Vortex Shedding from Spheres at Subcritical Reynolds Number in Homogeneous and Stratified Fluid

Abstract

The structure of the wake of a sphere at subcritical Reynolds numbers and the frequencies associated with the vortex shedding and oscillatory modes were studied in homogeneous and stratified fluid. Flow visualization techniques and local probe measurement were used to obtain the Strouhal number dependence on the Reynolds and Froude numbers. The interaction between the two instability modes has been characterized. It is found that stratification suppresses the oscillatory mode when Froude number F < 3 or 4 depending on the Reynolds number and the vortex shedding mode when F < 2. The Fronde number is defined with the sphere radius, R, its velocity, U,and the Brünt Väisälä angular frequency, N = $\sqrt {\left| {g/\rho d\rho /dz} \right|}$, (F = U/ RN).