Vortex streets generated by a moving momentum source in a stratified fluid

In this paper we compare the wakes of various bluff bodies in a stratified fluid at moderately high Froude numbers (Fϵ2U B /NDϾ8) and Reynolds numbers ͑ReϷ5000͒. The size and amplitude of the long-lasting wakes clearly depend on the shape of the bluff body, the wake width being small for a streamlined object and large for an object with sharp edges. However, the wake width can be collapsed when it is normalized by an effective diameter based on the drag force, often called the momentum thickness. General laws for the wake width, the velocity defect, and the Strouhal number are thus deduced and fit the data well. Finally, the cross-fluctuations of the velocity and the turbulent kinetic energy are analyzed. Their amplitudes and widths are proportional to those of the mean profile. Thus, the wake remembers only the momentum flux given by the bluff body to the fluid and not any other aspects of its geometry.

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“…The same conclusion was recently obtained by Voropayev and Smirnov 16 in the case of a towed jet, by introducing the same length scale as in ͑2͒.…”

Section: B Toward Universalitysupporting

confidence: 77%

“…Such a behavior has been already revealed by dye visualizations for various bluff bodies 4 and for a moving momentum source. 16 Mean profiles are analyzed in Sec. III A.…”

Section: B Definition Of the Flow Characteristicsmentioning

confidence: 99%

A loss of memory in stratified momentum wakes

Meunier

Spedding

2003

Physics of Fluids

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“…We will show in particular that the finite size of the area where the force is applied is an important control parameter when the shedding of vorticity is concerned. A similar approach with localized forcing was realized in the experiments by Voropayev et al (2002) and Voropayev & Smirnov (2003) where the source of forcing was a jet ejected from a nozzle horizontally in a stratified fluid. The nozzle was translated horizontally in the fluid in their experiments.…”

Section: Introductionmentioning

confidence: 85%

Wakes and vortex streets generated by translating force and force doublet: laboratory experiments

Afanasyev¹,

Korabel²

2006

J. Fluid Mech.

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Wakes and vortex streets such as those occurring behind towed or self-propelled bodies are generated by moving localized forces in a viscous fluid at moderate values of the Reynolds number, $\hbox{\it Re}\,{\sim}\,10^{2}$. The forcing is provided by an electromagnetic method and allows us to create a ‘virtual’ body without introducing any solid objects into the fluid. Characteristics of stable and unstable wakes, in particular the shedding frequency, are measured in the space of control parameters, namely the magnitude of the forcing and the speed of translational motion of the forcing. The results for a single force presented in the dimensionless form of the Strouhal number demonstrate quantitative similarity to those for the classical flow around a cylinder. The problem considered here has an extra degree of freedom compared to the problem of the flow around a cylinder and exhibit a wider array of different regimes. These regimes are documented in both our visualization experiments and particle image velocimetry measurements.

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“…In Voropayev et al (1999) and Voropayev & Fernando (2010), a finite amount of momentum is added into the wake which forms a jet-like front region resulting in a region of concentrated strong vorticity that forms several pairs of eddies which pair and merge, often leading to the formation of a single large dipole. The excess momentum wakes considered here occur behind a body moving at constant speed which leads to the formation of a steady system of eddies of comparable size similar to the case of a moving momentum source as discussed in Voropayev & Smirnov (2003). Nevertheless, we note that Voropayev et al (1999) and Voropayev & Fernando (2010) employ a significantly larger initial excess momentum at the beginning of their manoeuvres: the initial thrust to drag ratio in Voropayev et al (1999) is 5, which results in a dipole of significantly larger size than the eddies generated in the present study where the excess momentum is comparatively small.…”

Section: B De Stadler and S Sarkarmentioning

confidence: 98%

Simulation of a propelled wake with moderate excess momentum in a stratified fluid

Stadler

Sarkar

2011

J. Fluid Mech.

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Direct numerical simulation is used to simulate the turbulent wake behind an accelerating axisymmetric self-propelled body in a stratified fluid. Acceleration is modelled by adding a velocity profile corresponding to net thrust to a self-propelled velocity profile resulting in a wake with excess momentum. The effect of a small to moderate amount of excess momentum on the initially momentumless self-propelled wake is investigated to evaluate if the addition of excess momentum leads to a large qualitative change in wake dynamics. Both the amount and shape of excess momentum are varied. Increasing the amount of excess momentum and/or decreasing the radial extent of excess momentum was found to increase the defect velocity, mean kinetic energy, shear in the velocity gradient and the wake width. The increased shear in the mean profile resulted in increased production of turbulent kinetic energy leading to an increase in turbulent kinetic energy and its dissipation. Slightly larger vorticity structures were observed in the late wake with excess momentum although the differences between vorticity structures in the self-propelled and 40 % excess momentum cases was significantly smaller than suggested by previous experiments. Buoyancy was found to preserve the doubly inflected velocity profile in the vertical direction, and similarity for the mean velocity and turbulent kinetic energy was found to occur in both horizontal and vertical directions. While quantitative differences were observed between cases with and without excess momentum, qualitatively similar evolution was found to occur.

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Vortex streets generated by a moving momentum source in a stratified fluid

Cited by 24 publications

References 18 publications

A loss of memory in stratified momentum wakes

A loss of memory in stratified momentum wakes

Wakes and vortex streets generated by translating force and force doublet: laboratory experiments

Simulation of a propelled wake with moderate excess momentum in a stratified fluid

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