Jm. Chomaz et al., VERTICAL DIFFUSION OF THE FAR WAKE OF A SPHERE MOVING IN A STRATIFIEDFLUID, Physics of fluids. A, Fluid dynamics, 5(11), 1993, pp. 2799-2806
The far wake of a sphere towed horizontally in a linearly stratified f
luid with Froude numbers F is-an-element-of (0.25,12.7) and Reynolds n
umbers Re is-an-element-of [150,30 000] is investigated. Regardless of
the initial Froude number, the wake becomes quasi two dimensional at
times large compared with the Brunt-Vaisala period. For F less-than-or
-equal-to 4.5, the horizontal motions are coherent over the whole dept
h of the wake. The wake takes the form of a regular von Karman street
for F less-than-or-equal-to 1.5 but is irregular at larger F. For F gr
eater-than-or-equal-to 4.5 the far wake consists of, vertically incohe
rent, horizontal motions in several layers. The transition occurs smoo
thly by a vertical decorrelation of the horizontal motion in the depth
of the far which starts at F=4.5. The most novel result is that the h
orizontal velocity and vertical vorticity diffuse vertically in a time
much shorter than predicted by a viscous diffusion law. The ratio of
the observed diffusion time to the viscous diffusion time is expected
to depend on Reynolds number as is indicated by a simple model.