Experiments on the merged vortex formation in a flapping plate

A two-dimensional flapping device was designed by consisting of a front flapping flat plate and a rear fixed horizontal plate in a rectangular tunnel. The flow characteristics near the plate and the downstream were studied under different flapping parameters. A high-speed camera was used to obtain the flow images to analyse the shedding vortex formation. Results show that a merged vortex is generated when the plate flaps and, relatively, an inertial vortex is generated when it stops. The vortex merger is in agreement with the Kovaznay model after the non-dimensional time / t T of 0.3, implying its non-equilibrium properties during energy transfer. The length-scale ratio of the merged vortex to the neighbour existing minor vortex is around 2, providing an experimental verification for the simulations of Cardesa et al. [Science 2017]. In addition, the most clear merged vortex is formed when the plate flaps between 30 and 60 degree at a tip flapping linear velocity 3 times bigger than the inlet flow velocity.