Experimental study on the frequency-dependent characteristics of freestream disturbance modes in a Ludwieg tube

The discrepancy between wind tunnel experimental data and numerical computation data stems from the complex disturbance composition in the wind tunnel's incoming flow, which directly affects the results of wind tunnel transition experiments. When conducting receptivity studies using numerical computations, the evolution of perturbations is generally investigated by introducing single-frequency disturbances. However, the freestream disturbances in wind tunnels are inherently broadband. This broadband nature means that the frequency dependence of disturbance modes is often overlooked in high speed wind tunnel measurements. This oversight hinders experimental research on key issues such as receptivity and also poses challenges in comparing experimental results with numerical simulations. This study is conducted in the Φ250 mm Mach 6 Ludwieg tube wind tunnel at Huazhong University of Science and Technology. Hot-wire anemometer, Pitot probes equipped with Kulite sensor, and Focused Laser Differential Interferometer(FLDI) were used to measure the quantities of the freestream, including fluctuations of mass flow, total temperature, pressure, and density. Using modal decomposition theory combined with the small disturbance assumption, three freestream disturbance modes, i.e., acoustic waves, entropy waves, and vorticity waves, were obtained. Experimental results indicate that in this wind tunnel, acoustic waves dominate with an amplitude share of approximately 60%, followed by entropy waves at around 30%, and vorticity waves at about 10%, This confirms that the facility is a typical noise-dominated wind tunnel. To further investigate the variation of the three sub-mode proportions with frequency across different bands, narrowband filtering was applied to the data. It is found that as frequency increases, the proportion of entropy waves rises rapidly, while the proportions of acoustic and vorticity waves gradually decrease. Nevertheless, the amplitude of acoustic waves remains higher than that of the other two sub-modes.