Dynamic Perturbation of a Turbulent Boundary Layer and Experimental Identification of Critical-Layer-Type Behavior

A zero-pressure gradient turbulent boundary layer is perturbed by a spatially impulsive patch of two-dimensional roughness elements, which are actuated dynamically to alternate between smooth and rough surface conditions, and the downstream response is measured by hot-wire anemometry and particle image velocimetry. The dynamic perturbation is observed to contribute a periodic signature to the downstream flow-field, which manifests itself in critical-layer type behavior. The downstream flow field is reconstructed in a phase-locked sense in order to compare the observed behavior with asymptotic representations of the expected behavior at matched flow conditions. Perturbation using a periodic disturbance is shown to reveal underlying features of the turbulent boundary layer which are intimately connected to the critical layer framework for turbulent pipe flow proposed by McKeon \& Sharma (\textit{see the DFD-2010 presentation on `Structure from the critical layer framework in turbulent flow' by Sharma \& McKeon}), while simultaneously providing practical insight on the manipulation of the structure of boundary layers.