[Agad15]

I am simulating a case where immediate slip wall region exists after no slip wall. Obviously wall shear stresses shoot up at the interface and collapse to zero when slip wall comes and these gradients of wall shear stresses are too steep.

I started with coarse mesh the pressure drop (actually its a channel with bottom wall has divided into slip and no slip regions) is exactly matching with analytical result!!
I tried with finer and finer mesh and pressure drop deviating from analytical result
I tried to refine the mesh at interface region but same result
I tried adaptive mesh refinement but couldn't apply successfully (I don't know why, when I wanted to adapt it based on wall shear gradient its not taking)

How can I solve my problem? Why the coarser mesh giving exact result?