- Motivation: Analysis of the dynamics of coherent structures around a flapping wing helps in explaining the flow physics, improve flow modeling, and predicting the aerodynamic forces, and will lead to a be er design of flow control surfaces for unsteady cases.
- Current work: Gives a deeper insight into the life-cycle of the flow features and their dynamics in a flapping cycle.
- Experiments: Phase locked particle image velocimetry (PIV) is carried out to a hovering flat plate wing that mimics hoverfly kinematics.
- Finite time Lyapunov exponent (FTLE) is used to identify and track salient features in the flow.
- The maximizing ridges of the FTLE field are effective at identifying coherent structure boundaries and evolution dynamics in vortex dominated flows.
- pFTLE gives the region where flow diverges locally.
- nFTLE gives the region where flow experiences local attraction.
- The intersection of nFTLE and pFTLE ridges gives topologically relevant saddles which help in understanding the flow dynamics.
- A flapping cycle is characterised by 4 stages based on the LEV development.
- The saddle lift-off from the trailing edge is the responsible mechanism for LEV lift-off and onset of reverse flow.
- The LEV circulation increases up to about 3.8 convective time scales before splitting into multiple concentrations.
- Lift and drag correlate with the vorticity production which is in turn dependent on stroke velocity.
- EMERGENCE: Vorticity accumulates to form a LEV, grows in chord-normal direction
- GROWTH: Full saddle and half-saddle emerge indicating that the LEV binds to the wing and grows in the chord-wise direction
- LIFT-OFF: Saddle lifts off of the wing allowing a strong reverse flow and formation of a strong secondary vortex
- BREAKDOWN AND DECAY: The LEV breaks down into multiple concentrations of vorticity and decays around the wing