Velocity Field Around a Rigid Flapping Wing with and Without Winglet in Quiescent Water

Abstract

In the present paper velocity field around a rigid flapping wing is obtained to study the effect of winglet. Two dimensional particle image velocimetry (PIV) measurements are carried out to capture the velocity field. The experiments have been performed in quiescent water. A comparison between wing with and without winglet has been carried out at two flapping frequencies, namely, 1.5 and 2.0 Hz. It is observed that wing with winglet has a nontrivial effect on evolution of wingtip vortex, added mass and residual flow from previous stroke. At the beginning of downstroke, wingtip vortex does not form clearly for winglet case which is in contrast to without winglet case, while during upstroke wingtip vortex is well defined for both with and without winglet cases. Added mass is found to be lesser in winglet case in comparison with no winglet case. Residual flow in the beginning of downstroke is spread close to wing tip for winglet case over a distance of approximately unit span at same level as wingtip while for no winglet case it is spread over twice wing span with an angular orientation. During the beginning of upstroke the residual flow orientation in either case found to be nearly horizontal. Residual flow for winglet case is found to be lesser than that of no winglet case due to added mass effect. With increase in flapping frequency, added mass and velocity magnitudes in the flow field have been increased which is quite intuitive.