The Investigation On The Rotor/Stator Interaction Noise With Subsonic Speeds

Rotor wake-stator interaction is an important source of turbomachinery noise.Theprincipal component of discrete tone noise,stems from the interactions caused by therelative motion of rotor blades and stator vanes.Rotor/stator interaction is undoubtedly theoldest and most frequently treated subject in turbofan noise research.In this paper,anefficient numerical model is developed for solving the interaction of high frequency,unsteady,three-dimensional incident disturbances with an annular cascade.The numericalscheme is very efficient by splitting the velocity field into nearly-acoustic andnearly-connected vortical components.A code(RSINoise)based on high order numericalschemes and accurate boundary conditions was developed for the investigation of subsonicrotor/stator interaction noise.With high order compact scheme,a direct numerical solver has been developed for theinvestigation of the mechanism of the generation of aerodynamic noise.Both compressibleand incompressible flow can be simulated with this solver.The code is implementedcompacted with artificial pseudo-compressibility method.To compare the numericalaccuracy HOC and low order scheme,the well known yon Karman vortex shedding from acircular cylinder and the vortex paring induced sound in a planar jet were simulated.Theresults showed that the HOC method is superior to the low order scheme when consideredthe small disturbance in turbulent flows.It is well known that boundary conditionimplementation,unresolved scales,mesh non-uniformities and nonlinearity of thegoverning equations will induce numerical instabilities.Filtering method is applied so as tosuppress unwanted high wave number numerical oscillation.A correct formulation of the boundary conditions is very important in computationalaero acoustics(CAA).Small non-physical fluctuations introduced at boundaries will havealmost no influence on the velocity field but can have a very large influence on the acousticpressure fluctuations which are many orders of magnitude smaller than the pressurefluctuations generated by the flow in low Mach number flows.The state art of twonon-reflection boundary conditions(NSBC)named sponge layers and perfectly matchedlayer were implemented respectively to truncate the computational domain.Noisegenerated in plannar jet and benchmark aeroacoustic problems were calculated to test theaccuracy of two kinds of NSBCs.Two dimensional gust-cascade interaction were investigated with methods that combined linearied Euler equation and mode matching techniques.Since RSINoise cannotbe easily modified to run in the two-dimensional mode,the cascade geometry was"wrapped"around an annulus with a mean radius to get a large hub-to-tip radius ratio.Inthis paper,we examined how different radial mean swirl distributions affect the cascadescattering response and compared with previous results.The mode analysis technique wasalso implemented to investigate the cutoff ratio in the cross flow fan(CFF).The geometrymodel of the CFF is so complicated that it is difficult to implement the mode analysismethod.An unique simplied method for CFF was put forward to calculate the cut-off ratioin the flow field and the related results was readily discussed.From the eigenmode spectrum,one can identify the sonic modes,convected modes andattenuated modes easily.The normal modes in an annular duct were calculated to instigatethe coupling between the pressure and vorticity.The results indicated that there was littlepressure in the convected mode and vice versa in uniform flow.The coupling of the sonicmode and convected mode only occurred on the wall.However,in non-uniform flow,theswirl produces centrifugal and Coriolis forces which coupled the pressure and vorticaldisturbances.Another benchmark problem was tested and the results show the RSINoisecode is efficiently,which provided an effective means to investigate the aerodynamic andacoustic response of the annular cascade to a general gust.In different non-uniform flow,the response of an unloaded annular cascade to incidentacoustic and vortical disturbances took on different shapes.The results indicated that theswirling flow changes the number of acoustic modes in the duct and causes significantamplitude and radial phase variations of the incident disturbance.Further more,a case ofincompressible flow was considered to study how rotor-stator interaction governed thefluctuating pressure on the duct wall.The broadband interaction noise was calculated based on the spectral representation ofthe impinging upstream homogeneous turbulence.Since the homogeneous turbulencemodel is so limited that it cannot represent the essence of turbulence in engineering.Thenthe proper orthogonal decomposition(POD)method was introduced to extract theinhomogeneous turbulent spectrum for the calculation of broadband noise.However,theessence of the turbulence has not been known,so a new more accurate broadband noisemodel should be developed.In present,the investigation of the broadband noise is mainlybased on experiment.In the last section of this paper,the broadband noise source in a crossflow fan was investigated by examining the turbulent boundary pressure with microphone.This paper is supported by the National Natural Science Foundation of China (No: 50176012)and the Doctorate Foundation of the Ministry of Education(No:20020487025)...