Counter Gradient Diffusion

Paul

i wonder if someone can give a quick idiots guide to ... Counter Gradient Diffusion (CGD)

what is CGD in reality?

why do we get CGD in reality?

is CGD important, i.e. should we include it in our models?

how do we model CDG? which models do and dont account for it?

does CGD just occur for scalars? what about CGD of the velocity field (momentum)?

what about in LES? do we capture CGD in resolved scales?

in LES do we capture CGD in sub-grid scales? which models do capture this and is it important?

sorry there are so many questions here. any comments would be very useful.

thanks.