Numerical Investigation of the Influence of Skewness and Gap Geometry on Sound Radiation of Axial Vehicle Cooling Fans

D1 - Prediction of Axial Fan Noise by Hybrid Methods (i)

The overall acoustic impression is an important factor in the decision of a customer to purchase a particular vehicle. To optimize vehicles, specific components and systems which are responsible for noise generation are considered individually. One dominant noise source and central part of the engine cooling is the axial cooling fan. CFD programs have been used successfully in the fluidic design of fans for years. For the acoustic design, however, numerical simulations have only seen limited use so far.
In the present numerical investigations, a hybrid approach is chosen where first a CFD simulation based on the Navier-Stokes-Equations is performed and the acoustic is calculated in a second step. An integral method described by Ffowcs-Williams-Hawkings (FW-H) is used for the acoustic simulation.
In the FW-H method, the flow quantities of interest are the velocity components, the pressure and the density. Using an in-house solver for the FW-H calculation, these quantities are interpolated to a so-called integration surface placed around the acoustic sources caused by the impeller and are exported from the CFD calculation at every time step. From this integration surface, Green's function is used to calculate the acoustically relevant parameters at a point that corresponds to the microphone position in experiments.
The radiation of sound of three different fan configurations is calculated. On the one hand, a forward skewed fan is compared to a backward skewed one. The influence of the blade skew on the gap flow and the radiation of sound of both configurations are demonstrated. On the other hand, a backward skewed fan with a modified gap geometry is derived from the already mentioned fan configurations which leads to a lower radiation of sound and a higher efficiency.
The acoustical results are validated with measurements based on sound pressure spectra. The results are found to be in good to excellent agreement with measurements dependent on frequency.