Flow Acoustic Effects on a Commercial Automotive Air Intake Silencer - A Numerical Study using Computational Fluid Dynamics

Noise generated by turbo-compressors in combustion engine air intake systems is often mitigated by broad-band high-frequency duct silencers. The acoustic performance of a developed silencer design is generally obtained numerically without influence of duct mean flow, using acoustic computer aided engineering (CAE) tools. Discrepancies are however created between the acoustics of the real air intake system and numerical model of the system since mean duct flow is always present in the real system, due to the aspiration of the engine. This Master’s thesis project aims to capture flow effect on silencer acoustic performance numerically using computational fluid dynamics (CFD). In cooperation with Volvo Car Group, a commercially existing part of an air intake system, including a silencer composed of two Helmholtz resonators, is studied. An already established academic CFD methodology is explored, used, expanded and streamlined to investigate flow effects on acoustic properties of the complex silencer-duct system. The acoustic properties from the CFD simulation are then compared to experimental data and acoustic CAE. The established CFD methodology is integrated and applied using the commercial CFD software Star-CCM+, studying the silencer acoustic behaviour with several mean inlet flow speeds. Using Star-CCM+, mean flow and acoustic wave propagation is simulated simultaneously in the defined computational domain of the given silencer-duct system. Acoustic waves in a frequency band of interest related to the eigenfrequency of the silencer, are inserted through a time-varying inlet boundary condition. The numerical setup mimics an acoustic experimental measurement setup where the propagating acoustic waves are captured as pressure signals in virtual microphones, to calculate the acoustic silencing property of transmission loss (TL). Turbulence is modelled using the unsteady Reynolds-averaged Navier-Stokes equations (URANS). Numerical parameters in the CFD setup are studied in regards to numerical accuracy and computational efficiency, to find the most optimal model in describing the flow effect on silencer acoustic performance. The most optimal resulting CFD methodology was able to capture transmission loss characteristics with reasonable accuracy, under predicting the resulting eigenfrequency shift with roughly 8.5 % difference for all flow speeds; as well as a 11.4 % transmission loss peak difference, both in comparison to experimental data. Different duct geometrical changes were then studied using the optimally developed CFD setup, in order to improve silencer acoustic performance under flow condition. Through a numerical process, six different geometrical changes were developed with varying strategies. The best design resulted in a 15 % increase in first order resonance peak transmission loss and removal of eigenfrequency shift as well as only increasing pressure drop by 1.1 %, in comparison to a reference simulation. Keywords: Computational fluid dynamics (CFD), Aeroacoustics, Air intake silencer, Helmholtz resonator, Flow effect, Transmission loss, Noise reduction, Star- CCM+, Unsteady Reynolds averaged Navier-Stokes equations (URANS), geometrical change.