For outflow, it is generally possible to specify the velocity, but it is more common to either set all gradients to zero or define the pressure. Since pressure in the Navier-Stokes equations only appears as a derivative, its absolute value is irrelevant for incompressible flows. Therefore, it is sufficient to set the pressure at a single point within the flow domain. The outflow boundary should be placed far enough away to ensure an almost undisturbed flow.