Experimental Analysis of the Aerodynamics of Long-Shrouded Contrarotating Rotor in Hover

Abstract

This paper aims to quantify the benefits of a shrouded coaxial rotor configuration through experimental comparisons with free (not shrouded) rotors in hover. The experiment shows that both the figure of merit of contrarotating rotors and the system power loading are improved by the shroud inclusion. Improvements are induced by a suction effect at the inlet, which can be optimized by a regulation effect of the mass flow. Compared to free rotors, the strong suction peak formed on the shroud leading edge by a 65% increase in mass flow, allowing the shroud to contribute up to 56% of the total thrust. More uniform pressure distribution in the downstream rotor and less contraction of the slipstream decrease losses and increase the rotor efficiency. The shrouded system efficiency is further improved if the upstream rotor rotates slower than the rear one, for a given total shaft power, because a stronger pressure depression occurs upstream of the rotors to generate more mass flow. On the other hand, the system behavior is insensitive to the interrotor distance.

Computational study on the aerodynamics of a long-shrouded contra-rotating rotor in hover

“…With regard to the experimental work, 14 the best performed SDR system with the upstream rotor located at 80 mm from the inlet and inner rotor distance (IRD) of 20 mm was also chosen in the computation work. As the SDR system involves two rotors rotating at the opposite direction, it needs two reference frames to define each rotating motion, as shown in Figure 2.…”

“…For the analysis of the hover performance of the SDR system in the study by Huo et al, 14 there are limitations for current experiments: first, due to the limited space and strong flow interactions between rotors, the probe is hard to test the flow at the middle plane of two rotors. The flow characteristics between rotors are thus not clear; second, the experiment was actually two dimensional since it cannot take the radial component of the flow into account; third, the flow features revealed by experiments were still from a global view.…”

“…15 The contra-rotating rotor was specifically designed for shrouded systems by Drela and Youngren 16 with nonzero circulation at the blade tip. The detailed geometrical parameters of both shroud and contra-rotating rotor can be found in Huo et al 14 In the study by Huo et al, 14 the experimental test bed was designed to understand the flow physics and the interaction between different components of the shrouded contra-rotating rotor, as shown in Figure 1. The test bed can not only test the individual performance parameters such as thrust and torque, but also measure the evolution of aerodynamic variables that characterize the flow passing through the shroud, such as total and static pressure.…”

Revisiting Froude’s Theory for Hovering Shrouded Rotor

“…In what follows, formulations will be derived with respect to hovering conditions [16,17]. Similar formulations for general inflow velocities are addressed in the appendix.…”

“…It is unlikely that conventional MAV designs reach the upper limit. This is partly due to the nozzle diffusion angle constraint (diffusion angle typically below 10-11 deg [9,17]) that prevents the occurrence of nozzle flow separation and deterioration of the global aerodynamic performance. However, due to the recent extension of MAV's field of application, unconventional designs may emerge.…”