Presentation on theme: "University of Greenwich Computing and Mathematical Sciences"— Presentation transcript:
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 carby Liang LaiSupervisors: Professor C- H Lai,Dr. G S Djambazov,Professor K A PericleousSponsored by University of Greenwich
2 Solution strategies for Computational Aeroacoustics IntroductionSolution strategies for Computational AeroacousticsDecreasing costsource+PropagationDNS/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/RANSSimpler equations *(near field)(near field + far field)sourcePropagationnon-linearised Euler equationsDecomposition + source-retrieval techniquesOther 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 disturbance1.2m0.4m1.1m0.8m0.03m0.9m0.5mCar as a Helmholtz ResonatorTime 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 PHOENICSThe pressure fluctuation along the open sun-roof can be calculated,where is the pressure distribution obtained by using the CFDcalculation and is the background pressure distribution due to theupstream velocity.
8 Car as a Helmholtz Resonator Analyse Acoustic Response by using FFTComparison to a Helmholtz Resonatortherefore, 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/st = 10-3 s , wave time per cycle ta = 20 t, f = 50 Hz25m/s1.28m0.52m1.35m0.35m0.05m1.2m0.6m0.85m0.3m###Mesh
11 9 observation points within computational domain Geometry and Observation points7 points along sun-roof9 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 Responsef = 13Hz
17 Helmholtz Equation, the FT of the Wave Equation Homogeneous Wave equationIntegrate with respect to time --- taking Fourier transform of the wave equationfinally, one getsApply inside car cavity – neglecting convective effects
19 ConclusionCoupling techniques offer a realistic alternative to a full CAA simulationA complete acoustic response can be obtained by the coupling of RANS and Helmholtz equationHigh 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 Z. K. Wang, “A Source-extraction Based Coupling Method for Computational Aeroacoustics”, PhD Thesis, University of Greenwich (2004) S. V. Patankar, Numerical Heat Transfer and Fluid Flow, (Hemisphere, 1980) G. S. Djambazov, “Numerical Techniques for Computational Aeroacoustics”, PhD Thesis, University of Greenwich (1998) E. Avital, “A Computational and Analytical Study of Sound Emitted by Free Shear Flows”, PhD Thesis, Queen Mary and Westfield College (1998) CFD Code PHOENICS, CFD Code PHYSICA,