Sensing Wall Shear Stress Fluctuations in Non-Equilibrium Flows

Techniques to measure wall shear-stress (WSS) fluctuations in low-speed high Reynolds number turbulent flows often rely on flow assumptions that are strictly valid for canonical smooth-wall configurations. Some recent MEMS-based direct sensors promise to address the gap, but further progress is needed for reliable spatio-temporally resolved measurements for a broad class of non-canonical flows (with surface roughness, pressure gradients, atmospheric surface layers, etc.). Seeking to address this challenge, we discuss the principle of photoelasticity, where certain materials display stress-induced optical anisotropy, that can be utilized to sense wall shear stress dynamics. While photoelasticity has been used for non-destructive testing in civil engineering applications with Giga-Pascal scale loads, we present an experimental design that is tuned to measure the much weaker, milli-Pascal shear stress induced by low-speed turbulence overriding a wall. We demonstrate our on-going work to visualize the shear-stress via the observed optical birefringence in a custom-made polymer membrane followed by a discussion on quantifying and validating the shear stress using recent Particle Image Velocimetry based methods.