Outflow of a channel

Hi,
I am facing a problem of outlet B.C. in a 2D open channel problem with turbulent flow. In physics, the outlet velocity and pressure should be unknown and controlled by a gradient condition du/dx=0 & v=0 ; which means the velocity of x direction remain unchanged and y component, the gravity direction v is zero. However Comsol can only give a known value in the outlet boundary, either v or p. How can I solve this problem? by weak form?or others? Please give me any suggestion.

Thanks, very very much.