Experimental evaluation of the time dependent surface shear stress or heat transfer in adverse pressure gradient turbulent boundary layers was studied. Surface shear stress pulses as great as three to eight times the mean values were indicated in the adverse pressure gradient flows. The minimum time dependent surface shear stress varied from 0.2 to 0.4 times the mean shear as Reynolds number increased in the adverse pressure gradient flows. Although it was not possible to evaluate the surface shear stress in the separation region, laser velocimeter measurements indicate independent positive and negative velocity pulses are present in the wall region.
The statistical characteristic times between the largest shear stress pulses was correlated using the mean skin friction coefficient and a function of the velocity profile form factor and Reynolds number for a wide range of flow conditions.