Time steps affect pressure magnitude for Stokes equation

Hi everyone!

I’ve been teaching myself NGsolve over the past week, and now I’m attempting to solve the time dependent Stokes equation in preparation for Navier-Stokes equation. I’ve based my code on example 3.2 from the documentation, the difference is that I’m modelling Poiseuille flow with boundary conditions which wind up from zero. Further, I’ve prescribed a dirichlet outflow condition, thus I’m using a Numberspace as a lagrange multiplier to enforce zero pressure mean. I decided to keep 1/dt at the mass matrix for simplicity.

The problem then, is that while studying convergence with regards to dt, the magnitude of the pressure seems to increase with decreasing dt. I’ve spent a day trying to find the source of error without luck, could anyone point me in the right direction?

Attached is my jupyter notebook:
unsteadyStokes.ipynb (4.5 KB)