Abstract
The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder; the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.
Highlights
In stratified fluids with vertical gradients of density and/ or temperature, buoyancy effects become significant
The present study investigates the influence of stratification on two-dimensional near wake flows around a circular cylinder with the use of a density-stratified wind tunnel
Near wakes behind horizontal circular and square cylinders were investigated in linearly density-stratified flows of varying stabilities
Summary
In stratified fluids with vertical gradients of density and/ or temperature, buoyancy effects become significant. Meunier [13] reported in the study of stratified wake of a tilted cylinder, that the tendency of stabilization of the von Karman vortex street which can be seen with a moderate stratifycation is not efficient in a strongly stratified flow due to the presence of a second unstable mode characterized by tiled vortices This leads to a discontinuity in the Strouhal number as a function of the Froude number which is related to the stratification through the Brunt-Väisälä frequency. The influence of stratification on the wake of an object is not yet well understood over a wide range of Reynolds numbers, for high Reynolds numbers Given this background, the present study investigates the influence of stratification on two-dimensional near wake flows around a circular cylinder with the use of a density-stratified wind tunnel. The present paper will investigate the mechanisms of these phenomena
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