Effect of phasing of rotation on Delayed stall in flapping flights related to MAVs at Re=10 4

Unsteady aerodynamics of flapping flight at Re=10000 for forward flight with advance ratio, J=0.5, pertaining to the development of micro air vehicles, is analyzed. The effect of phasing of rotation is analyzed using three flapping kinematics advanced, symmetrical and delayed rotation. The flow structures obtained and time variation of lift and thrust are used to evaluate their effects. The power required for flapping and rotation are computed to evaluate the propulsive efficiency. In all the cases simulated, a strong spiral LEV is formed during the downstroke resulting in peak of lift and thrust. A strong spanwise flow from base towards tip is observed in the middle of the vortex. However due to the presence of a tip vortex, a strong negative spanwise flow forms near the tip, which prevents the removal of vorticity from the LEV. Hence the LEV gets unstable during the middle of the downstroke and separates from the wing resulting in a drop in lift. The results show that supination results in LEV formation near the base and the strength of the LEV depends on the flapping velocity. A stronger vortex is formed for advanced rotation and it generates high lift. The delayed rotation affects the thrust production during the translation and results in low propulsive efficiency. In terms of average value of thrust advanced rotation is preferred, while symmetric rotation generates slightly higher average lift than that of advanced rotation.

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