Analysis of the SPL Spectra Generated by Axial Flow Fans Working under Similarity Conditions

In the present paper a technique based on the similarity theory is shown which is aimed to study the trailing edge noise generated by axial flow fans. Given a set of SPL data measured on rotors of different diameter, once the analitycal structure of the SPL power spectrum is assumed, the technique allows to determine the dependence of the spectrum on the rotational speed and on the diameter. Such a technique has a general usefulness since it allows to verify the main features of measured and predicted SPL spectra. In the present case it has been developed to test trailing edge noise prediction methods and, possibly, to develop new semi-empirical correlations. After a description of the basic theory, the technique is applied to preliminary measurements taken in a hemi-anechoic chamber. The application shows that, for the present rotors, the measured OASPL follows the expected scaling law and slightly decreases as the Reynolds number is increased. Such a result needs to be confirmed by further investigations taken under more controlled inflow conditions. Nomenclature a0 = speed of sound D = rotor tip diameter f = frequency