Turbulent Couette Flow_2

[Samseg99]

Hi everyone! I'd like to simulate the Couette turbulent flow.
Starting from the example of the turbulent flat channel I put the top wall in motion like the answer in the Q&A (#153), but I've a problem. The Ux velocity of the top wall is set to 1 and in the points near the wall the velocity increase, it goes to 1.2 and keep this value also after same times (in a condition of stationarity statistics).
I saw that the problem is visible also in the answer I quoted before.
How can I solve this problem? Is the problem related to the code/numerical method or is something I'm doing wrong?

Thanks