Near-wall Influence of Large-scale Motions in High Reynolds Number Turbulent Boundary Layers

A spanwise array of surface hot-film shear-stress sensors and a traversing hot-wire located directly above one of these sensors are used to examine the relationship between skin-friction fluctuations and the velocity fluctuations in the boundary layer. Experiments are performed in a high Reynolds number turbulent boundary layer at Reθ ≈ 39000. The shear-stress data indicate the presence of large-scale structures that extend to large streamwise distances. These structures appear to be the footprint of the large-scale superstructures that are present in the logarithmic region. Conditional averages computed based on low and high skin-friction events indicate the presence of a forward-leaning low-speed and high-speed structure, respectively. The velocity fluctuations from the hot-wire are decomposed in to large-scale and small-scale components respectively. The decomposed signals are used to compute conditionally-averaged turbulence intensities conditioned on low and high skin-friction events. The results show that the low-speed structure associated with the low skin-friction event consists of weak small-scale fluctuations near the wall and intense small-scale fluctuations farther away from the wall. Conversely, the high-speed structure associated with high skin-friction event possesses intense and weak small-scale activity near and farther away from the wall, respectively.