MECHANISMS FOR AERODYNAMIC FORCE GENERATION AND FLIGHT STABILITY IN INSECTS
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The motivation of the current study is to explore two major aspects of flapping flight in insects: the aerodynamic mechanisms that are employed by flying insects to generate lift, and the strategies employed by insects to stabilize their flight in quiescent as well as perturbed aerodynamic environments. Regarding the former, despite intense study by physicists, biologists and engineers, we do not yet fully understand the unsteady aerodynamics that relate insect wing morphology and kinematics to lift generation. Here, we formulate a force partitioning method (FPM) and implement it within a computational fluid dynamic model to provide an unambiguous and physically insightful division of aerodynamic force into components associated with wing kinematics, vorticity, and viscosity. Application of the FPM to hawkmoth and fruit fly flight shows that while the leading-edge vortex is the dominant mechanism for lift generation, there is another, previously unidentified mechanism, the centripetal acceleration reaction force, which generates significant net lift. The centripetal acceleration reaction lift is power-efficient, and insensitive to Reynolds number and to environmental flow perturbations, making it an important contributor to insect flight stability and miniaturization. The FPM method developed here has wide ranging applications to virtually all fields of fluid dynamics, and in particular, to vortex dominated flows and flows with dynamically moving bodies. Similarly, the centripetal acceleration reaction force that has been identified here likely plays an important role in flows that involve bodies undergoing complex motions such as those encountered in the flying and swimming of animals, flow-induced vibration and deformation in biology and engineering, and multiphase flows. In a quest to explore strategies employed by hovering insects to stabilize their flight, the intrinsic stability of a hovering hawkmoth is analyzed. Analysis starts with the simplest model - a three degree-of-freedom (3DoF) linear time-invariant (LTI) model, and proceeds through 6DoF LTI to linear time-periodic (Floquet) models, and ends with a fully coupled fluid-body interaction (FBI) model which couples a Navier-Stokes solver with the 6DoF equations of motion of a freely flying hawkmoth. The well-accepted notion that the most unstable mode is a longitudinal (pitching) mode is challenged by the 6DoF LTI analysis that shows that there exists a lateral mode that is as unstable as the unstable longitudinal mode. Comparison of the flapping wing model with an equivalent revolving wing model also shows that the revolving wing model is more unstable than the flapping wing flyer. The results of the FBI model indicate that the hovering hawkmoth is more unstable in pitch than that predicted by the LTI model and that the location of center-of-mass (CoM) of hawkmoth relative to the neutral stability axis is the crucial element for stability. High speed videos of a freely hovering hawkmoth indicate that control of the CoM location relative to the neutral axis may be accomplished by the hawkmoth via rotation of the body relative the wing plane. Motivated by this, a simple sensory-motor control strategy for hover stabilization that relies on visual and mechanosensory feedback to drive small changes in the relative pitch between the body and the wing is hypothesized. Simulations are used to explore the viability of this control strategy as well as to determine the gains and latencies required by the sensory-motor control system to accomplish stabilization. Results suggest that the proposed strategy is indeed effective and viable given our current knowledge of insect response to aerodynamic perturbation and their sensory-motor control apparatus. The same strategy could potentially be employed in bio-inspired flapping wing micro-aerial vehicles.