error about solving darcy equation with mixed FEM
 

Hi, I use the mixed galerkin finite element to sovle darcy equation. pressure is linear interpretation, velocity is quadratic interpretation。 The domain is square. The result is correct when I apply no-slip b.c on top and bottom side, zero pressure on right-bottom corner, Ux=1.0 on left side. But I can not get correct result if I apply no-slip b.c on top and bottom side, zero pressure on the right side, P=1 on the left side. Why can I get correct result with this b.c.?