MTV for instantaneous wall shear stress measurements in turbulent flows

Measurements of the instantaneous wall shear stress (WSS) in turbulent flows are particularly challenging at high Reynolds numbers due to the small spatial and temporal scales involved. Among the existing non-intrusive optical diagnostics, molecular tagging velocimetry (MTV) gives continuous velocity profiles and is hence well suited to measure velocity gradients. The traditional limitations of spatio-temporal resolution of MTV techniques in liquids are overcome by a novel photobleaching approach of an inexpensive dye with Talbot-effect structured illumination using a pair of common pulsed neodymium-doped lasers (355 and 527 nm). The system is made compact and modular by using motorized optical mounts and small-pixel low-noise cameras. With those improvements, MTV reaches unprecedented spatio-temporal resolutions and becomes a very flexible scheme to study a broad range of liquid flows. Here, it is demonstrated in a turbulent channel flow with viscous length scales on the order of 10 µm. Special attention is paid to the tails of the probability density function of the fluctuating WSS.