dear xueying thanks for your help about my question I have another question: in a two dimensional cavity on the bottom side when we say no shear stress it means u(i,1)=u(i,2).is it right? I think if we have been had two fluid (molten,air)we use slip boundary condition in the molten mode and use no slip boundary in the air mode.is it right? please describe for me thanks

In my situation, if there's no shear stress, I put tn:T =0. And u(i,1) is not equal to u(i,2) in my case.

Here you said two fluid, I suppose you means that there's free interface between these two fluid. Then, for momentum equation, you need to put the stress jump across the interface; and in position equation(equation to calculate position), you put n.v=0. These are the general free interface BCs

However, if the two fluid are molten and air, if the interface shape is already known and you only need to calculate the molten part's velocity and pressure, then you might put slip BC there. In my research , only when I try to guess an initial free surface and calculate velocity and pressure on the guessed free surface, I put slip BC there.