Experimental study and velocity scaling of the tip-leakage noise generated by low-speed axial flow-fans

The present paper reports of an experimental study of the broad band and the tip leakage noise generated by axial flow fans employed for automotive cooling systems. Two operating points and three different configurations, with and without tip leakage flow, have been considered. The measurements have been taken in a hemi-anechoic chamber at constant rotational speed and during speed ramps, and the problem of the scaling of the different noise components has been faced. The acoustic response function of the test configuration, computed by means of the spectral decomposition method, has been compared with the SPL at constant Strouhal number. The comparison has shown that the acoustic response strongly affects the trend of the SPL spectra and results in a complicated dependence on the rotational speed. However, the analysis of the acoustic propagation function demonstrates that the broad band noise and the tip leakage noise scale with different power laws but are both affected by the same propagation effect. The scaling law also depends on the operating point.