HYPERSONIC FREESTREAM VELOCIMETRY USING FEMTOSECOND LASER ELECTRON EXCITATION TAGGING (FLEET)

Ground-based wind tunnel experiments serve as a crucial tool for examining the characteristics of hypersonic flows, with precise velocity characterizations assisting in understanding complex flow phenomena and validating numerical results. Compared to other molecular tagging velocimetry techniques, femtosecond laser electron excitation tagging (FLEET) employs N₂ as a tracer, thereby significantly simplifying the experimental design. Previous researches have been primarily focused on velocity measurements using pure N₂ flows. However, the presence of O₂ would markedly reduce the effective lifetime of FLEET signals. In addition, weak focusing can further decrease the signal-to-noise ratios of FLEET images, potentially impacting the accuracy of velocity measurements. The objective of this work is to conduct hypersonic (Ma 6.5) freestream velocimetry in the JF-8A hypersonic wind tunnel using air as the flow medium. Static experiments indicate that the presence of O₂ molecules and O atoms significantly reduces the lifetime of the FLEET signal compared to its longevity under pure N₂ conditions. Concurrently, the tagged lines broaden as the delay time increases. A similar phenomenon is observed for FLEET tagged lines under shock tunnel and gun operation modes. The measured freestream mean velocities are 1045.2 and 995.2 m/s, with 2σ standard deviations of 1.98 and 1.77 m/s, respectively. These findings demonstrate the feasibility of FLEET in velocity measurements under conditions of strong collisional quenching environments and weak focusing. They also provide an effective method for velocimetry in large-scale wind tunnels and N₂-containing flows.