From mathematical point of view, fixed value boundary conditions are Dirichlet boundary conditions. NOTE: For incompressible fluid flow, all the pressure dimensions are always [ /{rho}]=[ ], sometimes called kinematic pressure. So, all the pressures are divided by the density. Also all the mass flow rates, torques and powers are like divided by density. For incompressible fluid flow, the pressure value is alway relative, not absolute (this is why the zero value is often prescribed at the outlet).