The Improved Research Of Viscous/Acoustic Splitting Method By Analyzing Source Effects

In this paper,the origin and effect of each term in the equations of the viscous/acoustic splitting method(VASM)are studied and to suppress the acoustic instability of VASM,an improved viscous/acoustic splitting method(IVASM-c)is proposed.The ?ASM-c is used to predict the typical free shear and wall shear flow induced noise in both laminar flows and turbulent flows.A high-efficiency parallel computation program of ?ASM-c is developed using different grids between the calculation of acoustic field and flow field.With its rigorous mathematical derivation,VASM could be a modified version of Lighthill's acoustic analogy method equation(referred to here as MLAAM)without the assumption of far-field pressure.Comparing VASM with LAAM,two new acoustic sources are considered,namely(?)the monopole source due to the time derivative of the incompressible pressure and(?)the quadrupole source due to the negative incompressible pressure and viscous stress.The pressure source in the acoustic continuity equation of VASM is the monopole source of MLAAM.The acoustic convection term and velocity source(S2)in the acoustic momentum equation of VASM correspond to the quadrupole source of MLAAM due to the increment in momentum flux from the compressible/incompressible flow.The acoustic diffusion term and viscous source in the acoustic momentum equation of VASM correspond to the quadrupole source of MLAAM due to the increment in viscous stress from the compressible/incompressible flow.The pressure source S1 and velocity source S2 of VASM are both the acoustic sources with physical meaning and should not be simply ignored.A source decomposition method is proposed for studying the sound wave due to each source in VASM.The results show that the pressure source(S1)and sub-velocity source(S2.1)caused by the interaction between the incompressible velocity and the divergence of the fluctuating velocity are both important acoustic sources,and the acoustic instability of VASM is mainly caused by the unstable pseudo-sound source term(S2.2)generated by the fluctuating velocity and the flow velocity gradient in direction of the fluctuating velocity.The acoustic diffusion term could decrease the instable pressure caused by S2.2 but some of the acoustic viscous sources could increase the acoustic instability generated by S2.2.We propose an improved viscous/acoustic splitting method,?ASMI(m),and suppress the pseudo-sound waves excited by the additional unstable source generated by the waves spreading in a non-uniform velocity field.With the use of ?ASMI(m),the main radiating monopole and quadrupole source in MLAAM(VASM)is retained to the utmost extent.?ASMI(m)could successfully predict the flow-induced noise of single circular cylinder and temporal mixing layer in laminar flows.The validity of IVASMI(m)is tested for the noise of turbulent flows with different turbulence models.With the use of ?ASMI(m),the noise induced by wall shear flow and free shear flow could be successfully obtained with unsteady Reynolds-Averaged Navier-Stokes(URANS)turbulence models and filtered Navier-Stokes equations(FNSE)method(eg.LES method),respectively.The complex vortex flow field near to the wall caused by FNSE could induce the vestigial instable pressure of ?ASMI(m).We propose a new Improved VAS method(IVASM-c)with velocity source control function to limit the fluctuating pressure in the field with high vorticity.With the use of velocity source control function,the noise of turbulent wall shear flow with FNSE could be successfully predicted by ?ASM-c.The multiple grid method which the acoustic grid and fluid grid are not same in the calculation are introduced in ?ASM-c to optimize the acoustic results and speed up the acoustic calculation.We study the influence of acoustic grid and interpolation function to the acoustic result of ?ASM-c using multiple grid method.The matching principle of acoustic grid is built that the interpolation function should make the reconstructed flow field continuous and the acoustic grid should ensure there are at least 16 grid point in one acoustic wavelength.Following the matching principle,the adverse effects of fluid grid which contains unevenly spaced grid,distorted grid and large-scale farfield grid to the acoustic calculation could be avoid and the computing efficiency of the acoustic calculation is improved.We develop a high-efficiency calculation program of flow-induced noise with IVASM-c.The calculation program adopts two sets of grid technologies and parallels with domain decomposition method using MPI to exchange messages between child processes.The calculation program also optimizes the processing program of acoustic convective term and uses the flow field results of Fluent to predict the acoustic field.The high-efficiency ?ASM-c calculation program is proved valid for the prediction of flow noise caused by various wall shear flow or free shear flow in this dissertation.