EXPERIMENTAL STUDY ON SCREECH SUPPRESSION OF SUPERSONIC FREE JET THROUGH RECTANGULAR NOZZLE

The paper conducted a series of experimental study on V-shaped grooves influence of supersonic free jet through rectangular nozzles, such as two different screech modes, changing rules of sound pressure level of the screech mode, 3D-dynamic spectrum of sound pressure and schlieren photographs of the flow field. The experimental results show that V-shaped grooves on rectangular nozzle could reduce the acoustic feedback of shock wave from jet flow, so as to eliminate one mode of screech completely). It is also suggest that V-shaped grooves performed differently on rectangular nozzle compared with circular one at high frequency when jet pressure was high. Several fundamental frequency differentials exist in both two rectangular nozzles at the same pressure, whereas fundamental frequency in non V-grooved nozzle is much smoother than the V-grooved one. The nozzle with V-shaped grooves had a significant noise suppression effect below 0.55MPa. An instantaneous spectrum comparison between V-grooved and non V-grooved nozzles in scope of 0.25 similar to 0.40MPa was given. The SPL (Sound Pressure Level) differential increased at beginning, then decreased with the decline of jet pressure in low frequency below 10kHz. Nozzle with V-shaped grooves had a lower SPL. Nevertheless, an entirely different phenomenon was discovered in high frequency above 10kHz. When jet pressure was 0.40MPa, V-grooved rectangular nozzle had a higher SPL, and it approached to SPL of non V-grooved nozzle with decline of jet pressure until below it. Moreover, differentials gradually became obvious due to the mixing effect with different nozzle structure. Noise pressure in V-grooved nozzle dropped rapidly in range of 0.40 similar to 0.80MPa, as well as lower SPL and widely frequency distribution. Impact of V-shaped grooves on schlieren photographs was analyzed. Flow at nozzle exit was strengthened by impact of V-shaped grooves. An obviously density gradient could be found in first shock-cell and length of cell was smaller than the one without grooves. The flow was more stable in other cells but in a weaker intensity. The V-shaped grooves could increase length of jet path as well as decrease expansion of the cells. In general, V-shaped grooves in rectangular nozzle performed differently from circular nozzle, which could suppress and even eliminate jet flow screech. During engineering application, the matching of V-shaped grooves in rectangular nozzle and pressure of jet flow should be taken into consider.