Event
PhD Defense Announcement - Jason Pereira "Hover and Wind-Tunnel Testing of Shrouded Rotors ..."
Thursday, May 22, 2008
12:00 p.m.
Judith Resnick Lecture Auditorium - GLM 1202
PhD Defense Announcement
Jason Pereira
"Hover and Wind-Tunnel Testing of Shrouded Rotors for Improved Micro Air Vehicle Design"
Thursday, May 22, 2008
Time: 12:00PM
Place: Judith Resnick Lecture Auditorium - GLM 1202
Committee Chair: Dr. Inderjit Chopra
Committee Members: Dr. James Baeder
Dr. Derek Boyd (Dean's Representative)
Dr. Darryll Pines
Dr. Norman Wereley
Abstract:
"The shrouded-rotor configuration has emerged as the most popular choice for rotary-wing Micro Air Vehicles (MAVs), because of the inherent safety of the design and the potential for significant performance improvements. However, traditional design philosophies based on experience with large-scale ducted propellers may not apply to the low-Reynolds-number (~20,000) regime in which MAVs operate. An experimental investigation of the effects of varying the shroud profile shape on the performance of MAV-scale shrouded rotors has therefore been conducted. Seventeen models with a nominal rotor diameter of 16 cm (6.3 in) and various values of diffuser expansion angle, diffuser length, inlet lip radius and blade tip clearance were tested at various collective angles in hover. Compared to the baseline open rotor, the shrouded rotors showed increases in thrust by up to 94%, at the same power consumption, or reductions in power by up to 62% at the same thrust. These improvements surpass those predicted by momentum theory, due to the additional effect of the shrouds in reducing the non-ideal power losses of the rotor. Increasing the lip radius and decreasing the blade tip clearance caused performance to improve, while optimal values of diffuser angle and length were found to be 10deg and 50% of the shroud throat diameter, respectively.
Measurements were also made of the shroud surface pressure distributions and wake velocity profiles. For high net shroud thrust, a favorable pressure distribution over the inlet was seen to be more important than in the diffuser. Strong suction pressures were created by vortical flow at the blade tips, while less than 20% of the inlet thrust was borne by the outer portions of the inlet lip. Optimizing the inlet design to take advantage of these phenomena could enable increases in thrust and reductions in vehicle weight. The uniformity of the wake was improved by the presence of the shrouds and by decreasing the blade tip clearance, resulting in lower induced power losses.
The open rotor and a single shrouded-rotor model were subsequently tested at a single collective in translational flight, at various angles of attack. In axial flow, the net thrust and the power consumption of the shrouded rotor were lower than those of the open rotor. In edgewise flow, the shrouded rotor produced greater thrust than the open rotor, while consuming less power. Measurements of the longitudinal shroud surface pressure distributions illustrated the extreme asymmetry of the flow around the shroud, with large suction pressures on the windward side and extensive overpressure regions on the leeward side; this resulted in pitch moments much greater than those exerted on the open rotor. Except at low airspeeds and high angles of attack, the static pressure in the wake did not reach ambient atmospheric values at the diffuser exit plane; this challenges the validity of the fundamental assumption of the simple-momentum-theory flow model for short-chord shrouds in translational flight."
