Aeroacoustic study of an axial engine cooling module using lattice-Boltzmann simulations and the Ffowcs Williams and Hawkings’ analogy

Abstract

An acoustic study of a typical automotive engine cooling fan module using Lattice-Boltzmann simulations is presented. The simulation mimics the test conditions in a semi-anechoic chamber closely. The importance of the simulation length and of the ground reflection on the tones is demonstrated. Direct numerical acoustic results compare very well with measurements obtained with an array of microphones located in front of the module. A consequent study based on the power spectral density of wall-pressure fluctuations filtered at specific frequencies and the use of the Ffowcs Williams and Hawkings’ (FWH) analogy has allowed identification of the main contributors on the module to the farfield noise. Both the rotor (inlet distortion) and the stator (rotor–stator interaction) are seen to contribute to the first two dominant tones. The origin of subharmonic humps and of order 18 tone in the acoustic spectra has been then deciphered: the former is traced to tip vortices periodically interacting with the fan blades and the latter to rotor–stator interactions within the fan hub. This also stresses the importance of tip noise in engine cooling modules. Finally this accurate noise-source identification is shown to lead to simple in-situ improvements of the module design.