An Experimental Study on Lift Force Generation Resulting from Spanwise Flow in Flapping Wings

Using a combination of force transducer measurement to quantify net lift force, high frame rate camera to quantify and subtract inertial contributions, and Digital Particle Image Velocimetry (DPIV) to calculate aerodynamic contributions in the spanwise plane, the contribution of spanwise flow to the generation of lift force in wings undergoing a pure flapping motion in hover is shown as a function of flapping angle throughout the flapping cycle. These experiments were repeated at various flapping frequencies and for various wing planform sizes for flat plate and spanwise cambered wings. Despite the previous identification of the importance of spanwise fluid structures in the generation of lift force in flapping wings throughout the existing body of literature, the direct contribution of spanwise flow to lift force generated has not previously been quantified. The lift force generated by the wingtip vortex in the spanwise plane resulting from the flapping motion has also yet to be directly quantified. Therefore, in the same manner as commonly applied to investigate the chordwise lift distribution across an airfoil in flapping wings, spanwise flow due to bulk flow and rotational fluid dynamic mechanisms will be investigated to validate the existence of a direct component of the lift force originating from the flapping motion in the spanwise plane instead. In addition, the wingtip vortex is investigated to determine precisely how it can augment the lift force at various phases in the flapping motion. The DPIV method was used to identify the presence of strong, stable spanwise vortices and the effect these vortices can have on the generation of lift force. Finally, when flapping a flat plate wing and a wing of identical wing area and aspect ratio, but cambered in span (both wings in hover with no change in pitch), the spanwise cambered wing was found to generate a greater lift force