Computational Method Improvement And Influential Assessment About Sound Propagation Problems In A Shear Layer

Shear layer is a fundamental fluid structure that has been proved to change the propagation characteristics of aerodynamic noise,produce the acoustic phenomena of refraction,reflection and scattering,as well as affect the measurement and separation of the sound source.In this paper,some computational methods are improved and the effects of a subsonic shear layer on sound propagation are assessed.The main contents include:(I)Progresses of computational method and influential assessment of sound propagation in a sheared flow are analyzed and summarized.Some existed problems are pointed out and the structure of this paper is briefly listed.(2)To eliminate the instable vortical wave induced in numerical simulations of sound propagation through a shear layer using linearized Euler equation,a gradient term suppression and filtering(GTSF)method is proposed,which has shown to effectively filter the numerical instability and concurrently provide an accurate far-field prediction.Results show that the computational error of GTSF method is about 10%smaller than traditional one.(3)As the 2D and 3D FW-H equations are separately solved with different types of Green function,a generalized framework is established for the solution of 1D to 3D FW-H equation.The equivalent source method is introduced to accelerate the generalized solution of FW-H equation,which is 20 times faster than traditional configuration when there are many receivers.This generalized framework is coupled with GTSF method to achieve a co-simulation of sound propagation from near-field to far-field.In addition,a generalized solution method of wall-induced sound scattering is extended.(4)To deal with the limitation of traditional methods on shear layer correction,a unified correction method for the acoustic refraction(UCMAR)caused by a threedimensional shear layer is proposed.The accuracy of UCMAR method for 1D to 3D shear layer is well validated and a methodology of inverse interpolation is introduced to accelerate the solution by 200 times.Finally,the UCMAR method is mathematically proved to include some traditional correction methods.(5)The influences of shear layer shape,spreading angle,intensity,and temperature gradient on sound propagation and source localization are studied.It is found that the refraction induced by different shear layer parameters mainly affects the acoustic phase rather than amplitude.The phase variation induced by shear layer refraction is further shown to determine the accuracy of source localization where the localization error is found to be linear with the product of Strouhal number and jet Mach number.(6)The influences of typical vortical flow such as single vortex,time-evolving shear layer,and rotor down-washing flow on sound propagation are studied.Results show that the influence of vortices on sound propagation mainly depends on the ratio of vortex size to acoustic wave wavelength,vortex intensity and motion status.When the vortex size is larger than the acoustic wavelength and the vortex intensity is strong,the vortex scattering is apparent.The vortex motion will change the sound propagation direction and weaken the directivity.In this paper,the computational method of acoustic propagation in shear flow is improved,whereas the GTSF method,generalized solution method of FW-H equation,and UCMAR method are proposed,and the effect of typical shear flow on acoustic wave propagation is studied and summarized.These studies have strongly promoted the development of numerical method for sound propagation in complex flow and the investigation of sound propagation mechanism,which accumulate technology to solve the aerodynamic noise problem in major national projects.