A computational fluid dynamic study of hawkmoth hovering

<p>A computational fluid dynamic (CFD) modellingapproach is used to study the unsteady aerodynamics of theflapping wing of a hovering hawkmoth. We use thegeometry of a Manduca sexta-based robotic wing to definethe shape of a three-dimensional ‘virtual’ wing model and‘hover’ this wing, mimicking accurately the threedimensionalmovements of the wing of a hoveringhawkmoth. Our CFD analysis has established an overallunderstanding of the viscous and unsteady flow around theflapping wing and of the time course of instantaneous forceproduction, which reveals that hovering flight is dominatedby the unsteady aerodynamics of both the instantaneousdynamics and also the past history of the wing.A coherent leading-edge vortex with axial flow wasdetected during translational motions of both the up- anddownstrokes. The attached leading-edge vortex causes anegative pressure region and, hence, is responsible forenhancing lift production. The axial flow, which is derivedfrom the spanwise pressure gradient, stabilises the vortexand gives it a characteristic spiral conical shape.The leading-edge vortex created during previoustranslational motion remains attached during therotational motions of pronation and supination. Thisvortex, however, is substantially deformed due to couplingbetween the translational and rotational motions, developsinto a complex structure, and is eventually shed before thesubsequent translational motion.Estimation of the forces during one complete flappingcycle shows that lift is produced mainly during thedownstroke and the latter half of the upstroke, with littleforce generated during pronation and supination. Thestroke plane angle that satisfies the horizontal forcebalance of hovering is 23.6 °, which shows excellentagreement with observed angles of approximately 20–25 °.The time-averaged vertical force is 40 % greater than thatneeded to support the weight of the hawkmoth.</p>