Capturing the Coupled Dynamics of Hair-Like Structures and Wall-Bounded Turbulence

Two complementary experimental approaches were developed to investigate the coupled dynamic interaction between wall-bounded turbulence and flexible micro-scale roughness elements, referred to as micropillars. Previous measurements in a zero-pressure-gradient boundary layer (nominal friction Reynolds number of 2400) demonstrated that these nature-inspired, hair-like structures vertically displace and modify the near-wall turbulence peak while also responding to the flow by vibrating or undergoing displacement. To understand these interactions, the two approaches were designed to capture the flow and micropillar dynamics simultaneously. The first approach combined hot-wire anemometry with high-speed imaging to capture velocity fluctuations and micropillar motion concurrently. Two hot-wire probes, positioned over the micropillar array, measured streamwise velocity fluctuations at two distinct wall-normal locations, while a top-mounted high-speed camera recorded the streamwise and spanwise motion of individual micropillars. The second approach integrated particle image velocimetry (PIV) with synchronized high-speed imaging to measure the carrier-phase flow field and micropillar dynamics simultaneously. PIV cameras resolved the velocity field in the region between and above a pair of micropillars in the streamwise direction, while also tracking their streamwise motion. Together, these complementary approaches provided velocity fields at high spatial and temporal resolution alongside micropillar motion, enabling spatio-temporal analysis of the fluid–structure interactions. The motivation for, advantages and limitations of each approach, as well as demonstrative measurements, are presented.