Numerical Investigations Of Sound Generation In Subsonic Jets

Jet noise has become more and more important in the realm of Aeronautics and As-tronautics due to it would cause environmental pollution, reduce aeroengine efficiency, and bring structural damage to vehicle itself, consequently, it attracts more and more attentions from scientific researchers. In the present study, numerical simulation and parabolized stability equation (PSE) are used to investigate sound generation mecha-nisms of jet noise, and the relationships between large-scale vortical structures, linear instability waves and far-field noise in laminar and turbulent jets are investigated in detail. Main aspects of present study are summarized as follow:(1) Vortex-pairing noise in axisymmetric jets is investigated numerically. The in-fluences of Mach number and heating on vortex-pairing noise are examined, and the sound generated by instability waves is discussed in detail. The results show:the flow field is dominated by vortex roll-up and pairing, the positions of vortex roll-up and pairing move upstream when heating the jet or reducing the jet Mach number. The far-field noise is dominated by subharmonic frequency which indicates that the vortex-pairing is the dominant sound source, and the acoustic intensity is enhanced and the propagation direction of the dominant sound waves move toward large polar angles as the increase of Mach number or jet core temperature. The decomposition of source terms of Lilley-Goldstein (L-G) equation shows the momentum and thermodynamic components lead to distinctly different beam patterns. Significant cancellation is found between the momentum and thermodynamic components at low polar angles for the isothermal jet and high polar angles for the hot jet. The cancellation leads to the min-imum values of the far-field sound. The disturbance energies and the radial profiles of pressure disturbance predicted by PSE are well in agreement with the directly com-puted results at the growth stage, but large deviations occur downstream because of nonlinearity. However, the sound generated directly by PSE modes is underestimated greatly. Based on linear PSE solutions, the nonlinear model in terms of interactions of instability waves for sound prediction is built in combination with the L-G equation. The dominant beam patterns and their original locations predicted by the nonlinear model are in good agreement with the direct simulation results, and the predictions of sound pressure level (SPL) by the nonlinear model are relatively reasonable.(2) Large eddy simulation (LES) is performed for two jets, i.e., the cold and hot jet. The far-field overall sound pressure levels (OASPL), noise spectra, turbulent fluc-tuations are predicted reasonably and are well validated against previous experimental results. Based on the LES database, the relationships between large-scale structures, instability waves and far-field noise are discussed in detail. The most energetic co-herent structures, denoted as the first POD modes, are extracted from the pressure field of turbulent jets using the frequency domain variant of the snapshot method of proper orthogonal decomposition (POD). The first POD modes are found to be more cleaner than LES flow fields, and reveal clear growth, saturation and decay pattern-s, look like a wavepacket. The growth rates, amplitudes, wavelengths and phases of pressure fluctuations predicted by linear PSE are close to that of the first POD modes up to the saturation, the radial shapes of pressure fluctuations are in agreement with the first POD modes reasonably, while deviations of decay rates and amplitudes occur downstream because of nonlinearity. It is rational to suppose that the most energetic coherent structures in present turbulent jets could be modelled as linear wavepackets characterized by solutions of linear PSE, at least at the growth stage. The far-field sound generated by the first POD modes is computed using the Kirchhoff extrapola-tion, which matches well with that of LES at shallow angles. Thus, the wavepackets associated with coherent structures are dominant sound sources in present turbulent jets. While, the sound generated by the linear PSE modes is far lower than that of LES, which indicates that linear wavepacket is insufficient for predicting far-field noise.(3) Large eddy simulation is performed for investigating the swirling effects on the jet flow development and far-field noise, the results show that:the swirling effect enhances the instability of the shear layer, promotes corruption of large-scale vortical structures, weakens the strength of vortex interactions, reduces the correlations in az-imuthal direction and decreases the peak of velocity and density fluctuations in the jet centerline As a result, the far-field sound pressure levels are suppressed apparently due to swirling effect, especially, at the shallow angles. The peak of OASPL is found to be reduced about 3.2dB in swirling jet. The decomposition of far-field sound shows that the far-field sound is dominated by lower order Fourier modes, i.e., m= 0,1,2. At shallow angles, the reduction of axisymmetric mode at low frequencies leads to the sup-pression of OASPL, while the reduction of OASPL at large polar angles is attributed to m= 1,2 modes. The most energetic coherent structures with specified frequencies and azimuthal numbers are extracted from pressure fields by POD in the swirling jet. It is found that these structures would be characterized by linear instability waves predicted by solutions of linear PSE at the growth stage.