A COMPUTATIONAL FLUID DYNAMIC STUDY OF HAWKMOTH HOVERING

A computational fluid dynamic (CFD) modelling approach is used to stud y the unsteady aerodynamics of the flapping wing of a hovering hawkmot h, We use the geometry of a Manduca sexta-based robotic wing to define the shape of a three-dimensional 'virtual' wing model and 'hover' thi s wing, mimicking accurately the three-dimensional movements of the wi ng of a hovering hawkmoth, Our CFD analysis has established an overall understanding of the viscous and unsteady flow around the flapping wi ng and of the time course of instantaneous force production, which rev eals that hovering flight is dominated by the unsteady aerodynamics of both the instantaneous dynamics and also the past history of the wing . A coherent leading-edge vortex with axial flow was detected during t ranslational motions of both the up-and downstrokes, The attached lead ing-edge vortex causes a negative pressure region and, hence, is respo nsible for enhancing lift production, The axial flow, which is derived from the spanwise pressure gradient, stabilises the vortex and gives it a characteristic spiral conical shape. The leading-edge vortex crea ted during previous translational motion remains attached during the r otational motions of pronation and supination, This vortex, however, i s substantially deformed due to coupling between the translational and rotational motions, develops into a complex structure, and is eventua lly shed before the subsequent translational motion. Estimation of the forces during one complete flapping cycle shows that lift is produced mainly during the downstroke and the latter half of the upstroke, wit h little force generated during pronation and supination, The stroke p lane angle that satisfies the horizontal force balance of hovering is 23.6 degrees, which shows excellent agreement with observed angles of approximately 20-25 degrees. The time-averaged vertical force is 40 % greater than that needed to support the weight of the hawkmoth.