1. University of Greenwich Computing and Mathematical Sciences
Experience of using a CFD code for estimating the noise generated by gusts along the sunroof of a car
by Liang Lai
Supervisors: Professor C- H Lai,
Dr. G S Djambazov,
Professor K A Pericleous
Sponsored by University of Greenwich

2. Solution strategies for Computational Aeroacoustics
Introduction
Solution strategies for Computational Aeroacoustics
Decreasing cost
source +
Propagation
DNS/LES/RANS
(Near field + far field)
Direct Numerical Simulation (DNS)
Large Eddy Simulation (LES or DES)
Reynolds-Averaged Navier-Stokes (RANS)
¤ High-order schemes in space and time
¤ Different length scales and time scales for aeroacoustic simulation in turbulent flows
¤ Computational cost is very high even for using RANS with high-order schemes !

3. Introduction Coupling Methods ?
The unsteady near-field is solved directly by LES or unsteady RANS, but acoustic solutions obtained by solving a set of simpler equations (e.g. wave equation,
Euler equations, and other perturbation equations). +
LES/RANS
Simpler equations *
(near field)
(near field + far field)
source
Propagation
non-linearised Euler equations
Decomposition + source-retrieval techniques
Other methods, e.g. Helmholtz equation?
Simpler equations *

4. Automobile Application Examples
CAA in the automotive industry

5. The Car Sunroof Problem
Buffeting noise is due to shear-layer instability in the opening of the cavity subjected to tangential flow.
Shear-layer vortices are produced and are convected downstream of the opening, eventually hitting the rear edge.
When the vortex breaks, a pressure wave is produced which enters into the cavity.
At a certain speed, the vortex shedding frequency in the shear layer will match an acoustic mode of the cavity leading to resonance is in the form of a standing wave.
Resonance is in the form of a Helmholtz mode

6. Problem setup and external excitation
A sinusoidal disturbance
1.2m
0.4m
1.1m
0.8m
0.03m
0.9m
0.5m
Car as a Helmholtz Resonator
Time step: Wave amplitude: Wave time per cycle:
dt = s P0= -0.1 kg/s ta= 10 dt

7. Car as a Helmholtz Resonator
Basic procedure with PHOENICS
The pressure fluctuation along the open sun-roof can be calculated,
where is the pressure distribution obtained by using the CFD
calculation and is the background pressure distribution due to the
upstream velocity.

8. Car as a Helmholtz Resonator
Analyse Acoustic Response by using FFT
Comparison to a Helmholtz Resonator
therefore, resonant frequency ( = 1.45) f = 6.32Hz

9. Alternative Problem Description
A hypothetical car with an open sun-roof
*The vortex strength W = A0 sin(ωt), where A0 = 1.2 m/s
t = 10-3 s , wave time per cycle ta = 20 t, f = 50 Hz
25m/s
1.28m
0.52m
1.35m
0.35m
0.05m
1.2m
0.6m
0.85m
0.3m
###Mesh

10. Use of nested sub-grids

11. 9 observation points within computational domain
Geometry and Observation points
7 points along sun-roof
9 observation points within computational domain

12. Differencing Schemes Affect Disturbance Decay
| | | |
| W w P | E |
|  | |

13. Effect of Differencing Scheme: (a) Hybrid
Source Input (fs=50Hz)

14. Effect of Differencing Scheme: (b) QUICK
Source Input (fs=50Hz)

15. Pressure time history at the sunroof LE
By QUICK scheme.
6.7m

16. FFT of Pressure Fluctuation – Resonance at 13Hz
---- Analyse Acoustic Response
f = 13Hz

17. Helmholtz Equation, the FT of the Wave Equation
Homogeneous Wave equation
Integrate with respect to time --- taking Fourier transform of the wave equation
finally, one gets
Apply inside car cavity – neglecting convective effects

18. Acoustic Pressure
x-axis direction

19. Conclusion
Coupling techniques offer a realistic alternative to a full CAA simulation
A complete acoustic response can be obtained by the coupling of RANS and Helmholtz equation
High order schemes are necessary to avoid numerical diffusion of fluctuations.

20. Acknowledgement References
The Helmholtz Equation program is coded by Professor Frederic Magoules.
Supported by The British Council Franco-British Alliance Programme.
References
[1] Z. K. Wang, “A Source-extraction Based Coupling Method for Computational Aeroacoustics”, PhD Thesis, University of Greenwich (2004)
[2] S. V. Patankar, Numerical Heat Transfer and Fluid Flow, (Hemisphere, 1980)
[3] G. S. Djambazov, “Numerical Techniques for Computational Aeroacoustics”, PhD Thesis, University of Greenwich (1998)
[4] E. Avital, “A Computational and Analytical Study of Sound Emitted by Free Shear Flows”, PhD Thesis, Queen Mary and Westfield College (1998)
[5] CFD Code PHOENICS,
[6] CFD Code PHYSICA,

21. 50Hz Disturbance Velocity Vectors [u(x,t)* - u(x,t)]