Computation of Unsteady Low Reynolds Number Free-Flight Aerodynamics of Flapping Wings

The observation that thrust is produced when a wing flaps above a particular reduced frequency at low Reynolds numbers has been associated with the formation of a reverse Karman vortex street. Through experiments, numerical computations, and analytical methods, scientists have been able to explain thrust generation in birds and insects. This has increased the interest in the development of micro aerial vehicles. There is, however, a paucity of experimental or computational studies that explain the basic mechanism of thrust generation from the viewpoint of the surrounding vortical structures and the effect of these vortices on free flight when the wing is accelerating in the direction of thrust. In the present work, three-dimensional free-flight computations are carried out on flapping wings that have rectangular planforms composed of NACA 0012 and an elliptical airfoil section, respectively, to demonstrate that thrust is generated as a result of vortex asymmetry. Based on the computed vortex dynamics, it is also shown that the relative convection rates of positive and negative vorticity are a key parameter involved in thrust generation. Numerical computations are performed using the unsteady three-dimensional Navier-Stokes solver composite grid incompressible Navier-Stokes, which is based on the Overture framework for overlapping grids. Comparative analysis with existing experimental/computational data is also presented.