Diffusion of a passive admixture settling in an isotropic random velocity field

The coefficient of diffusion of a passive admixture settling under the action of buoyancy and gravity forces in a random homogeneous and isotropic velocity field is calculated in the diffusion approximation, It is shown that the diffusion tensor is anisotropic with respect to the direction of the velocity of settling, The characteristic parameters are estimated. The effect of the anisotropy is essentially associated with the finiteness of the correlation time of the velocity field; this effect is absent in the approximation of delta-correlated-in-time velocity field.