Experiments and numerical simulations of flapping wing flight

Abstract

Aerodynamic theory for fixed wing aeroplanes, in the high Reynolds number limit, has now been well established. In nature however, it is the flapping motion that dominates, and it is observed that the frequency of flapping increases with decrease in size of the wing. In insects, unsteady flapping motion increases the lift produced by wing, above and beyond that generated at constant velocity or that predicted by steady state aerodynamic theory. Thus if one has to make use of the flying mechanisms of these tiny flying objects, for example to build a Micro Air Vehicle (MAV), one has to look in to the theory of unsteady aerodynamics. Engineering principles needed for an optimum design of small mechanical objects, which can use unsteady aerodynamics for their propulsion and lift, have not yet been established. In this thesis work we investigate the aerodynamics of flapping wing using experimental and computational techniques. This thesis consist of four chapters. In the first chapter the motivation to study flapping-wing flight is discussed, along with a literature survey and definition of the problem. In the second chapter, experimental techniques used and the experimental setup is elaborated. Results from the experiments is also included in this chapter. Numerical simulations of flapping wings were also carried out, and the technique and the results from these simulations will be discussed in chapter three. Results from numerical simulations will be compared with that from experiments. In the last chapter the consolidated results and conclusions of the work is given. In the following paragraphs the contents of these chapters are explained in brief. The experimental setup consisted of two rigid flapping wings made of perspex, and were given the desired kinematics using a stepper motor. The wings were made to flap in a water tank, and direct dye injection and streak flow visualization techniques were used. Two kinds of kinematics were given to the wings : one in which the downstroke time is same as the upstroke time, and the other in which the downstroke time is less than the other. We found qualitatively different flow fields for these two cases. We have also looked into the near field pictures to comment on the qualitative change in the flow field that is observed. Another mechanical model with four wings and five degrees of freedom was also built, and flow visualization carried out. Numerical simulations of 2-D flapping wings using inviscid discrete vortex method was done. We observe switching of large eddy formation from formation during downstroke to that during upstroke after a few flapping cycles. The net force generated in the vertical direction was computed using control volume approach.