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.

A classification of Aero-Acoustic problems can be made using the following categories:
External wind noise transmitted to the inside through a structure
In the automotive industry, a pillar, side mirror and windshield wipers noise are typical problems of this category.
Internal flow noise transmitted to the outside through a structure
Examples of this class of problems are exhaust, HVAC and Intakes noises.
Rotating machine noise
Axial and centrifugal fans are noisy components that bring with them many interesting Aero-Acoustic problems.

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.

An alternative methodology is to incorporate in a single numerical tool, right from the beginning, the ingredients that are necessary to perform direct Aero-Acoustic numerical simulation. They are:
Compressible Navier Stokes
To be able to propagate pressure waves and therefore take into account in a single simulation the flow and the noise, including all possible cavity modes.
Fluid structure coupling
To be able to treat the problems involving a turbulent flow on one side of the structure and the noise radiation on the other side.
Small time step
To be able to deal accurately with frequencies going up to several thousand Hertz.
Transient turbulence modeling
Unlike the Reynolds Averaged Navier Stokes (RANS) methods that makes the assumption that the flow is a combination of a steady state and turbulent fluctuations. Aero-Acoustic noise is directly linked to the small scale turbulence fluctuations and strongly time dependant.
Acoustic boundaries with prescribed impedance
This is a critical point of a good Aero-Acoustic simulation. Boundaries need to be able to perform tasks such as giving a free field impedance to an inlet with fixed velocity, prescribing a specific impedance at the outlet of a duct to make sure long wavelength stay trapped inside, treat exterior air impedance effect on a vibrating structure and be used to model absorbing materials (carpet, foams,and so on) that are used to coat many components.

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.

1 Nicolopoulos D., Périé F., and Jacques A., 「Direct numerical simulation of Aero-Acoustic phenomena」, M-CUBE, Internal Report, February 2004.
2 Lighthill M.J., 「On Sound Generated Aerodynamically, Part I: General Theory」, Proc. Roy.Soc., A211, 564-587, 1952.
3 Lighthill M.J., 「On Sound Generated Aerodynamically, Part II: Turbulence as a source of sound」, Proc. Roy.Soc., A222, 1-32, 1954.
4 Curle N., 「The influence of solid boundaries upon aerodynamic sound」, Proc.Roy.Soc.Lond., A23, 505-514, 1955.
5 Ffowcs Williams, J.E. and Hawkings D.L., 「Sound Generation by Turbulence and Surfaces in Arbitrary Motion」, Phil.Trans.Roy.Soc., A, Vol. 264, No. 1151, pp. 321-344, 1969.
6 Siegert R., 「Numerical simulation of aero-acoustic sound generated by a simplified side mirror model.」, SIA, 1999.