Fabio Mazzocut Zecchin

Experimental study of the effect of the rotor-stator gap variation on the tonal noise generated by low-speed axial fans

In the present paper the effect of the rotor stator axial gap on tonal noise generated by axial flow fans employed for automotive cooling systems is studied. A fan equipped with either a 18 vanes stator or a 3 struts one, positioned at several axial gaps, is tested in a hemianechoic chamber during rotational speed ramps. To analyze the acoustic pressure properties three different quantities have been employed: the velocity scaled SPL at the BPF harmonics, the phase of the acoustic pressure at the BPF harmonics and the propagation function obtained by means of the spectral decomposition. The useful properties of these quantities and their joint use are first analyzed. Then they are employed for two purposes: investigating propagation effects and source strength fluctuations and identifying contributions to tonal noise due to random sources (large scale inlet turbulent structures) and due to deterministic sources (aerodynamic rotor-stator interaction). The results of the study show that the stator causes unexpected propagation effects which affect the measured SPL. These effects depend on the axial gap and on the considered BPF harmonic and they also affect the feasibility of a SPL velocity scaling.

An experimental investigation of the spectral properties of the tonal noise generated by axial flow fans with and without rotor-stator interaction

The present paper is aimed at studying the spectral properties of the tonal noise generated by axial flow fans. The phase of the acoustic pressure FFT is considered as a means to integrate the usual plots of SPL at BPF harmonics. Four different configurations of a fan, obtained combining an upstream bar and a downstream stator in presence of large inlet turbulent structures, have been tested in a hemi-anechoic chamber. In order to vary the strength of the acoustic source, measurements are taken during rotational speed ramps at low angular acceleration. The joint analysis of SPL and phase at BPF harmonics shows that it is possible to distinguish between tonal noise due to inlet turbulence from tonal noise due to rotor-stator aerodynamic interaction. Furthermore, information about the presence of important propagation effects, which result in a non linear phase trend, may also be gained.

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

The present paper reports an experimental study of 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. Different noise components have been identified and the properties of the generating mechanisms have been separated from the acoustic propagation effects. To this aim, the acoustic response function of the test configuration has been computed employing the spectral decomposition method, and then it has been compared with the constant-Strouhal number SPL. The low frequency broadband part of the spectra and the peaks related to the tip-leakage flow are affected by the same propagation effects but show a different dependence on the rotational speed. The scaling exponent also depends on the operating point.

Aerodynamic noise from cooling and HVAC systems features important to land vehicles in urban traffic conditions

The present paper deals with the aerodynamic noise generated by cooling and HVAC systems employed in land vehicles. After a brief description of the physical origin of aerodynamic noise, the features important to system engineers dealing with fans installation and operation are considered. First, the main scaling laws useful to estimate the sound level spectrum are reported. Then, the main characteristics of the sound pressure level spectrum are related to the typical aerodynamic generating mechanisms and to the quality of the perceived sound, and some guidelines for noise reduction are listed. Finally, a method for the employ of results obtainable from laboratory tests is proposed which aims to identify the origin of annoying fluctuations in the received noise.