Influence of the Gap Flow of Axial Vehicle Cooling Fans on Radiated Narrowband and Broadband Noise

Numerical investigations on the sound radiation of low-speed axial vehicle cooling fans with different blade skew and gap geometries were carried out and validated by experiments. To reproduce similar installation conditions to those found in a vehicle, an overall system is considered which contains a radiator, the cooling fan and a combustion engine dummy. By modifying the gap geometry of a backward skewed fan, broadband noise between 150Hz and 2500 Hz can be reduced measurably. To investigate both effects, the different broadband noise emission and the mechanisms which lead to the narrowband subharmonic peaks, a hybrid approach is chosen. Therefore a computational fluid dynamics (CFD) simulation is performed as a first step and the acoustics are computed afterwards. Initially, the sound radiation is calculated using an integral method described by Ffowcs-Williams and Hawkings (FWH) and is validated by measurements. Due to the limited possibility to locate the acoustic sources, a volume-based method is carried out. Thereby, the acoustic source terms are calculated based on the APE-2 formulation to separate acoustic from hydrodynamic effects in the acoustic near field. Afterwards the propagation of these acoustic sources are calculated and compared to measurement results as well. Using this approach, different gap flows and its mechanisms on sound production can be identified by correlating flow field with the acoustic field. The influence of the gap flow on tonal and broadband noise of the considered fan configurations is clarified.