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

摘要

The observation that thrust is produced when a wing flaps above a particular reduced frequency at low Reynoldsnnumbers has been associatedwith the formation of a reverseKármán vortex street. Through experiments, numericalncomputations, and analyticalmethods, scientists have been able to explain thrust generation in birds and insects.Thisnhas increased the interest in the development ofmicro aerial vehicles. There is, however, a paucity of experimental orncomputational studies that explain the basic mechanism of thrust generation from the viewpoint of the surroundingnvortical structures and the effect of these vortices on free flightwhen thewing is accelerating in the direction of thrust.nIn the present work, three-dimensional free-flight computations are carried out on flapping wings that havenrectangular planforms composed of NACA 0012 and an elliptical airfoil section, respectively, to demonstrate thatnthrust is generated as a result of vortex asymmetry. Based on the computed vortex dynamics, it is also shown that thenrelative convection rates of positive and negative vorticity are a key parameter involved in thrust generation.nNumerical computations are performed using the unsteady three-dimensional Navier–Stokes solver composite gridnincompressible Navier–Stokes, which is based on the Overture framework for overlapping grids. Comparativenanalysis with existing experimental/computational data is also presented.