By Sandeep Sovani, Fluent Inc. and Bipin Lokhande, Fluent India

View the pdf of this article

The mesh in the vicinity of the noise-producing object has a strong impact on the quality of sound that can be simulated using CFD

From a fluid dynamics point of view, an automobile's side view mirror (SVM) is a bluff body exposed to a high speed flow. The flow structure in the wake of an SVM is highly transient and subjects the vehicle surfaces in its vicinity, such as doors and windows, to significant unsteady pressure fluctuations. This unsteady pressure variation ultimately propagates inside and around the vehicle as noise. Sound generated in and propagated through a fluid domain can be simulated using two methods:

• Computational aeroacoustics (CAA), defined as a direct simulation of acoustic fields generated by flow, and the interaction of acoustic fields with flow. 'Direct' implies that computation is only based on fundamental physical principles without reliance on empirical results.
• Aeroacoustics models for propagation of sound from the source to the receiver.

The CAA and aeroacoustics model are both in good agreement with experiment for a receiving point not far from the mirror²

FLUENT can conduct aeroacoustic simulations using both of these approaches. CAA is handled by FLUENT through its well-established and extensively tested transient flow capability. In addition, two aeroacoustic models have been implemented and tested. FLUENT 6.0's Lighthill-Curle acoustic module is capable of propagating sound generated by pressure fluctuations on wall boundaries to far-field observation points. FLUENT 6.1 has a built-in acoustic module based on the Ffowcs-Williams-Hawkins theory that can calculate sound radiated by boundary and interior surfaces towards observation points inside or outside the computational domain. In addition, FLUENT 6.1 results can now be imported to SYSNOISE, an acoustics modeling tool from LMS International.

For the SVM, the sound generated by the turbulent flow field in the wake of the mirror has been simulated using CAA and the Ffowcs-Williams-Hawkins formulation in FLUENT. The generic mirror shape consists of a half cylinder topped with a quarter sphere of the same diameter.

The CAA approach is executed by conducting a transient simulation of the flow around the mirror with the LES turbulence model. Monitor points are put at locations where microphones were placed in experiments reported in the literature1 and the transient static pressure signal is recorded at these points. The Fast Fourier Transform (FFT) tool newly introduced in FLUENT 6.1 is used to convert the transient pressure signals into frequency spectra.

The analysis based on the Ffowcs-Williams-Hawkins model starts with a transient simulation of the flow field around the mirror. At the beginning of the simulation, source surfaces for the sound and receiver (microphone) locations are input. For the SVM, the mirror body and flat plate on which it is mounted are selected as source surfaces. During the calculation, FLUENT creates plots or files of sound pressure vs. time.

Sound pressure spectra show that both methods are in good agreement, qualitatively and quantitatively, with experiment. The accuracy of aeroacoustic simulations is heavily dependent on that of the underlying transient flow simulation. Time-step, grid resolution, and grid quality not only determine the accuracy of the predicted sound pressure level, but also the frequency band over which the simulation results are meaningful.

Reference

1 Siegert R., Schwarz V., and Reichenberger J., AIAA Paper no. 99-1895, 5th AIAA/CEAS Aeroacoustics Conference, Seattle WA, May 10-12, 1999.

2 Lokhande B.S., Sovani S.D., and Xu J., "Computational Aeroacoustic Analysis of a Generic Side View Mirror," Paper no. 2003-01-1698, SAE Noise and Vibration Conference, Traverse City, MI,

Contours of velocity illustrate the transient nature of the flow around the mirror

FluentNEWS