Dynamic Response of Flexible Hair-Like Structures to Turbulence

The interactions between a turbulent boundary layer and flexible roughness arrays were investigated using concurrent high-speed imaging and hot-wire anemometry. The flexible roughness elements comprised bioinspired hair-like arrays of micropillars. Two arrays of different streamwise fetch were considered, a shorter array extending approximately two boundary layer thicknesses and a longer one extending to about ten boundary layer thicknesses. All other physical characteristics of the individual arrays, including the spacing between micropillars, were identical. The streamwise and spanwise displacements of selected micropillar tips were tracked using high-speed imaging, while hot-wire measurements were acquired simultaneously at two distinct wall-normal locations, directly behind a micropillar tip and at the approximate center of the logarithmic region. In the case of the longer array, the streamwise development of the flow was also measured. All experiments were carried out at a nominal smooth-wall friction Reynolds number of 2400. The micropillar structures acted as micromechanoreceptors, dynamically responding to the structures in the flow. The micropillar response included resonance at its natural frequency, which was amplitude modulated by large-scale flow structures. In addition, there was a direct superposition of the large-scale structures of the flow on the micropillar deflection, revealing a complex multiscale interaction between flow and structure. These observations established a mechanistic link between turbulent structures and the flexible roughness elements.