wallGradU and wallShearStress, inconsistent result for RAS model
 

Hi,

I am doing a simple simulation of water flowing on a plate with RAS k-epsilon model.
My goal is to verify the simulation results with different roughness height on the surface of the plate and to calculate the wall shear stress of each case.

However, the utility wallGradU gives (36.5451 -8.58721e-05 0.000132998) for one point on the bottom wall and the utility wallShearStress gives (-0.000808236 3.08435e-09 -2.97856e-09) at this point. As I am doing a simulation of water so the dynamic (shear) viscosity is 8.9e-4 Pa·s. And the wall shear stress can be calculated by the equation:

http://www.cfd-online.com/W/images/m...d495dddff5.png

So I think at this point ,the wall shear stress should be 8.9e-4*36.5451=0.0325 pa.

I would like to ask why there is the difference between the results of wallGradU and wallShearStress ? If possible, could anyone tell me the equations used in OpenFoam to calculate the wall shear stress ? Thanks !

To add some details. The results I got is for a smooth plate with nukRoughWallFunction and Ks=0 Cs=0.5 as the boundary condition at the bottom wall. And I did not find a place to add the density of the fluid into the simulation so I guess the wall shear stress is calculated with the kinematic viscosity rather than the dynamic viscosity. Or it assumes that the density is 1 kg/m^3. I am a starter of CFD and OpenFoam so can someone help me please. Thanks !