Unsteady Hydrodynamic Investigation Of Flapping-Wing Propulsion And Interference Effects

In aerodynamics and hydrodynamics, bird flight and f\sh swimming have inspired and guided the development of aircraft and underwater vehicles. It is interesting, however, to note how primitive these man-made machines seem compared with their natural counterparts in terms of intelligence, efficiency, agility, adaptability and functional complexity. The interest in flapping-wing propulsors stems from long-standing observations of fish and birds which all use some oscillating-wing mechanism for thrust generation. With the development of micro air vehicles (MAVs) and unmanned underwater vehicles (UUVs), the flapping-wing propulsion and interference effects have received considerable attention.Under the assumption of incompressible, inviscid, and irrotational flow, the governing equation and boundary conditions are formulated. Panel method is invoked to solve the governing equation in space- fixed frame. Apiecewise linear, continuous distribution of vorticity over the airfoil surface is used to generate the disturbance flow. A small straight-line wake element of unknown length and orientation, which are to be determined as part of the solution, is attached to the trailing edge. A downstream wake of concentrated vortices is formed from the vorticity from the vorticity at earlier times, which is assumed to be concentrated into discrete vortices. Constant source and doublet singularities are distributed over the surface of wing and its wake. The length and orientation of shed wake sheet are designated. Desingularized scheme is used to calculate velocity induced by a discrete vortex or discrete vortex filament. Each wake point is,moved with local fluid particle.Panel method with linear distribution of vorticity is applied to simulate the impulsively started problem, heaving, pitching, combined heaving and pitching motion of a two-dimensional airfoil. Comparing calculated lift coefficients with Wagner function for sudden started airfoil and with Theodorsen function forflapping airfoil, there exists little difference for small disturbance. The effect of vortex blob radius on forces is also examined. Moreover, the effect on thrust and propulsive efficiency is considered for reduced frequency, non-dimensional heaving amplitude, pitching amplitude, phase difference between heaving and pitching motion, wake velocity profile, and the extent of wake rollup.Panel method with linear distribution of vorticity is applied to deal with two impulsively started airfoils in close proximity. The effect of fore-airfoil starting vortex on the lift history of aft-airfoil is explored. Both the enhancement of propulsive performance by two airfoils flapping out of phase and the recovery of wake power are simulated. Considerable attention is directed to investigate interference effects of two flapping airfoils by changing relative position and phase difference. Relative position and phase difference for constructive interaction and destructive interaction are analyzed. The propulsive performance dependence of aft-airfoil on wake velocity profile and vortex interaction modes is also studied.To verify three-dimensional numerical scheme, calculated lift coefficients for impulsively started problem of wings with different aspect ratio are compared with linear theory. Furthermore, the effect of reduced frequency, non-dimensional heaving amplitude, pitching amplitude, phase difference between heaving and pitching motion on propulsive performance is irfvestigated for wings with aspect ratio of two and six. For intereference effect of three-dimensional wing, the dependence of aft-wing's propulsive performance on streamwise distance and phase difference is emphatically analyzed. The relationship between typical vortex interaction modes and aft-wing's propulsive performance is investigated.