Abstract
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.
Highlights
In the near future, intra-city transportation may well be revolutionized by urban air mobility (UAM), i.e., new technologies of vertical takeoff and landing (VTOL) manned and unmanned aerial vehicles (UAVs)
In this paper, an experimental characterisation of low Reynolds number rotors is performed in an anechoic room
This change in directivity leads to a similar change of directivity of the overall sound pressure level (OASPL)
Summary
Intra-city transportation may well be revolutionized by urban air mobility (UAM), i.e., new technologies of vertical takeoff and landing (VTOL) manned and unmanned aerial vehicles (UAVs). They found that, for a given target thrust, rotor performance was enhanced for a number of blades of 3 and that, despite the increasing influence of blade-vortex interactions, acoustic noise monotonically decreased with the number of blades. This pendulum type test bench consists of two 10 N S100 load cells from Strain Measurement Devices (SMD) (Wallingford, CT) for thrust and torque measurements, as shown in Fig. 2 [see Desert (2019) and Desert et al (2019)] It is placed outside of the anechoic room; load measurements were not performed simultaneously to acoustic measurements for 3D-printed rotors.
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