THE STRUCTURE OF THE NEAR WAKE OF A SPHERE MOVING HORIZONTALLY IN A STRATIFIED FLUID

We present experimental results for the wake structure of spheres movi ng in homogeneous and stratified fluid. In homogeneous fluid, the resu lts of Kim & Durbin (1988) are confirmed and it is shown that the two characteristic frequencies of the wake correspond to two instability m odes, the Kelvin-Helmholtz instability and a spiral instability. For t he stratified wake four general regimes have been identified, dependin g principally on the Froude number F. For F > 4.5 the near wake is sim ilar to the homogeneous case, and for F < 0.8 it corresponds to a trip le-layer flow with two lee waves, of amplitude linear in F, surroundin g a layer dominated by quasi-two-dimensional motion. Froude numbers cl ose to one (F is-an-element-of ]0.8,1.5[) give rise to a saturated lee wave of amplitude equal to half the sphere radius, which suppresses t he separation region or splits it into two. Between F = 1.5 and 4.5 a more complex regime exists where the wake recovers progressively its b ehaviour in homogeneous fluid: the axisymmetry of the recirculating zo ne, the Kelvin-Helmholtz instability and, finally, the spiral instabil ity.