Measurement of freestream disturbance in hypersonic wind tunnel with hot-wire anemometer

The types and levels of disturbances in hypersonic freestream have significant effects on the flow phenomena such as boundary layer transition and shock/boundary layer interactions. However，the understanding of the disturbances in hypersonic wind tunnels is currently limited，which restricts the relevant research. In response to this situation，two methods，the high-temperature hot-wire with a high over-heat ratio and the conventional hot-wire with variable over-heat ratios，were conducted to measure the freestream disturbances in the Mach number 6 hypersonic wind tunnel at Huazhong University of Science and Technology. The relative error of the disturbance amplitude derived by the two methods does not exceed 15%，and the normalized root mean square values of the mass flow rate and total temperature fluctuations at different Reynolds numbers are between 0. 77%-1. 25% and 0. 1%-0. 18%，respectively. The hypothesis that acoustic disturbances dominate in conventional wind tunnels was verified by using the fluctuation diagram of compressible hot-wire. Meanwhile，the direction of the acoustic disturbance was determined to be between 120° and 140°，which translates to a sound source velocity between 0. 67 and 0. 78 of the freestream velocities. Additionally，the spectrum of static pressure caused by the acoustic wave below 40 kHz was obtained using the mass flow spectrum of the high-temperature hot-wire. It emerged that the amplitude in low-frequency disturbance below 10 kHz had a magnitude of 10−8 and the high-frequency spectrum roll-off had a slope of −5/3. This study verified the viability of using constant-temperature anemometers for freestream disturbance measurement in hypersonic wind tunnels，and obtained the disturbance types and amplitudes of distribution characteristics in the frequency domain. © 2024 Chinese Society of Astronautics. All rights reserved.