Experimental and Computational Analysis of Rigid Flapping Wings for Micro Air Vehicles
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© 2014 by David Mayo. Published by the American Institute of Aeronautics and Astronautics, Inc. Targeted experiments in parallel with a systematic computational-fluid-dynamics analysis were performed for a micro-air-vehicle-scale rigid flapping wing in forward flight. Two-component time-resolved particle-imagevelocimetry measurements were performed in an open-circuit wind tunnel on a wing undergoing pure flap-wing kinematics at a fixed wing-pitch angle. Chordwise velocity fields were obtained at equally spaced spanwise sections along the wing (30 to 90% span) at two instants during the flap cycle (middownstroke and midupstroke) for the reference Reynolds numbers of 15,000. The flowfield measurements were used for the validation of the threedimensional computational-fluid-dynamics model. The computational-fluid-dynamics analysis used a compressible Reynolds-averaged Navier-Stokes solver to resolve the complex, highly vortical, three-dimensional flow. The objectives of the combined efforts were to understand the unsteady aerodynamic mechanisms and their relation to force production on a rigid wing undergoing an avian-type flapping motion. Overall, the computational-fluiddynamics results showed good agreement with the experimental data for resolution of the overall highly unsteady and vortical flowfield. A control-volume approach used to calculate the strength of the leading-edge vortex from the particle-image-velocimetry measurements and from the computational-fluid-dynamics-generated flowfields showed comparable results. A hybrid momentum-based method was used to estimate the sectional vertical force coefficient from the particle-image-velocimetry-measured flowfield, which agreed well with the computational-fluid-dynamics force prediction over a range of flapping frequencies and wing-pitch angles. In general, it was observed that the flow over the wing was highly susceptible to changes in induced angle of attack resulting from the flapping motion and variations in reduced frequency, which manifested in the predicted airloads. Based on the computational analysis, the spanwise flow component was not significant, except near the wing tip, and therefore most of the vertical force and propulsive thrust produced could be explained using the magnitude and direction of the sectional lift and drag forces acting on the wing. For the present wing kinematics, most of the upward vertical force was produced during the downstroke and positive propulsive thrust during the upstroke, which shows the need for appropriate temporal and spanwise pitch modulation of the wing along with flapping to produce positive vertical force and propulsive thrust during the entire flap cycle.
author list (cited authors)
Mayo, D. B., Lankford, J. L., Benedict, M., & Chopra, I.