Molecular Tagging Velocimetry of high-speed flow around a modified NACA airfoil

Molecular tagging velocimetry (MTV) is deployed for the first time in a high-speed anechoic wind tunnel over a modified NACA 0019-94 airfoil. A two-laser system is used to 'write' a continuous line of tracers in the flow and subsequently 'read' them as they are advected downstream. The laser beams are launched from within the airfoil using motorized optical mounts and a sCMOS camera, equipped with an image intensifier, located below the facility floor directly images continuous velocity profiles at the wall. The technique relies on the availability of water vapor content in air. An excimer laser at 193 nm performs the 'write' step by photo-dissociating water vapor molecules from the ambient air, producing OH radicals. Those tracers are then 'read' through laser-induced fluorescence by a tunable dye laser at 281.905 nm. The 'write'/'read' time interval is optimized between 30 μs to 160 μs for the three different speeds studied: 10, 20 and 30 m/s.