A Computational Study on Diffuser Length Variation for a Ducted Rotor in Hover and 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 the SpalartAllmaras RANS model implemented in a stabilized finite element method. A sliding mesh was used to conveniently account for the large-scale motion associated with rotor revolutions. The simulation results were analyzed to understand the flow physics and quantify the contributions of the rotor and various sections of the duct interior surfaces on the total thrust, drag, and pitching moments. Performance comparisons were made between the different diffuser length configurations, varying from 144% to 18% rotor radius. In hover, the short diffuser configurations produced 3% lower thrust and required 1.5% higher power than the long diffuser configurations. 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. At 10m/s edgewise flight, the rotor was the dominant generator of thrust (in the range of 80% of the total), Reducing diffuser length did not affect the thrust generated by the rotor, but the duct thrust reduced by about a third. The short duct completely eliminated the ram-pressure induced H-force on the rear diffuser, significantly reducing the drag of the entire system. Since 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 small tip clearance of the rotor causes a local interaction between the blade tip and duct that is the dominant contributor to vibratory H-force on the ducted rotor. 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.


Misiorowski, M., Gandhi, F., and Oberai, A., "A Computational Study on Diffuser Length Variation for a Ducted Rotor in Hover and Edgewise Flight ,"

Proceedings of the AHS Specialists’ Conference on Aeromechanics Design for Transformative Vertical Flight, San Francisco, California, Jan 16-19, 2018.