On the 'stratification drag'

It is well known that particles introduced into density stratified regions of atmosphere and ocean settle slower than they would in neutrally stratified environments. It is shown that this retarded descent can be due to the presence of an additional drag force, known as the "stratification drag", as a result of the existence of a fluid tail attached to the particle at low and moderate Reynolds numbers. The fluid tail contains low density fluid incorporated from upper layers, and hence is subjected to an additional buoyancy force that is responsible for the stratification drag. Laboratory experiments carried out with three-layer fluids, containing a thick interfacial layer sandwiched between two homogeneous layers, indicate that the stratification drag is pronounced in the Reynolds number range 1.5 < Re-1 < 15, where Re-1 = Ud(p)/nu, U and d(p) are the velocity and diameter of particles, respectively, and nu is kinematic viscosity of the fluid. For a given Re-1, the stratification drag on a particle is an order of magnitude larger than the total drag on the particle in a homogeneous fluid.