Flow visualization and aerodynamic load calculation of three types of clap-fling motions in a Weis-Fogh mechanism

A flow visualization and aerodynamic load calculation of three types of clap-fling motions in a Weis-Fogh mechanism are performed through experiment and computation. In the experiment, the flow development is visualized using a smoke-wire technique for two wing motion types: ‘fling followed by clap and pause’, and ‘clap followed by fling and pause’. For the computation, the two-dimensional Navier–Stokes equations are solved for the same type of wing motions of the experiment. A good agreement between the experimental and numerical result is observed regarding the main flow features, such as the sequential development of the two families of separation vortex pairs and their movement. For the ‘fling followed by clap and pause motion’, a strong separation vortex pair of counter-rotation develops in the opening between the wings in the fling phase and then moves out from the opening in the following clap phase. For the ‘clap followed by fling and pause motion’, the separation vortex pair develops in the outside space in the clap phase, and then moves into the opening in the following fling phase. Results show that the leading edge separation vortices are dominant features of the flow pattern and cause a large negative pressure distribution near the leading edge. In the ‘cyclic fling and clap motion’ case, the aerodynamic loads of the fling phase after the second cycle of the wing motion when the periodicity of the flow pattern and the aerodynamic loads are established, are much greater than those of the first cycle. However, the aerodynamic loads of the clap phase in the later cycles are not much different from those of the first cycle.