A Possible Mechanism for Boundary Layer Mixing and Layer Formation in a Stratified Fluid

Abstract

A plate tilted at 42° from the horizontal is oscillated along its length with frequency ? and amplitude a in a linearly stratified fluid (salt water) of kinematic viscosity ?. If the Reynolds number Rc = a(?/?)½ is small, a parallel oscillating boundary layer is set up. As Re is increased from zero the parallel flow becomes unstable. If ?2?N?2 (where N? is a transverse Brunt-Väisälä frequency), the motion appears to be a convective overturning associated with periodic reversals of the density gradient. If ?2?N?2, these plumes just mix up the fluid next to the plate. If, however, ?2≈N?2 and Re is not too large, then the plumes interact with the oscillating shear flow and mix in such a way as to set up layers which extend into the interior of the fluid. It is suggested that these effects might also be present when internal waves propagate over a slope.