Femtosecond laser electronic excitation tagging applied to velocity measurement in hypersonic boundary layer flows

The flight process of hypersonic vehicles involves complex flow interactions, such as boundary layer transition and shock-wave/boundary-layer interactions, making velocity measurement within the boundary layer crucial for understanding aerodynamic characteristics. Femtosecond Laser Electronic Excitation Tagging (FLEET) offers significant advantages over traditional Molecular Tagging Velocimetry (MTV) by measuring hypersonic flows without the need for tracer particles. However, measuring velocity within the boundary layer remains challenging due to near-wall effects and flow disturbances. This study investigates the applicability of FLEET in hypersonic boundary layer flows, aiming to enhance understanding of boundary layer characteristics and provide data for numerical simulations. The JF10 wind tunnel generates high-temperature and high-pressure gases by detonating a hydrogen-oxygen mixture, producing a shockwave that compresses nitrogen test gas. The gas is then expanded through a nozzle to create a hypersonic flow field. A femtosecond laser focuses near the surface of a blunt-cone model, and an intensified ICCD camera captures the measurements. Velocity profiles were obtained under Ma=8.9 inflow conditions, considering factors like femtosecond filament imaging resolution. The results show that FLEET is effective for velocity measurement in the hypersonic boundary layer, with fitting coefficients exceeding 99.2%. The measurement uncertainty was determined to be 12.4%. The results validate FLEET as a feasible technique for velocity measurements in hypersonic boundary layer flows, and future research will integrate it with other laser diagnostic techniques for improved accuracy.