The structure of the viscous sublayer and the adjacent wall region in a turbulent channel flow

Hot-film anemometer measurements have been carried out in a fully developed turbulent channel flow. An oil channel with a thick viscous sublayer was used, which permitted measurements very close to the wall. In the viscous sublayer between y + ≃ 0·1 and y + = 5, the streamwise velocity fluctuations decreased at a higher rate than the mean velocity; in the region y + lsim 0·1, these fluctuations vanished at the same rate as the mean velocity. The streamwise velocity fluctuations u observed in the viscous sublayer and the fluctuations (∂ u /∂ y ) 0 of the gradient at the wall were almost identical in form, but the fluctuations of the gradient at the wall were found to lag behind the velocity fluctuations with a lag time proportional to the distance from the wall. Probability density distributions of the streamwise velocity fluctuations were measured. Furthermore, measurements of the skewness and flatness factors made by Kreplin (1973) in the same flow channel are discussed. Measurements of the normal velocity fluctuations v at the wall and of the instantaneous Reynolds stress −ρ uv were also made. Periods of quiescence in the − ρ uv signal were observed in the viscous sublayer as well as very active periods where ratios of peak to mean values as high as 30:1 occurred.