why cfd must always have pressure

Today after half a year running, i finally admit my mistake and decide to give myself a restart. as in cfd to stabilize the equation, you must have a
http://comments.gmane.org/gmane.science.analysis.femlab/7099

dP without a velocity or mass flow Direchlet constraint is not well-posed. This can be proven mathematically by examining the eigenvalues of the Jacobian matrix of partial derivatives wrt state variables.

You can specify velocity at inlet or outlet and pressure must be specified at the outlet in order to get a well-posed boundary conditions. That is why the boundaries are set up the way they are in COMSOL. They do it correctly, and what you are getting is expected.

http://www.physicsforums.com/showthread.php?t=351724