Second leading-edge vortex enhancement: a novel vortex-interaction mechanism for stall flutter suppression

Stall flutter is suppressed on a NACA0012 airfoil at a low Reynolds number ( Re

1000–5000) through a counter-intuitive strategy: targeted enhancement of the second leading-edge vortex (sLEV) via phase-optimised surface morphing, in contrast to the conventional paradigm of suppressing vortices. Two distinct flow topologies are identified: (I) boundary-layer-eruption-induced leading-edge vortex (LEV) shedding at large amplitudes (

upper A Subscript alpha A α $A_\alpha$

≥ 10°), accompanied by sLEV formation, and (II) bluff-body-type shedding at small amplitudes (

upper A Subscript alpha A α $A_\alpha$

≤ 5°) without sLEV. A new circulation scaling is introduced to identify topology I, which drives severe structural oscillations. Flow-structure analysis reveals a novel sLEV–trailing-edge vortex (TEV) interaction mechanism that mitigates stall flutter: strengthening sLEV triggers a chain effect that weakens the TEV, elevates the aerodynamic moment trough and disrupts the feedback loop sustaining the oscillation. An energy-based map is constructed to characterise the boundary of stall flutter across control parameters and to enable rapid prediction of control performance. By combining the clustering-based vortex-induced load partitioning method with this energy framework, an optimal phase indicator is established: actuation synchronised with the peak sLEV-induced moment consistently yields near-optimal control performance, providing a direct quantitative link between pre-control flow features and optimal control parameters. A phase-locked-loop strategy further demonstrates that surface morphing with an amplitude an order of magnitude smaller than the characteristic LEV scale can nonetheless achieve complete suppression of stall flutter within the topology I regime. These findings not only demonstrate the efficacy of surface morphing control but also offer new physical insight into stall flutter dynamics and their precision control via targeted vortex interactions.