Sir, I am solving channel flow problem, At the exit I used fully developed boundary condtion du/dx=0.

I have seen one paper mentioned at the exit convective boundary condtion i.e du/dt+u(avg)*du/dx=0 please help me how to use this as boundary condtion. Thank you

The second equation also implies the fully developed conditions at the exit (du/dt = 0 for steady flow, and second term du/dx = 0 as you have specified). So you are all set. No problem there.

Thanks. Actually I am solving time dependent problem I understood. One more doubt in vortex shedding problem at the top wall u[i][j]=-u[i][j-1] and at the bottom wall u[i][j]=-u[i][j+1]. Could you please tell me how do we name this type of boundary conditions.

I assume it has to deal with periodic boundary conditions.