verification of 3D channel
 

Hi, I am new to CFD. I want to verify the volumetric flow rate and shear stress but I just found expressions for the 2D rectangular channel and flow is due to pressure difference. I am using the expression for shear stress as tw=frictional force/ area(length of wall*width ) and Cf=tw/(0.5*rho*Uavg^2)? values are the same as a 2D result.is that ok? but what is the analytical expression for tw?