Is it necessary to applied Zero pressure b.c. on one pressure dof When solve the stokes equation with fem? The Stokes equation can be solve without set the zero pressure point in MSC.Marc.

The governing equation for the pressure involves its gradient only (unless there is some pressure-dependent material property involved). This gradient equation must be integrated, giving an integration constant to be determined. Some mathematicians resolve this by requiring the the average of the pressure over the problem domain be zero. Otherwise it can be resolved by setting the pressure to an assigned value at some point in the domain. The flow equations can be solved without reference to the pressure, so the reference pressure becomes an issue only if you want to evaluate the pressure (as a function of the flow velocity).