Axial flow fan broad-band noise and prediction

Two prediction methods for broad-band noise of low-pressure axial fans are investigated. Emphasis is put on the interaction noise due to ingested turbulence. The numerical large eddy simulation (LES) is applied to predict the unsteady blade forces due to grid generated highly turbulent inflow; the blade forces are then fed into an analytical two-dimensional acoustic ducted source model. A simple semi-empirical noise prediction model (SEM) is utilized for indicative comparison. Finally, to obtain a database for detailed verification, the turbulence statistics for a variety of different inflow configurations are determined experimentally using hot wire anemometry and a correlation analysis. In the limits of the necessary assumptions the SEM predicts the noise spectra and the overall sound power surprisingly well without any further tuning of parameters; the influence of the fan operating point and the nature of the inflow is obtained. Naturally, the predicted spectra appear unrealistically “smooth”, since the empirical input data are averaged and modeled in the frequency domain. By way of contrast the LES yields the fluctuating forces on the blades in the time domain. Details of the source characteristics and their origin are obtained rather clearly. The predicted effects of the ingested turbulence on the fluctuating blade forces and the fan noise compare favorably with experiments. However, the choice of the numerical grid size determines the maximal resolvable frequency and the computational cost. As contrasted with the SEM, the cost for the LES-based method are immense.