Experimental investigation of low Reynolds number rotor noise

In this paper, an experimental characterisation of low Reynolds number rotors is performed in an anechoic room. Two commercially available two-bladed rotors as well as four three-dimensional (3D)-printed rotors with different numbers of blades (from two to five) are tested. The latter have canonical geometry, with an NACA0012 blade section profile, extruded in the radial direction with constant chord and constant 10° pitch. The experimental setup and the 3D printing strategy are first validated using results from the literature for the commercially available rotors. For all the tested rotors, four noise characteristics are analysed: the overall sound pressure level (OASPL), the amplitude of the blade passing frequency (BPF), and the amplitude of its first harmonic and the high-frequency broadband noise. For all the rotors, an increase in all noise characteristics is observed with the rotational speed (rpm) for all directivity angles. Moreover, an interesting change of pattern is observed for the amplitudes of the BPF and of its first harmonic, with, in the vicinity of the rotor plane, a minimum value for low rpm and/or high number of blades, and a maximum value for high rpm and/or low number of blades. This change in directivity leads to a similar change of directivity of the OASPL. For the broadband noise, a dipole-like pattern is obtained with a minimum value at θ=-10°, i.e., aligned with the trailing edge and thus indicating the generation of trailing edge noise. Finally, scaling laws that characterise the amplitude of the different noise components with respect to the rpm are proposed.