This work presents an experimental and analytical investigation of unsteady loading of a low Reynolds number propeller and its relation to aeroacoustics. The propeller is positioned at an angle of attack with respect to the freestream which causes a variation in loading along the azimuth. The propeller is operated at 4000 RPM with an advance ratio of J = 0.4, corresponding to a free stream velocity of 8 m/s. The chord-based Reynolds number of the propeller is of the order of 10⁴ and is investigated at two angles of attack: α_r = 7.5° and α_r = 15°. For reference, a steady case, α_r = 0◦, is presented as well. Loads and noise measurements were carried out in the A-Tunnel of TU-Delft. Two microphone array configurations were used to mimic the observers positions above and underneath the flightpath of the propeller. Underneath the flightpath, an increase in SPL was found with increase of the rotor angle of attack. Above the flightpath, the opposite effect was observed, resulting in an asymmetrical directivity profile when the rotor is at an angle of attack. A quasi-steady loading model, based on the lifting line code Xrotor, is used to determine the azimuthal variation in loading. The variation in tip speed along the azimuth due to the propeller angle of attack is prescribed to determine the variation in loading in one rotation. Far-field loading and thickness noise are computed using an aeroacoustic model based on the Ffowcs-Williams & Hawkings’ equation to predict the harmonic noise in the frequency domain of a rotating dipole and monopole, respectively. The numerical predictions for the steady case show good agreement for both the aerodynamic and aeroacoustic performance when compared with loading and noise measurements. For the α_r≠ 0° cases it was shown that unsteady loading noise increases the SPL for all harmonics, with a larger contribution at higher harmonics.

Noise is a major issue in Urban Air Mobility vehicles, especially due to its operation in the urban environment. One of the major sources of noise is the rotor, and thus, there is a need to optimize the rotor design for aerodynamics and aeroacoustics. A possible technological solution is represented by Co-rotating rotors (i.e., two rotors connected through the same shaft with a prescribed axial and azimuthal separation). Such configuration can offer significant potential in noise reduction without affecting thrust and power distribution in a vehicle. At present, there is a lack of knowledge on the mutual interaction between the two rotors which limits physics based optimal designs.
The purpose of the present study was to investigate the behaviour of a co-rotating rotor when its geometrical parameters are varied, such as: axial and azimuthal separation, collective pitch, etc. The simulation was performed on commercial software 3DS PowerFLOW which uses Lattice-Boltzmann Very Large Eddy Simulations (LB-VLES) for flow-field simulation and Ffowcs-Williams and Hawkings (FWH) analogy for far-field aeroacoustic post-processing. Results show that, a higher angular separation results in higher thrust and lower noise, while a higher axial separation increases thrust but with noise increase with respect to a single rotor. However, thrust of co-rotating rotors is still smaller than the sum of two isolated rotors. This is due to the potential effect due to mutual interaction of the streamtubes of each rotor, blade-vortex interaction and flap effect (i.e. the lower rotor acts as a flap for the upper rotor when angular separation is small). Noise reduction is instead caused by destructive interference between the sound scattered by each rotor. Thus, a co-rotating rotor with higher azimuthal angle will produce less noise than a single rotor, both producing the same thrust. These findings suggest that the design of the two rotors can be optimized to account for the interaction effects, thus potentially doubling the effective thrust but still reducing noise.