An Investigation Of Aeroacoustics In High Speed Jets

A systematic study has been undertaken to quantify the mechanisms of noise in high speed jets.Computation approaches of jet noise based on Lighthill Acoustic Analogy are introduced in detail.Multiple sets of calculation conditions are set to get the information of jet flow and acoustic field in both subsonic jets and supersonic jets.The main work of this study is shown as follows:Six sets of calculation conditions in subsonic jets and four in supersonic jets are considered.The results show that the flow field structure is influenced by the Mach number,ideally expanded Mach number,jet temperature and Reynolds number;Analyses of flowfield and related near-sound field reasonably identify two major noise generation mechanisms,that is,noise from jet core area by Mach wave and jet shear layer;The jet noise is mainly propagated in two directions,which propagates downstream along the jet direction and spread from the jet boundary layer to the surroundings.The sound pressure level at several observation points is compared with the experimental data in the reference.For the subsonic jets,the low frequency part of the spectrum of SPL is in good agreement with the experimental results,but the middle and high frequency parts are quite different.For the supersonic jets,the error of SPL between numerical results with experimental values at observation points is acceptable.The noise characteristics in supersonic jets are studied.Mach number and temperature effects of ideally expanded supersonic jets are analyzed.Mach number effects are as follows:As the Mach number increases,the potential core length becomes longer,the high SPL region of the near field becomes wider;As the temperature of jet increases,the potential core length becomes basically no change;the high SPL region of the near field becomes wider;the angle of the direction of maximum acoustic emission becomes larger.Aeroacoustic mechanisms of an overexpanded supersonic jet impinging on a flat plate are investigated.The results show that the high temperature and low Mach number region is formed in the center of the plate;Analyses of flowfield and related near-sound field reasonably identify three major noise generation mechanisms,that is,noise from jet core area by Mach wave,jet shear layer and the center region of the plate;The jet noise is mainly propagated in three directions,which propagates downstream along the jet direction,spreads from the jet boundary layer to the surroundings and propagates upstream in a certain angle after imping on the plate.