CFD Analysis and Design Optimization of Flapping Wing Flows.

摘要

The main objectives of this research work are to perform the CFD analysis of the 3-D flow around a flapping wing in a gusty environment and to optimize its kinematics and shape to maximize the performance. The effects of frontal, side, and downward wind gusts on the aerodynamic characteristics of a rigid wing undergoing insect-based flapping motion are analyzed numerically. The turbulent, low-Reynolds-number flow near a flapping wing is governed by the 3-D unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the Spalart-Allmaras turbulence model. The governing equations are solved using a second-order node-centered finite volume method on a hexahedral mesh that rigidly moves along with the wing. Our numerical results show that a centimeter-scale wing considered is susceptible to strong downward wind gusts. In the case of frontal and side gusts, the flapping wing can alleviate the gust effect if the gust velocity is less than or comparable to the wing tip velocity. The second objective is to optimize the wing kinematics and shape to improve its aerodynamic characteristics. To our knowledge, this is the first attempt to perform high-fidelity combined optimization of flapping wing kinematics and shape in 3-D unsteady turbulent flows. For our optimization studies, an adjoint-based gradient method using the method of Lagrange multipliers is employed to minimize an objective functional with the 3D URANS and grid equations as constraints. It has been shown that some unsteady phenomena such as the clap and fling mechanism found in use by flying insects (e.g., a wasp Encarsaria formosa, or greenhouse white-fly Trialeurodes vaporariorium), maximize the wing propulsive efficiency. These results indicate that the time-dependent adjoint-based optimization method is an efficient tool for design of a new generation of micro air vehicles.