This paper presents recent progress in a continuing investigation of the aeromechanical aspects of unsteady flapping wings for micro air vehicles (MAV). Numerical simulations were performed for two-dimensional (2D) pitching-plunging airfoils and three-dimensional (3D) flapping wings, mainly at hover conditions, using an in-house code called INSflow. The results were compared with available experimental data obtained in the water tunnel at the NRC-IAR. The investigation revealed that, at hover conditions, the vortices formed during the airfoil plunging motion may remain near the airfoil and affect new vortex formations, and thus the integral aerodynamic performance. In addition, the flow around the 3D insect-like wing is fully three-dimensional. The tip flow affects the flow separation, reducing the separation intensity. Two-dimensional calculations may over-predict the separation and the shedding vortices, thus affecting the generation of aerodynamic forces.

Figure 1. Instability and transition in a laminar separation bubble, with a sketch of time-averaged streamlines.

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The Reynolds-averaged Navier-Stokes equations with one-equation turbulence models are used to simulate the flow field past a cone-flare geometry in the Mach number range from 5 to 8 with emphasis on the interaction region of the flare shock with the upstream boundary layer. A model based on the physics of shock unsteadiness is used to correct the standard Spalart-Allmaras turbulence model to improve the prediction of the extent of the separation bubble arising from the shock/turbulent boundary-layer interaction and its accompanying peak pressure and aerothermal loads on the surface. The computed results are validated against the experimental data. The limitations of the shock-unsteadiness model and the extent of the improvement in predicting the heat flux are discussed. ARTICLE HISTORY