flow in the mirror


PhD student (2000-02): Johan Larsson
PhD student (2002-2008): Jonas Ask
Supervisor: Lars Davidson
Co-supervisor: Hans Enwald*
Cooperation: Volvo Cars Corp., ANSYS Fluent Sweden*
Sponsors: Volvo Cars Corp., Vinnova, ANSYS Fluent
Publications: [1-5]
Start of project: October 2000

Aerodynamic noise is the dominating noise source at car speeds above 100 km/h. The major noise sources are noise generated by the flow around A-pillar, mirrors, wheel housings, ventilation channels and fans.
Aeroacoustics (AA) is an attribute that has a large impact on product image and is directly experienced by the customer. Aeroacoustic noise problems are traditionally solved by addition of damping material and increased weight. This is in direct conflict with the goal to reduce the fuel consumption. The present project aims at reducing aeroacoustic noise at the source and thus avoid increased weight.
The work in [1,2] focus mainly on the scalar wave equations pioneered by Lighthill. A thorough review, including the inherent limitations in each method, is conducted. The equation by Curle is identified as the one that meets the objectives of this study the best, and it is implemented and solved for an open cavity. Since no experimental data is available for this case, a Direct Simulation (two-dimensional), resolving both the flow and the acoustics, is performed.

The sound field computed by Curle's equation agrees well with the Direct Simulation, but the computational cost involved is considerably smaller. This makes Curle's equation, together with the simplifications introduced and validated in this thesis, suitable for engineering use. The acoustic noise generation is studied in detail. The wall pressure fluctuations are found to account for roughly 90% of the radiated sound intensity, and the viscous and entropy fluctuations are found to be negligible. The downstream region of the cavity is found to generate the most sound, a fact which can be used to explain the directivity of the radiated sound.

Currently the unsteady fluid flow in the 2D cavity is computed using an incompressible flow. The acoustic noise generation at walls is compared with the 2D Direct Simulation data. The agreement is found to be good. For low Mach number flows where walls are present the dominating source terms in Lighthill-Curle's equation are terms involving the wall pressure fluctuations. Since these terms are mainly affected by hydrodynamic phenomena one can assume that the sound sources can be captured in an incompressible flow field. A modified version of the Lighthill-Curle's analogy is applied to study the near field acoustics of a laminar flow past an open cavity at a Mach number of 0.15 with the length-to-depth ratio of L/D=4. The aim of the work is to study the differences in compressible and incompressible sources to Lighthill-Curle's equation and their influence on the radiated sound.

We are currently working on the flow around a generic mirror. The flow field is computed with DES and we're using STAR-CD. The turbulent instantaneous fluctuations obtained from these simulations are then imported to our acoustic code based on the Lighthill-Curle's analogy. With this code the radiated sound is computed.


  1. J. Larsson
    "Computational Aero Acoustics for Vehicle Applications", thesis of Lic. of Engng, Dept. of Thermo and Fluid Dynamics, Chalmers University of Technology, Göteborg, 2002.
    Download PDF file (41MB)
  2. J. Larsson, L. Davidson, M. Olsson and L.-E. Eriksson
    "Aero Acoustic Investigation of an Open Cavity at Low Mach Number", AIAA paper 2003-3237, 9th AIAA/CEAS Aeroacoustics Conference, Hilton Head, May, 2003.
    View PDF file
  3. J. Ask, L. Davidson, H. Enwald and J. Larsson. An acoustic analogy applied to incompressible flow fields",Computational Aeroacoustics: From Acoustic Sources Modeling to Far-Field Radiated Noise Prediction, Colloquium EUROMECH 449, Chamonix, France, Dec 2003.
    View PDF file
  4. J. Larsson, L. Davidson, M. Olsson and L.-E. Eriksson
    "Aero Acoustic Investigation of an Open Cavity at Low Mach Number", AIAA J. Vol. 42. No. 12, pp. 2462-2473, 2004.
  5. J. Ask and L. Davidson, "An acoustic analogy applied to incompressible flow fields", Comptes Rendus Mecanique, Vol. 333, Number 9, pp. 657-732, 2005.
  6. J. Ask and L. Davidson
    "An Investigation of Outlet Boundary Conditions for Incompressible Near Field Acoustics", 11th AIAA/CEAS Aeroacoustics Conference, AIAA 2005-2992, May 23-25, Monterey, California, 2005.
    View PDF file
  7. J. Ask
    "A Study of Incompressible Flow Fields for Computational Aero Acoustics", thesis of Lic. of Engng, Dept. of Thermo and Fluid Dynamics, Chalmers University of Technology, Göteborg, 2005.
    View PDF file of thesis
  8. J. Ask and L. Davidson
    "The Near Field Acoustics of a Generic Side Mirror based on an Incompressible Approach", Division of Fluid Dynamics, Dept. of Applied Mechanics, Dynamics, Chalmers University of Technology", Göteborg, Sweden, Report 05/05, 2005
    View PDF file
  9. J. Ask and L. Davidson
    "The Sub-Critical Flow pas a Generic Side Mirror and its Impact on Sound Generation and Propagation", 12th AIAA/CEAS Aeroacoustic Conference, Cambridge, Massachusetts, 2006.
    View PDF file
  10. J. Ask and L. Davidson
    "Sound Generation and Radiation of an Open Two-Dimensional Cavity", AIAA Journal, vol.47, no.6 pp. 1337-1349, 2009.
  11. J. Ask and L. Davidson
    "A Numerical Investigation of the Flow Past a Generic Mirror and its Impact on Sound Generation", Journal of Fluids Engineering, Vol.131, number 061102, 2009.
  12. J. Ask, L. Davidsonn
    Flow and Dipole source evaluation of a generic SUV,J. Fluids Eng., Vol. 132, No. 051111, 2010.

This page, Computational Aero-Acoustics for Vehicle applications, should be part of a frames system at www.tfd.chalmers.se/~lada/projects/proind.html
Ingalena Ljungström