Wall shear stress measurements in water by molecular tagging velocimetry at high spatio-temporal resolution

Wall shear stress is one of the most important (if not the most important) quantities to resolve for many applied fields of fluid mechanics, including naval hydrodynamics and aerodynamics. In high-Reynolds number flows, it cannot be simulated directly and one is forced to use models. The latter typically rely on experimental dataset for their development and subsequent validation. However, measuring wall shear stress at high Reynolds numbers is very challenging, especially in water. Historically, molecular tagging velocimetry (MTV) has been demonstrated to measure instantaneous wall shear stress, but it suffers from limited temporal and spatial resolutions. By generating fine and prompt patterns (thanks to Talbot-effect structured illumination and photobleaching rhodamine 6G, respectively), we demonstrate MTV with unprecedented spatio-temporal resolution. Here, the measurement of instantaneous wall shear stress is reported in a transitional boundary layer and in a turbulent channel flow at Re Τ ≈ 800 and 1,200.