Computational Aero-Acoustic

This section presents a resumed state-of-the-art of simulation in aero-acoustic domain. ¹ Aero-Acoustics is the engineering field dealing with noise generated generally (but not necessarily) by a turbulent fluid flow interacting with a vibrating structure. This field differs from the pure acoustic domain where the object is the propagation of acoustic pressure waves, including reflections, diffractions and absorptions, in a medium at rest. Aero-Acoustic questions arise in many industrial design problems and are heavily represented in the noise nuisances related to the transportation industry.

Most of the Aero-Acoustic R&D works are performed experimentally but this method has some critical pitfalls. Although it is relatively simple to setup a microphone, measure a noise level and derive a spectrum at any given location in space, the correct analysis of an Aero Acoustic problem involves the use of advanced experimental techniques and is complex to use. The Aero Acoustic engineering community seeks more and more the help of CAE tools as they become available. Those tools complement the experimentations and allow a thorough visualization and understanding of the pressure and velocity fields as well as the structural vibrations. Furthermore, parametric studies can be carried out with little added cost since a numerical model modification is often straightforward and the CPU time is becoming cheaper and cheaper.

CFD codes are available since over several years, able to predict with a reasonable precision steady state flows (drag and lift) and slow transient flows like heating and defrosting. Highly transient flows involved in the Aero-Acoustic phenomena have not been treated since they were not in the bulk of the needs and they required way too much CPU to be industrially feasible. Acoustic Propagation numerical tools have also been industrially available since quite a few years. These tools operate in the frequency domain and are able to propagate a given boundary condition signal in a fluid at rest, including the noise reflections, diffractions, transmissions and attenuations thanks to the various geometrical obstacles and different materials.

Attempts have been made to combine existing CFD and Acoustic propagation tools to predict Aero-Acoustic problems. Most methodologies are based on the Lighthill and Curle method, developed in the mid 1950's and Ffowcs Williams and Hawkings contributions made in the late 1960's. ^{2 3 4 5} The ideas underlying these methods are to decouple the flow pressure field and the acoustic pressure field. The fluid flow can then be computed by a standard CFD code and the noise derived from the curvature and turbulent intensities of the flow. A propagation tool is then used to compute the noise on a sub grid of the CFD computational domain loosing therefore quite some local information and high frequency content. First attempts were made with incompressible steady state CFD simulations and were not able to deliver valuable result in many cases. A good example of these limitations is highlighted by the study of the noise generated by a simple side mirror shape written by R. Siegert. ⁶ Recent developments of this family of techniques require the use of transient simulations and filtering to avoid loosing to much information on the coarser acoustic mesh. Reasonable success has been met in specific areas involving low frequencies (up to a couple hundred Hz) and considerable CPU time is needed.

These ingredients have been implemented in a single numerical code. The outcome is Radioss solver which is different from the existing CFD codes in its capabilities and particularly well suited to short time transient analysis.