Swathi’s PhD work

Flowfield and force evolution for a symmetric hovering flat-plate wing

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AIAA Journal, Vol. 56, No. 4 (2018)


  • 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

Effect of pitch on the flow behavior around a hovering wing

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Experiments in Fluids, Vol. 60, No. 5 (2019)