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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 University of Greenwich Computing and Mathematical Sciences

2 Introduction Solution strategies for Computational Aeroacoustics ¤ High-order schemes in space and time 1. Direct Numerical Simulation (DNS) 2. Large Eddy Simulation (LES or DES) 3. Reynolds-Averaged Navier-Stokes (RANS) source + Propagation DNS/LES/RANS (Near field + far field) ¤ 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 ! Decreasing cost

3 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). Introduction Simpler equations * non-linearised Euler equations Decomposition + source-retrieval techniques Other methods, e.g. Helmholtz equation? + + LES/RANS Simpler equations * (near field) (near field + far field) source Propagation

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 Car as a Helmholtz Resonator 25m/s A sinusoidal disturbance 1.2m0.4m1.1m0.8m 0.03m 0.9m 0.5m 0.4m Time step: Wave amplitude: Wave time per cycle: dt = s P 0 = -0.1 kg/s t a = 10 dt Problem setup and external excitation

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 A hypothetical car with an open sun-roof *The vortex strength W = A 0 sin(ωt), where A 0 = 1.2 m/s t = s, wave time per cycle t a = 20 t, f = 50 Hz Alternative Problem Description 25m/s 1.28m0.52m1.35m0.35m 0.05m 1.2m 0.6m 0.52m 0.85m 0.3m ###MeshMesh

10 Use of nested sub-grids

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

12 |||| |WwP|E| | || |||| Differencing Schemes Affect Disturbance Decay

13 Effect of Differencing Scheme: (a) Hybrid Source Input (f s =50Hz)

14 Effect of Differencing Scheme: (b) QUICK Source Input (f s =50Hz)

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

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


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