Computational Study of Diffuser Length on Ducted Rotor Performance in Edgewise Flight

This study examines the effect of diffuser length variation on the performance of ducted rotors in hover and edgewise flight. The flow over a three-dimensional model of ducted rotor configurations was simulated using a detached-eddy simulation model implemented in a stabilized finite element method. Performance comparisons were made between the different diffuser length configurations, varying from 144 to 18% rotor radius. In 10  m/s edgewise flight, the duct inlet at the front generates high lift, and the rotor generates higher lift over the front as well due to the upwash at the front of the disk. These factors combine to generate a large nose-up pitching moment. Additionally, ducted rotors typically see large drag from ram pressure on the rear diffuser. The short duct completely eliminated the ram-pressure induced H-force on the rear diffuser, significantly reducing the drag of the entire system. Because the ram pressure on the aft diffuser generated a nose-down moment, partially counteracting the nose-up moments from the rotor and duct inlet, its absence in the shortest duct resulted in a net increase in steady nose-up pitching moment. The rotor is the primary source of vertical vibratory forces as well as vibratory pitching moment. The shortest diffuser configuration was shown to significantly reduce the magnitude of the vertical and H-force vibrations, but the magnitude of the vibratory pitching moments increased.