Propagation characteristics of typical acoustic disturbances in nozzle contraction of supersonic wind tunnels

Studying the influence of acoustic disturbances on nozzle flow fields is key to improving wind tunnel design and aerodynamic experimental accuracy. This study uses numerical simulations to examine the propagation of disturbances with different modes through the contraction of a supersonic nozzle. It also explores the influence of the mean flow on acoustic propagation. Results show strong mode conversion in the contraction, adding complexity to the sound field. The mean flow promotes conversion toward lower-order modes and suppresses the formation of higher-order modes. Higher frequencies cause higher amplitudes and greater spatial non-uniformity. Excited-state modes show much higher amplitudes than cutoff-state ones. For excited modes, amplitude peaks appear near the throat. For cutoff modes, they appear near the inlet. The mean flow enhances the amplitude near the inlet but attenuates it near the throat. For cutoff modes, the mean flow extends the effective propagation region and amplifies the overall amplitude. Transmission loss exhibits a clear dependence on frequency. Higher-order modes correspond to higher optimal attenuation frequencies. This study reveals the propagation laws of acoustic modes in the nozzle contraction, providing guidance for noise control in the design of quiet wind tunnels.