I am developing a FEM fluid solver using the Taylor-Hood algorithm, i.e. quadratic interpolation for velocity, and linear for pressure.
I have developed the code for 2-D quadrilaterals and triangles, and everything worked fine. I am trying to do it in 3D, using a combination of 20-noded and 8-noded hexahedrals, and I have the following problem. The velocity solution is correct and converging, but the pressure solution is very unstable. Pressure is not converging. It doesn't diverge either. I tried with under-relaxation and the problem remained.
The code looks alright (I checked it several times), but there's always a possibility to have a problem somewhere. Am I missing something in the physics? The pressure is zeroed at one point, and the problem I am trying to solve is the simple 3D lid driven cavity flow, where velocity is prescribed on all boundaries.
Can you suggest me any relative document?