The momentum equations in the
 

The momentum equations in the incompressible Navier Stokes equations are given by (with vectors denoted in capitals):

rho*d/dt(V) + rho*(V dot DEL)V = -GRAD(p) + mu*laplacian(V)

The velocity at a wall is zero, which gives the following boundary condition:

GRAD(p) = mu*laplacian(V)

Yet, we prescribe the zeroGradient boundary condition at the walls in OpenFOAM, i.e.

GRAD(p) = 0

Why is this so?