Numerical Integral Prediction Methods Of Aerodynamic Noise For Non-compact Bodies

Aeroacoustic noise induced by noncompact bodies are extensively existed in several areas of aerospace and aviation, industrial production and transportation. Especially for aircrafts with large configuration, the impact of aerodynamic noise is serious during the landing and take-off stage or flight stage. Since Hybrid Computational Aeroacoustics(HCAA) method has low computational complexity, high computational efficiency and low cost, it is widely used to study the aeroacoustic noise in the fields of practical engineering applications. The flow and the noise fields are separately solved with CFD method in the fluid flow field and the wave equation in the acoustic field. Lighthill’s acoustic analogy has some advantages, such as simplicity, strong applicability and high efficiency.Based on Ligthill’s hypothesis, the computational aeroacoustic methods of noncompact bodies are presented, and these method could simulate the scattering effect caused by wave/surface interaction. The research work and innovations mainly include the following points:(1) In combination with Theory of Vortex Sound and pressure decomposition method, an integral equation of acoustic pressure is proposed to calculate the aeroacoustic noise induced by low Mach number flow. The acoustic calculation includes the compution of scattered pressure on surfaces and that of acoustic pressure in far field. The aeroacoustic noise produced by low Mach number flow around cylinders are predicted, and the distribution of acoustic field is in agreement with Curle’s method. The characteristicses of noise field are analyzed in detail, and the variation of aeroacoustic noise with frequency is coincident with that of turbulent fluctuations. Numerical studies indicate that the present method could calculate acoustic noise induced by low Mach number flow accurately, and describe the relationship between flow and noise directly.(2) A unified aeroacoustic computational method of noise radiation and scattering is presented. The fluctuations in the flow field are divided into the hydrodynamic component mainly induced by fluid flow and the acoustic component mainly induced by wave propagation. The unified integral equation is deduced with Lighthill’s acoustic analogy and wave equation for Green’s function. The compressible flow calculation is performed with two-order method, and the scattered pressure on boundary is firstly computed, and then the far field pressure of arbitrary observer is solved. Verification work is developed with the numerical calculation of two-dimensional and three-dimensional circular cylinder. The noise distributions obtained with the present method agree well with references. Numerical studies demonstrate that the present method could overcome the huge workload induced by high-order methods, and accurately simulate the scattering effect of noncompact bodies and the characters of aeroacoustic noise induced by compressible flow.(3) In order to slove the aeroacoustic noise induced by complex configurations, physical models composed with multiple bodies or oscillating structures, the computational aeroacoustic integral method of nomcompact permeable boundarides is presented. The smooth boundary surrounding all the bodies is chosen as the permeable boundary, aeroacoustic noise calculation includes the calculation of scattered pressure on permeable boundary and that of far field pressure. Numerical studies are carried out for circular cylinder, Rod-Airfoil configuration and 30P30 N airfoil, and the acoustic noise of permeable boundary agrees well with that of body surface. It is demonstrated that the permeable boundary could simulate the the scattering effect of noncompact bodies and reduces the computational complexity, and it is also helpfulto improve the computational efficiency.(4) During the landing and take-off stage or low altitude flight for airfoils, the huge workload produced by calclation for scattering sources is urgently to slove. A half-space aeroacoustic computational integral method of noncompact bodies is established with the help of image source method. The flow field and the scattered acoustic field are respectively solved with two-order DES turbulent model and noncompact boundary integral method. Firstly, the acoustic noise of circular cylinder, NACA0012 airfoil and 30P30 N airfoil in half space are studied, and the scattering effect of the ground causes the severe variations. Then, the subsonic cavity noise is studied, and the numerical results are in good agreement with high-order CAA method. The sound pressure level reduces with frequency, and the acoustic field varies the same as fluid fluctuations. Numerical studies demonstrate that the present method could simulate the scattering effect produced by the nonconpact bodies and the ground. Meanwhile, the present method avoid the acoustic calculation of scattering sources on the ground, and the computation efficiency is greatly improved.(5)With the help of free-space wave equation, an integral solution of time-domain noncompact Green’s function is established, and the far-field acoustic pressure could be obtained with the integral solution and Lighthill’s acoustic analogy. The aeroacoustic noise of stationary and moving point sources located nearby the cylinder is studied, and numerical results show that the present studies are in good agreement with analytical solution. The time-domain aeroacoustic noise of low Mach number flow around circular cylinder is predicted, and the numerical results agrees well with the that of frequency-domain method and experimental datas. The far-field aeroacoustic noise varies periodically with time, and the distribution of noise varies with the fluid fluctions. Numerical results demonstrate that the present method could take into account the transient noise variations, and simulate the scattering effect produced by noncompact bodies.