Aero 2-2_Paper 1

A review of recent advances in the use of surface integral methods in Computational AeroAcoustics (CAA) for the extension of near-field CFD results to the acoustic far-field is given. These integral formulations (i.e. Kirchhoff’s method, permeable (porous) surface FfowcsWilliams Hawkings (FW-H) equation) allow the radiating sound to be evaluated based on quantities on an arbitrary control surface if the wave equation is assumed outside. Thus only surface integrals are needed for the calculation of the far-field sound, instead of the volume integrals required by the traditional acoustic analogy method (i.e. Lighthill, rigid body FW-H equation). A numerical CFD method is used for the evaluation of the flow-field solution in the near field and thus on the control surface. Diffusion and dispersion errors associated with wave propagation in the far-field are avoided. The surface integrals and the first derivatives needed can be easily evaluated from the near-field CFD data. Both methods can be extended in order to include refraction effects outside the control surface. The methods have been applied to helicopter noise, jet noise, propeller noise, ducted fan noise, etc. A simple set of portable Kirchhoff/FW-H subroutines can be developed to calculate the far-field noise from inputs supplied by any aerodynamic near/mid-field CFD code. 1 BACKGROUND—AEROACOUSTIC METHODS For an airplane or a helicopter, aerodynamic noise generated from fluids is usually very important. There are many kinds of aerodynamic noise including turbine jet noise, impulsive noise due to unsteady flow around wings and rotors, broadband noise due to inflow turbulence and boundary layer separated flow, etc. (e.g. Lighthill1). Accurate prediction of noise mechanisms is essential in order to be able to control or modify aeroacoustics volume 2 · number 2 · 2003 – pages 95 – 128 95 *Presented at the CEAS Workshop “From CFD to CAA” Athens, Greece, Nov. 2002. †Professor, e-mail: [email protected]. Aero 2-2_Paper 1 28/10/03 5:37 PM Page 95