The Computations And Investigations Of Compressible Shear Flow Noise

Sound generated aerodynamically has gradually become a serious and nonnegligible problem in the realm of Aeronautics and Astronautics, it not only causes environmental pollution, but also leads to damage of vehicle itself, consequently, it attracts more and more concerns from scientific researchers and engineers. Aiming at two different types of typical noise problems existed in an airplane, the work focuses on improving existed noise computation schemes and studying production mechanisms of shear flow noise numerically, including following aspects in detail:(1) The DRP scheme is usually used in conjunction with specified filters, therefore, we reselect the integral interval and obtain new coefficients of7-point DRP scheme which can improve resolution at the interval of effective wavenumber space, considering the band-width of10-th order central filter. Borrowing the method of hybridization, a new variant of hybrid scheme DRP-WENO is constructed based on two kinds of sub-scheme:DRP and WENO, and the flags are determined by Multi-Resolution algorithm. The DRP-WENO gains higher computational efficiency than pure WENO scheme, bet-ter resolution than the hybrid scheme based on the WENO-JS scheme, and it can be applied in noise problems containing shocks, since the dispersion relation in smooth re-gion is well preserved. Numerical experiments demonstrate that DRP-WENO achieves a good robustness in the flows of weak/strong shocks and vortices.(2) Direct numerical simulation is performed for2D open cavities of L/D=2,4with different inflow boundary layer thickness and Mach number at a low Reynolds number, to investigate the near-field dynamics and far-field noise. The results show:there are two types of oscillation modes for a2D cavity, namely shear mode and wake mode, and the ratio of L/D and boundary layer thickness are two important parameters for the mode switching. In the shear mode, the dominating oscillating frequency is occupied by Rossiter Ⅱ mode. As the decrease of boundary layer thickness, a low frequency component near Rossiter I mode appears, the noise is intensified and the pattern of wavefronts is also varied. According to the analyses of low-dimensional model and routes of vortex motion, the low frequency component is produced by the switching of vortex-edge impingement. As the increase of Mach number, the Strouhal number is reduced, however, the frequency doesn’t drift much at the same Mach number; In the wake mode, large vortices keep shedding from the cavity leading edge, the oscillation frequency is around St=0.24and shows little variation with Mach number.(3) With a given laminar velocity profile, the large eddy simulation is utilized to study the near-field dynamics and far-field noise of free jets with a Mach number0.9. Both the Eulerian and Lagrangian method are used to indicate the large coherent struc-tures, several kinds of typical coherent structures are found in the flow field of the free jet, and furthermore we find that the Lagrangian coherent structures detached from the shear layer will intrude into the jet core region near the end of the potential core, shown by the evolution of the Finite Time Lyapunov Exponents(FTLE). In addition, as illustrated by the motion of the physical sound sources obtained by space-time con-volution filtering, such a kind of intrusion will intensify the sound source near the end of the potential core, and then lead to strong sound emission from this region; The time correlation between the sound source signals and the far-field pressure shows:the noise at aft angles are produced in a region of a width of3～4radii behind the end of the potential core, which is related with the intermittent intrusion. The core region contributes more than the shear layer region to this part of noise. The correlation between the sideline noise and sound source in the region is much weaker than that of the shear layer, which means that sideline noise is mainly produced by small-scale turbulent fluctuations.(4) By solving the compressible boundary layer equations, the velocity profiles of swirling jets with different swirl intensity are obtained. The stability characteristics are firstly analyzed through linear stability theory, and then large eddy simulation is performed to study the changes of near-field dynamics and far-field noise caused by swirling effect at a moderate Reynolds number, compared with the free jet. Due to the higher amplification rate, the flow entrainment and mixing of the swirling jet is much stronger in the circular shear layer at the stage of the potential core, faster corruption of large structures is achieved, the turbulent fluctuations in the shear layer and at the centerline saturate earlier, and the peak of the latter turbulent intensity is also decreased. The sound pressure level at aft angles can be suppressed in swirling jets, the OASPL at the aft angle of30°is reduced up to3dB in the strong swirling case, and the vortex-pairing noise is also reduced due to faster corruption of large structures; The statistical analyses of sound sources have shown that the distribution of overall sound source is moved forward, and the convection velocity at the initial stage has also been changed because of swirl. Moreover, the peak location of the intermittency factor at the centerline appears earlier, and the intermittency in a region of5～10radii to the nozzle is suppressed in the swirling cases.