Propagation of perturbations in the viscous sublayer and adjacent wall region

The fluctuating velocity components, u and w, and their gradients at the wall, \[ (\partial u/\partial y)|_{w}\quad {\rm and}\quad (\partial w/\partial y)|_{w}, \] were measured in a fully developed turbulent channel flow using hot-film probes. The data were taken in an oil channel at a low Reynolds number which allowed velocity measurements very close to the wall. From simultaneous measurements of these signals it could be deduced that coherent flow structures, inclined to the wall, travel downstream. Space-time correlations imply that these are rotating structures. The spanwise distance between these structures was found to be z⁺ ≈ 50 and they could be observed over a streamwise distance of Δx⁺ > 1000.