Numerical Investigation of Noise Generation by Automotive Cooling Fans

Abstract

Electrical vehicle technology is sharply rising and expected to dominate the international market in the coming 10 years. In electric vehicles, axial cooling fans are commonly used to cool batteries with high heating load. One drawback of axial cooling fans is the high aeroacoustic noise level resulting from the fan blades and the obstacles facing the air flow. To create a comfortable cabin environment in the vehicle, a low-noise installation design of the axial fan is required. The purpose of the current project is to develop an efficient computational aeroacoustics (CAA) simulation process to assist the cooling-fan installation design. The current thesis focuses on a hybrid CAA approach where the aerodynamic field is obtained through the unsteady Reynolds-averaged Navier-Stokes equations (URANS) model, and the acoustic field is computed via Lighthill’s analogy. The simulation process is applied to a benchmark case in the literature, where the aerodynamics and aeroacoustics of a low-speed axial fan are investigated separately. Initial steady state simulation is performed using the moving reference frame (MRF) approach to quantify different simulation model parameters before running the main URANS simulation. The URANS model provides good prediction of the mean flow quantities. The computed unsteady wall pressure fluctuations succeeds to resolve the fan blade passing frequency and the sub-harmonic frequency resulted from the interaction of the gap flow with the fan blades. The CAA approach studied in this thesis is limited to the prediction of narrow-band components of the fan noise, i.e., the blade-passing-frequency (BPF) noise and the noise generated by distinct coherent vortex structures. The aeroacoustic simulation model successfully captures the narrow-band acoustic signal and gives promising results for extending the approach to predict broad-band fan noise by replacing the URANS model with more advanced CFD models, such as the large eddy simulation.