Convergence issues incompressbile 2d channel flow

Dear PyFR-Developers,
I’m trying to simulate an incompressbile turbulent 2d channel flow at Re=10000 (L=10D, D=0.016m, u_mean=0.6275, nu=1e-6, 561 Elements). I provided a simple 1/7-power-law velocity profile as an inlet condition and an arbitary pressure of 1 at the outlet, which should not matter as it is an incompressible case.

Unfortunately however I play around with the ac-factor (varying from 1-10*u_mean), the timestep and the pseudo-timestep the simulation blows. I attached my initialisation files. Do you have any hints what could be the matter?
Kind Regards,
Hendrik

pipe_2d.msh (36.1 KB)

pipe_2d.ini (1.54 KB)