Numerical simulation of the flow over flapping airfoils in propulsion and power extraction regimes

Inspired by the natural flying and swimming creatures, the application of flapping wings has attracted a great deal of attention from the scientific community in recent years. On one hand, flapping wings are expected to replace conventional rotor systems to build Micro and Nano Aerial Vehicles (MAVs and NAVs) operating at low Reynolds number while on the other hand they are also being investigated for the role of power generation in conjunction with conventional rotary wind turbines. For the development of MAVs/NAVs and understanding of unsteady aerodynamics, the current thesis fills the gaps in the current state of the flapping wing aerodynamics research. Firstly, at Reynolds number Re = 20000, the effect of large amplitude motions on the plunging airfoil propulsion is investigated with both two-dimensional (2D) and three-dimensional (3D) Navier-Stokes (NS) simulations. For a given plunging frequency, it is shown that increasing the amplitude of plunging airfoil motion causes the flow to be chaotic. From 3D simulations, it is shown that the chaotic force generation is not an artefact of 2D assumption. Secondly, the effect of airfoil shape (thickness and camber) variation on thrust and efficiency of a flapping airfoil at Reynolds number 200, 2000, 20000 and 2 × 106 is assessed with 2D NS and Unsteady Panel Method (UPM) simulations. It is found that for different Reynolds numbers, there exists an optimum thickness of symmetric airfoil section for maximum thrust and propulsive efficiency. The role of leading edge vortices is shown to be key to the observed performance variation. It is also shown that varying camber does not provide any benefit in terms of thrust or propulsive efficiency. Flapping airfoils can also be exploited to function as power generators, NS simulations are performed to study different configurations of flapping airfoils as power generators which include single and two foils in tandem arrangement with prescribed sinusoidal and non-sinusoidal flapping motions at Re = 20000. Also, a numerical strategy is developed to model a fully flow-driven flapping foil power generator by utilising the aero-elastic response to the lift and moment acting on it. It is found that non-sinusoidal motion produces around 20% more power than the sinusoidal counterpart and the fully flow-driven simulation results show the practicality of flapping wing power generators