[Vishwa_mitra]

I am not sure of boundary condition (BC) types, Dirichlet, Neumann, Cauchy and Robin in our transport phenomenon. How are they classified and how do they differ from each other? I know that when the conditions at the surface is defined it is Dirichlet type BC (for example no-slip) but the moving plate boundary condition (Couette flow, the gradient at wall = constant) is Neumann BC, isn’t this BC was defined at the wall shouldn’t it be Dirichlet? what about BC in the annular fluid flow case, where we define one BC that gradient is equal to shear stress for both fluids (dv/dr|_Ri = τ_1; dv/dr|_Ro = τ_2), so isn’t this should be Dirichlet boundary condition?