pressure fluctautions on a plate

oe

I am solving for pressure fluctuations on a plate due a supersonic impinging jet. The plate is at 2 jet diameters and pressure ratio = 1.2 (p_exit/p_ambient = 1.2) and the jet issues from c-d nozzle (Design mach 2.2). I compare the computational results with experimental. The pressure on the plate center seem to be underpredicted and the pressure drops sharply and then rises again to the almost the correct experimental value and falls off then rises again and then levels off. I am using Roe 2nd order upwind-biased flux difference splitting with TVD MINMOD. Does anyone have any ideas why the changes in pressure are not smooth as observed in the experimental results. Is it do with the explicit scheme? Also I use RK time stepping. This is a steady flow case I considered.

Please suggest.