Transient velocity measurement of shear flow using Femtosecond Laser Electronic Excitation Tagging

The measurement accuracy of flow velocity affects the prediction accuracy of aerodynamic performance of the aircraft. However, the current laser-spectroscopy-based velocity non-intrusive measurement technology can’t fully satisfy the requirement of high-precision flow velocity measurement, but the Femtosecond Laser Electronic Excitation Tagging （FLEET） measurement technology can. In this work, a FLEET system is developed based on a Ti:sapphire femtosecond laser. The electronic fluorescence of N₂, which is excited by the femtosecond laser, is analyzed. Based on the FLEET system, transient velocities of the shear flow within 30 m/s to 170 m/s, which is adjusted by the gas pressure or volume flow rate in the high-speed channel, are measured. Finally, the effect of delay time on velocity measurement is investigated. With the delay time increasing, the fluorescence image widens due to the diffusion of plasma, and the signal to noise ratio decreases due to the fluorescence intensity decay. However, the velocities measured at different delay times are basically consistent. The results show that FLEET is practicable to measure the velocity of the shear flow.