  The effects of sweep angle and reduced frequency on the leading-edge vortex
(LEV) structure over flapping swept wings in the Reynolds number ($Re$) range
of $\mathbf{O}(10^4)$ are yet to be completely understood. With increasing
interest in designing bio-inspired micro-air-vehicles (MAVs), understanding LEV
dynamics in such scenarios is imperative. This study investigates the effects
of three different sweep angles ($\Lambda = 0^\circ$, $30^\circ$ and
$60^\circ$) on LEV dynamics through high-fidelity improved delayed detached
eddy simulation (IDDES) to analyze the underlying flow physics. Plunge ramp
kinematics at two different reduced frequencies ($k = 0.05$ and $0.4$) are
studied to investigate the unsteady motion effects on LEV characteristics. The
leading-edge suction parameter (LESP) concept is applied to determine LEV
initiation, and the results are verified against flow field visualization for
swept-wing geometries. The force partitioning method (FPM) is used to
investigate the spanwise lift distribution resulting from the LEV. Distinct
peaks in the lift coefficient occur for the high reduced frequency case due to
the impulse-like plunging acceleration. This causes the LEV to detach from the
leading edge more quickly and convect faster, significantly affecting the lift
generated by the wing. As reduced frequency increases, the LEV breakdown
mechanism switches from vortex bursting to LEV leg-induced instabilities. These
results provide insights into the complex vortex structures surrounding swept
wings at $Re = 20,000$, and the impact both sweep angle and reduced frequency
have on the lift contribution of these flow features.
