Validating LES for Jet Aeroacoustics

Abstract

I. AbstractEngineers charged with making jet aircraft quieter have long dreamed of being able to see exactly how turbulent eddies produce sound-and this dream is now coming true with the advent of large eddy simulation (LES). Two obvious challenges remain: validating the LES codes at the resolution required to see the fluid-acoustic coupling, and the interpretation of the massive datasets that result in having dreams come true. This paper primarily addresses the former, the use of advanced experimental techniques such as particle image velocimetry (PIV) and Raman and Rayleigh scattering, to validate the computer codes and procedures used to create LES solutions. It also addresses the latter problem in discussing what are relevant measures critical for aeroacoustics that should be used in validating LES codes. These new diagnostic techniques deliver measurements and flow statistics of increasing sophistication and capability, but what of their accuracy? And what are the measures to be used in validation? This paper argues that the issue of accuracy be addressed by cross-facility and cross-disciplinary examination of modern datasets along with increased reporting of internal quality checks in PIV analysis. Further, it is argued that the appropriate validation metrics for aeroacoustic applications are increasingly complicated statistics that have been shown in aeroacoustic theory to be critical to flowgenerated sound. II. IntroductionA. The Problem Typical aeroacoustic problems are bedeviled at several levels. First, the fluid flow generating the acoustic field is turbulent, and the description of this flow has a high degree of freedom. That is to say in classical engineering terms that the flow and its associated sound is described by a large number of modes over many scales. A subset of aeroacoustic problems, typically involving resonance, can be described more simply, but from an engineering perspective if the flow is simple then the method of manipulating the flow to enhance or remove the sound is also simple. Second, the sound is the result of slight phase imperfections from the compressibility of the flow. Indeed, if the flow were truly incompressible there would be no noise as all dilatations would cancel out. Third, and most significant for engineers attempting to predict the acoustic field, the sound is such a small part of the solution that it is easily lost to approximations used in analysis. Thus, the field of aeroacoustics is still a research topic instead of being an engineering science.Of the approaches used on aeroacoustics all run into the problems mentioned above. Empirical approaches are largely successful for cases where the turbulence has simple scaling with the geometry such as simple jets and turbulent boundary layer noise. However, make a change to the geometry that changes the turbulence beyond the simple description underlying the noise model and the empirical noise model falls apart. Think modifications to nozzle geometries or fuselage-wing interfaces. Most theoretical approaches have pro...