ACTA AERONAUTICAET ASTRONAUTICA SINICA ›› 2022, Vol. 43 ›› Issue (8): 125857-125857.

• Fluid Mechanics and Flight Mechanics •

### Acoustic performance of an under-expanded supersonic jet with different impinging distances to an inclined plate

QIN Chen1,2, ZHANG Rongping2, ZHANG Junlong2,3, YUE Tingrui2, WU Songling2

1. 1. State Key Laboratory of Aerodynamics, China Aerodynamics Research and Development Center, Mianyang 621000, China;
2. Key Laboratory of Aerodynamic Noise Control, China Aerodynamics Research and Development Center, Mianyang 621000, China;
3. School of Aeronautics, Northwestern Polytechnical University, Xi'an 710072, China
• Received:2021-05-24 Revised:2021-07-21 Online:2022-08-15 Published:2021-07-20
• Supported by:
State Key Laboratory of Aerodynamics Foundation (SKLA2019020101)

Abstract: An experiment of supersonic under-expanded jet impingement was carried out in a full anechoic chamber, in which a jet of Mach number is 1.23 from a convergent nozzle with the diameter D of 56 mm impinged on an inclined plate with the length of 600 mm, and the impingement angle is β of 65°. Arc array of microphones and high frequency Particle Image Velocimetry (PIV) were used to observe the far-field sound and flow structure at different impact distances L, respectively. Microphones were located on the side of the horizontal axis of the jet with the far-field distance of 64.3D. The sampling frequency of PIV was high enough to capture the shedding frequency of large-scale vortex structure. The gray scale image of PIV snapshots shows that the potential flow core of the free jet extends about 9D, and the large-scale vortex structure is dominated by asymmetric mode. The effects of the impact distance of nozzle to inclined plate on the overall sound pressure level (OASPL), turbulence mixing noise, impingement noise, broadband shock-associated noise and shock screech were carefully analyzed. It is found that broadband shock-associated noise has almost no change if the impact distance is larger than that of the length of potential flow core (L > 9D). For L ＜ 9D, the aeroacoustic results show that the broadband shock-associated noise decreases, and the radiation ability of impact noise to the side line is very weak while the impingement noise dominates the spectrum of upstream, resulting in the increase of upstream OASPL and the sharp decrease of side line OASPL. The spectrums for L > 5D indicated that the jet impingement causes a small frequency shift of screech. Due to the limitation of inclined plate, the shear layer instability wave cannot develop downstream normally. It is indicated from the flow visualization of L = 7D that the large-scale vortex structure of shear layer switches between asymmetric mode, axisymmetric mode and transition mode. A higher frequency screech is detected when L = 4D, and its dimensionless wavelength corresponds to axisymmetric mode. The quantitative analysis of the shock screech amplitude for L > 5D shows that the inclined plate mainly affects the upstream screech intensity. As for side line direction, the intensity of the second harmonic of screech is stable, while the impingement on the inclined plate reduces the average sound pressure level.

CLC Number: