Any reason why you couldn’t continue the discussion in Convergence problem for poiseuille flow of stokes equations ?
Purposely opening duplicate posts without an appropriate justification may lead to us closing down the second one, i.e. this one.
(And, no, the fact that the domain is not a square anymore but a rectangle is not an appropriate justification, in general).
what recommendations would you suggest to address this issue effectively
Pick a u and p which are not quadratic and linear, respectively, but still solution of the Stokes problem.