Stratified wakes of spheroids at moderate pitch angle: influence of Reynolds number*

With rapid developments in the technology of aerial vehicle and under-sea exploration, the study of wakes in density-stratified fluids has become crucial for understanding vehicle dynamics and their environmental impact. We report results from LES of a 6:1 prolate spheroid placed at a moderate pitch, α = 10°. Cases at different stratification levels (Froude number, Fr = U/ND = 1.9,6,∞) are considered. Reynolds number (Re = UD/ν) of 5000 and 15000, both with laminar flow separation to form a counter-rotating streamwise vortex pair, but different wake Re are studied.

At Re = 5000, the unstratified (Fr = ∞) wake becomes turbulent right after the body. The Fr = 6 wake is initially structurally similar but, after turbulence forms, it decays faster - a buoyancy effect. In contrast, the strongly stratified (Fr = 1.9) wake remains initially laminar for some distance before transition and a prolonged increase in turbulence. Lee waves (internal gravity waves generated by the body) modulate the wake at both Fr = 6 and Fr = 1.9. At the higher Re = 15000, wakes at all the stratification levels show symmetric flow separation, unlike the asymmetric separation at Re = 5000, Fr = ∞ and 6. Despite the several differences between the two Re cases in turbulence statistics and vortex dynamics, both also exhibit some similarities in the mean wake.