Abstract:Jet flow is one of the main noise sources of aircraft. Traditional method of increasing the bypass ratio is reaching its technical limit. Revealing the sound sources in the jet flow and finding an effective method to control it are urgent tasks for researchers. Porous materials are found to slightly change the near surface flow condition and influence the vortex structure in the flow field, so it is possible to use it as a passive strategy for flow and noise controlling. In this paper, porous treatments on the inner wall of the nozzle near the exit are used to control the jet flow. A testing rig for jet flow and noise testing with the nozzle outlet diameter D_j=10 mm was set up. The flow characteristics and the acoustic field of subsonic jet with Ma=0.6 were studied experimentally. Results showed that the jet noise consisted of two parts, i.e., the low frequency noise in the small radiation-angle range produced by the large vortical structures, and the high frequency noise in the large radiation-angle range generated by the small scale vortices. The large vortical structures were the dominant source of the jet noise. According to the noise propagation and distribution characteristics, some extensions with pores of blind holes on the inner wall were designed and attached to the exit of the baseline nozzle in an attempt to destroy the stability of the ring vortices from the outlet and to suppress developments of large scale vortical structures. The influence of the pore size, namely, the hole depth H and diameter D respectively, on both near and far acoustics fields was carefully measured and analyzed. The results showed that the hole depth H had two opposite effects on the noise spectra. For shallower cases with H=1, the noise in the frequency range of f>6 000 Hz was cut down by about 2 dB, while the noise in f<5 000 Hz was increased by about 1 dB. For the cases of deeper holes with H=4, the lower frequency noise was reduced by about 1 dB, but the higher frequency noise increased. Such tendencies on the noises were more obvious for the cases with bigger D values. The present results provided a reference for attenuation of jet noise.