Progress on molecular tagging velocimetry (MTV) for wall shear stress measurements in water

Measuring the instantaneous wall shear stress (WSS) in high-Reynolds-number wall-bounded turbulent flows is challenging greatly due to the small spatial and temporal scales involved. Molecular tagging velocimetry (MTV) is a non-intrusive optical diagnostic that gives continuous velocity profiles and is hence well-suited to measure velocity gradients. A novel photobleaching scheme, combined with Talbot-effect structured illumination, overcomes the traditional limitations of spatio-temporal resolution of MTV techniques in water. The approach has been demonstrated in a turbulent channel flow with viscous length scales on the order of 10 μm. The technique's performance is assessed with various pairs of common pulsed neodymium-doped lasers (351-355 nm and 527-532 nm) and cameras. Special attention is paid to making the MTV system compact and remotely operated to allow for the upcoming in-situ deployment from within a model in very large water facilities.