Noise characteristics in two overexpanded jets at different nozzle-exit turbulence conditions are investigated by using large-eddy simulations. One simulation has a clean inflow turbulence condition, but another has a highly disturbed nozzle-exit turbulence condition introduced by boundary-layer tripping. The boundary-layer effect inside the nozzle is simulated by using the equilibrium wall model. This wall-model approach has been validated with available experimental data and theoretic predictions. A strong helical screech tone is observed in the simulation where the inflow turbulence level is low, but the simulation where the nozzle-exit turbulences are highly disturbed presents a weaker screech tone. The maximum lip-line pressure wave intensity of the stronger screech tone is observed around the 6~(th) compression wave, but the lip-line maximum intensity of the weaker screech tone is around the 8~(th) compression wave. In the case of the strong screech tone, there is a phase-speed match between the dominant instability waves and the most amplified upstream propagating wave traveling along the lip line. It is believed that this phase-speed match generates a strong standing wave rendering shock-cell oscillations highly coherent. On the other hand, there is a substantial phase-speed mismatch between instability waves and the most amplified upstream propagating wave in the case of a weaker screech tone. It is proposed that the phase-speed match between instability waves and the most amplified upstream propagating wave is critical to the generation of a strong helical screech tone. The reasons that the highly disturbed inflow turbulences reduce the screech intensity are also discussed.
展开▼
