Free and Scattered Acoustic Field Predictions of the Broadband Noise Generated by a Low-Speed Axial Fan

F1 - Noise Prediction and Investigation for Axial Fans

This paper deals with the free and scattered acoustic field of the broadband noise generated by a low Mach number axial fan analytically and experimentally. The numerical model proposed is a semi-analytical solution to predict the acoustic field of an industrial fan operating in a turbulent stream. The model employed is based on Amiet's theory on the turbulence-interaction noise for a stationary airfoil. The theory for the stationary airfoil is then applied to the rotating blades of the fan.
However, the model includes far and free-field assumptions, and is derived for spanwise uniform incoming flow properties impinging to the leading edge of the airfoil. For most of the industrial applications and in the present configuration these assumptions then become invalid.
The theory has been therefore extended in order to predict the acoustic response of the airfoil in its geometrical near-field. The extended analytical model is compared with the numerical results and a good improvement is satisfied. The model is also applied to a stationary optimized airfoil located downstream of the turbulent flow in the anechoic chamber. Improved results are obtained in the comparison with the measurements.
In order to take the spanwise varying incoming flow conditions into account a segmentation has been applied.
The improved model has been combined with a Boundary Element Method (BEM) accounting for the acoustic scattering from an obstacle. Due to the statistical characterization of the turbulence, an innovative approach called Acoustic Transfer Vectors (ATV) in the BEM framework is employed for the broadband noise scattering.
The numerical validation of the broadband noise scattering has been performed through a comparison of the results obtained by the extended semi-analytical model combined with the ATV approach with a flat scattering screen, and the analytical model which contains the acoustic scattering from an infinite flat plate.
The model combined with the ATV approach is also applied to an industrial low-speed axial fan operating next to a flat scattering screen. An agreement is observed in the comparison with the measurements in both free and scattered acoustic field.