关键词: 高超声速, 稀薄气体, 低密度风洞, FLEET, 速度测量

Abstract: Hypersonic low-density wind tunnels are critical facilities for ground-based experiments and precise aerodynamic design of near-space and very-low-orbit aircraft. These tunnels feature extremely high flow velocities (Ma > 20) and extremely low densities (10?? kg/m3), where molecular vibrational energy frozen readily occurs, leading to significant measurement errors in conventional intrusive velocimetry methods. Femtosecond Laser Electron Excitation Tagging (FLEET) enables high-precision flow velocity measurement by ionizing gas molecules and tracking the displacement of the ionized channel. However, due to constraints imposed by the external tunnel configuration, direct velocity measurements in large-scale hypersonic low-density wind tunnels have not yet been achieved in China. In this study, we propose a shaped electric field-enhanced molecular excitation method and establish a precisely tunable femtosecond laser transmission and focusing system in the Φ1 m hypersonic low-density wind tunnel at the China Academy of Aerospace Aerodynamics. A series of FLEET velocimetry experiments were conducted, and the morphological characteristics of the filament under varying pressure and delay conditions were analyzed. FLEET velocity measurements were successfully obtained in a large-scale flow field characterized by high Mach numbers (Ma > 20) and extremely low static pressure (on the order of 0.1 Pa). The results were compared with conventional Pitot tube measurements. At Ma = 10.58, the deviation between FLEET and Pitot tube measurements was 1.24%, demonstrating the applicability of this technique in low-pressure environments on the order of 1 Pa. At Ma = 22.18, the deviation increased to 10.65%. Severe fluorescence diffusion and significant vibrational energy frozen under extremely low pressure are likely the main causes of this discrepancy. The successful implementation and operation of this experimental system will further advance the development of ground-based testing technologies for near-space and very-low-orbit applications in China.

Key words: hypersonic, rarefied gas, low-density wind tunnel, FLEET, velocimetry measurement