Numerical Investigation of Narrow-Band Noise Generation by Automotive Cooling Fans

Axial cooling fans are commonly used in electric vehicles to cool batteries with high heating load. One drawback of the cooling fans is the high aeroacoustic noise level resulting from the fan blades and the obstacles facing the airflow. To create a comfortable cabin environment in the vehicle, and to reduce exterior noise emission, a low-noise installation design of the axial fan is required. The purpose of the project is to develop an efficient computational aeroacoustics (CAA) simulation process to assist the cooling-fan installation design. This paper reports the current progress of the development, where the narrow-band components of the fan noise is focused on. Two methods are used to compute the noise source. In the first method the source is computed from the flow field obtained using the unsteady Reynolds-averaged Navier-Stokes equations (unsteady RANS, or URANS) model. In the second method, the azimuthal modes of the flow field obtained using the steady RANS with the moving reference frame model are treated as the sound source. While the first method is able to resolve any narrow-band noise source that is related to large-scale harmonic oscillations, the second method may only be able to estimate the sound source at the blade passing frequency and its harmonics. The acoustic field generated by the sound source is computed by solving the inhomogeneous Helmholtz equation. The simulation process based on both methods is applied to a benchmark case in the literature and the simulated results are compared with experimental data. Satisfactory agreement is observed from the comparison.