Experimental study on the statistics of wall shear stress in turbulent channel flows

Measurements of local wall shear stress have been undertaken using a laser gradient meter that enables direct evaluation of the wall velocity gradient with high spatial resolution. Mean values as well as the statistical parameters of fluctuating wall shear stress, such as third and fourth moments, have been obtained in fully developed air flow in a two-dimensional channel at moderate Reynolds numbers. Agreement with the available direct numerical simulation (DNS) and experimental results is satisfactory. The dependence of the turbulence statistics on the Reynolds number is masked by the experimental ambiguity. Although the linear near-wall velocity profile is the principal requirement for the application of the present system. it is found that the measurement of the mean velocity gradient is less influenced by the nonlinearity of the velocity profile more distant from the wall than are the turbulence statistics.