Structure of high and low shear-stress events in a turbulent boundary layer

Abstract

Simultaneous particle image velocimetry (PIV) and wall-shear-stress sensor measurements were performed to study structures associated with shear stress events in a flat plate turbulent boundary layer blackat a Reynolds number Re_\tau \approx 4000}. The PIV field-of-view covers 8\delta (where \delta is the boundary layer thickness) along the streamwise direction and captures the entire boundary layer in the wall-normal direction. Simultaneously, wall-shear-stress measurements were taken using a spanwise array of skin-friction sensors (spanning 2\delta). Based on this combination of measurements, the organization of the conditional wall-normal and streamwise velocity fluctuations (u and v) and of the Reynolds shear stress (-uv) can be extracted. Conditional averages of the velocity field are computed by dividing the histogram of the wall-shear-stress fluctuations into four quartiles, each containing 25\% of the occurrences. The conditional events corresponding to the extreme quartiles of the histogram (positive and negative) predominantly contribute to a change of velocity profile associated with the large structures and in the modulation of the small-scales. A detailed examination of the Reynolds shear stress contribution related to each of the four quartiles shows that the flow above a low wall-shear-stress event carries a larger amount of Reynolds shear stress than the other quartiles. The contribution of the small- and large-scales to this observation is discussed based on a scale decomposition of the velocity field.