While landing, birds often perform a perching maneuver, which involves pitching their wings upwards while decelerating to a complete stop. By performing this perching maneuver, the birds can continue generating higher lift and drag force while slowing down, resulting in a smooth landing. The present study is motivated by the perching maneuver and aims to investigate two critical aspects of it. First, we want to explore how the proximity of the ground affects the unsteady forces and the flow field during the perching flight; and second, we want to analyze how a wing sweep influences a perching maneuver. To explore the first aspect of this dissertation, we investigated the finite flat plate undergoing a perching maneuver in the ground effect. Our results showed that the instantaneous and time-averaged lift force increased as the plate came close to the ground, while the instantaneous peak drag coefficient stayed relatively constant with changes in the ground height. However, the negative drag force, or the parasitic thrust, at the latter stages of the perching maneuver increased with the increase of the ground proximity. We found that performing rapid pitching at the end phase of the decelerating motion, which is done by introducing the time offset between the decelerating and pitch-up motion, significantly reduced the parasitic thrust even when the perching plate was in close proximity to the ground. Our results revealed that the dipole jet induced by the counter-rotating vortices was lower for the pitching case executed at the latter stage of the decelerating motion, which affected the advection of the shed vortices, acceleration of the fluid between the wing and the ground, and varied the unsteady forces during the perching maneuver. For the highest shape change number considered in this study, at a time offset of 0.5, the wing generated a positive averaged drag force and near zero averaged lift force, which is appropriate to land smoothly on the initial perching location without gaining altitude. The second aspect of this dissertation is motivated by the observation that some birds fold their wings to create a wing sweep during such perching. This study aims to find out whether such a wing sweep helps during a perching maneuver. We use two flat plates: one with a sweep and another without any sweep, and consider a deceleration maneuver where both decelerate to a complete stop from a Reynolds number, Re = 13000. We consider two cases: one, where the wings undergo only heaving, and another, where the wings perform both heaving and pitching. The latter maneuver was designed to mimic perching. By performing experiments and simulations, we compare the temporal evolution of the instantaneous forces and the vortex dynamics of both these plates. We show that during a major part of the deceleration, the instantaneous lift forces are higher in the case of the plate with sweep compared to the plate with no sweep during both kinematics. Our results indicate that the higher lift in the swept plate case was contributed by a stable leading edge vortex (LEV) which remains attached to the plate. This increase in stability was contributed by the spanwise vorticity convection caused by a distinct spanwise flow on the swept plate, as revealed by the numerical simulation. We also show that combined pitching and heaving resulted in higher force peaks, and the forces also decayed faster in this case compared to the heave-only case. Finally, by using an analytical model for unsteady flows, we prove that the higher lift characteristics of the swept plate were entirely due to higher circulatory forces. We also developed an analytical model that accounts for the variation of unsteady forces on a flat plate undergoing a perching maneuver. We model the flat plate using unsteady lifting line theory while the effect of ground height is incorporated using image vortices. We used Wagner's theory and the unsteady Kutta condition to model pitching and gradual deceleration. To include the ground effect, we updated the added mass force by accounting for the increase in flow acceleration between the wing and the ground. The model's accuracy was tested against the experimental results on a finite wing undergoing identical kinematics. Our result demonstrates that the present analytical model captures the unsteady variation of forces during a perching maneuver.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Mechanical and Aerospace Engineering
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Adhikari, Dibya Raj, "The Effect of Wing Shape and Ground Proximity on Unsteady Fluid Dynamics During the Perching Maneuver." (2023). Electronic Theses and Dissertations, 2020-. 1497.