From Coherence to Bursting: Ensemble Modal Analysis of LEVs on a Six-Axis Robotic Flapper

Leading-edge vortices (LEVs) are a key mechanism for delaying stall in flapping flight, preventing flow separation at high angles of attack and producing lift beyond typical fixed wing flight. At high Reynolds numbers LEVs burst and become incoherent, making study and characterisation difficult. As flapping flight is leveraged at larger scales, understanding of how LEVs behave in turbulent states is key to designing effective kinematic control of flapping wings. Experimental investigation of flapping flight can be challenging due to the multiple degrees of freedom involved, making systematic studies complex to perform. Using a 6-axis industrial robot, we perform programmatic variation of complex wing kinematics to explore a pitching-translating figure of 8 stroke path in forward flight at Re=15,000. Flowfields have been captured using time-resolved planar particle image velocimetry (2D2C-PIV) at the half span. Up to 500 periodic ensembles of the full flapping cycle have been collected, allowing resolution of timescales from 10^-3 – 10³ seconds to explore multiscale behaviour. We perform space-time proper orthogonal decomposition to decompose the vorticity, such that deviations in the LEV and turbulent wake from the periodic mean are uncovered. Hence, LEV behaviour phase-locked to the kinematics are compared against dynamics where real-time actuation may be required to maintain stability and coherence of unsteady lift-enhancing structures.