake instability and not caused by a stagnation‐line or a boundary‐layer instability.