The fluctuating wall-shear stress and the velocity field in the viscous sublayer

The fluctuating wall-shear stress was measured with various types of hot-wire and hot-film sensors in turbulent boundary-layer and channel flows. The rms level of the streamwise wall-shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of the streamwise velocity fluctuations in the viscous sublayer. Heat transfer to the fluid via the probe substrate was found to give significant differences between the static and dynamic response for standard flush-mounted hot-film probes with air or oil as the flow medium, whereas measurements in water were shown to be essentially unaffected by this problem.