Aerodynamic and Aeroacoustic Interaction Effects of a Distributed-Propeller Configuration in Forward Flight

Abstract

The Advisory Council of Aviation Research has set the future goals of sustainable development in aviation by reducing pollutant emissions of CO2 by 75% and 90% in NOx emissions per passenger kilometre and noise emission by 65% of the perceived noise level compared with the measurements in 2000. This demand has led to the development of new aircraft concepts with alternative propulsive systems. The development of the all-electric and hybrid-electric aircraft concepts has regenerated the interest in distributed propulsion systems with propellers. Distributed-propeller systems are propulsive configurations with multiple propellers located along the wing or/and aircraft's airframe. Among the benefits of these systems is their electrical connectivity with the power generating sources or energy sources, like batteries, that make them attractive for vehicles performing electrical vertical take-off and landing for urban air mobility.
Previous studies' investigation of multi-rotor systems has been primarily focused on the aerodynamic performance changes in hover conditions. The thrust decrease of these propellers during their operation at a close distance is accompanied by oscillations associated with the flow structures interactions. This results in a noise emission increase attributed to the thrust fluctuations. The investigation, therefore, of the distributed-propeller system in forward flight will contribute to the in-depth analysis of the performance of these systems and illustrate the root of potential changes, assessing the changes in thrust as well as the unsteady loading of the blades. Also, the present analysis will study the mechanisms that lead to noise emission increase and the potential benefits of the relative phase angle variation between adjacent propellers. The effect of the relative phase angle has shown a positive impact on noise emission of multiple-propeller systems without considering the interference between adjacent propellers. Thus, the aim of the present computational study is to investigate the aerodynamic interference and aeroacoustic signature of a distributed-propeller configuration in forward flight, including the effects of the relative phase angle.
The comparison of the distributed-propeller model with the isolated propeller case showed minor differences in the time-averaged performance. The mean thrust coefficient increases slightly by 0.0004, whereas the time evolution plot reveals the thrust oscillation with amplitude equal to 5.3% of the mean value. The trailing vortex systems of adjacent propellers induce velocity components that alter the propellers' flow field upstream and downstream. These induced velocities result in axial velocity increase upstream of the propeller and in non-zero values of the tangential velocity, resulting in thrust oscillations. This results in a change of thrust as the propeller rotates. During the motion of a blade, its thrust is reduced when it approaches the adjacent propeller and subsequently increases during its retreat, resulting in an unsteady loading on the blades. The slipstream of the systems also presents differences from the respective one of an isolated propeller. The symmetric and circular shape of the wake flow behind the isolated propeller is broken and turned into a deformed wake flow behind the distributed-propeller system, as a result of the interference of the tip vortices. The tip vortices emitted by adjacent blades stay close with each other during their downstream motion, with their interaction results in change of trajectory, shape deformation and fast dissipation.
The noise emission of the distributed-propeller system shows a noise level increase in front of the propellers while along the propeller plane, this increase is smaller. The comparison of the middle propeller of the system with the isolated one reveals an increase of 13.5 dB along the propellers axis, while the increase along the plane of rotation is 1.8 dB. The increase along the propeller axis is associated with enhanced tonal components at frequencies up to the 5th blade passing frequency. The directivity of the noise emission and the augmentation of tonal components imply that the unsteady loading is the reason for the noise levels increase. The increase of the broadband component (3.2 dB) at oblique angles with respect to the propeller axis is attributed to the interaction of adjacent flow structures.
The variation of the relative phase angle between adjacent propellers results in blades passing from the region between adjacent propellers at different times. This has positively impacted the unsteady loading as it is decreased compared to the case without relative phase angle variation. The oscillatory behaviour of the thrust is reduced as the standard deviation of the thrust coefficient drops from 0.001 to 4.5e-04. On the contrary, the time-averaged performance of the propellers remains unaffected. The reduction of the unsteady loading results in noise emission reduction in the upstream direction by 5 dB, while at oblique angles in the downstream direction, there is a 1.8 dB increase. This increase is attributed to a different tonal component distribution than in the case without relative phase angle variation. Thus, the impact of the relative phase angle variation could be beneficial on noise emission angles normal to rotor plane due to the reduced unsteady loading.