Shear stress fluctuations by turbulent flow
 

Dear all:

I am trying to plot the shear force as a function of flow time on & near a flat plate at a fixed point. The flow in contact with the plate surface is turbulence therefore I am expecting random time history because the flow velocity of a turbulent flow should be random. However my CFD result shows a quite smooth sin wave.

Its a 2D simulation, I used k-e model, standard wall treatment, no slip, Re=3.6e6, Simplec method and second order for every option. sufficiently small time step, the mesh is ok and I got reasonably good Cl and Cd results in the steady case. For the unsteady simulation I have two monitor points one on the plate and one slightly above. The point-velocity time history for both points follow the same trend (sin wave) with only a little differences in magnitude.

I was hoping to see some randomness and I have been stuck for days. any input on this matter from you guys is much appreciated!:(