An experimental study of the effect of rotor-stator interaction on tonal noise in low-speed axial fans

The present paper concerns the tonal noise generated by axial flow fans employed for automotive cooling systems, with and without downstream stator. A 9 blades rotor and three different stators have been employed (asymmetric production stator and symmetric 18 and 19 blades stators), while the rotor-alone has been assumed as reference configuration. Measurements have been taken in the DIMSET hemi-anechoic chamber. Relevant tonal noise peaks of unknown origin are present even in the rotor-alone configuration and, as expected, the downstream stator increases the SPL at the blade passing frequency (BPF) harmonics. The comparison of the SPL measured during speed ramps in the various configurations shows that the results are in agreement with the trends expected basing on the theory of acoustic interference: depending on the stator blade number some harmonics are not modified showing that the phenomena are linear and that the active noise generation mechanisms at the related harmonics are unchanged. Then the measurements with the 18 blades stator have been repeated at 7 different stator angular positions thus changing the phase of the tonal noise contribution due to the rotor-stator interaction. This did not result in any variation of the SPL at the 2nd BPF harmonic, showing that the other contribution to the tonal noise component has a random phase and therefore is of random origin, e.g. due to large turbulent structures chopped by the rotor blades. These results provide a deeper insight in the physical phenomena and will guide more detailed aerodynamic investigations.