Scaling of wall shear stress fluctuations in a turbulent duct flow

Instantaneous fluctuations of the wall shear stress have been measured in a fully developed turbulent duct flow over a factor of 10 variation in the Reynolds number. Distributions for the average frequency of detections obtained using the variable-interval time averaging technique suggest that scaling on outer variables or perhaps a mixture of outer and inner variables is more appropriate than scaling on inner variables when the Reynolds number is sufficiently large. The Reynolds number behavior of the average frequencies of zero crossings and local maxima of the shear stress fluctuation rules out the possibility of outer scaling and indicates a weak Reynolds number dependence when scaled on inner variables. These apparently conflicting trends are discussed in the context of published results on the scaling of wall shear stress fluctuations.