Filament flexibility delays separated-vortex-ring instability in dandelion seeds

Existing aerodynamic models of dandelion seeds have treated the filament as rigid or statically prescribed, leaving flexibility-induced wake stabilization unresolved. A three-dimensional partitioned fluid–structure interaction framework coupling large-eddy simulation with an absolute nodal coordinate formulation solver examines compliance at ReD≈470, with additional fixed-compliance cases at ReD=350 and 600. At ReD≈470, Ca=3.06×10−7 retains a centered separated vortex ring and reduces the standard deviation of the resultant transverse-force coefficient by 40.1%, whereas Ca=3.83×10−7 develops late-stage wake inclination and bilateral shedding similar to the rigid case, with the standard deviation reaching 3.0% above the rigid value. At intermediate compliance, azimuthally non-uniform breathing-like motion repositions local passages and is associated with bilaterally balanced leeward low-pressure cores, a reverse-flow centroid offset 19.2% below the rigid value, and weaker transverse velocity gradients, despite little change in the sectional forward-flow partition. Together with the shared low-frequency structural–aerodynamic response, this reorganization characterizes a lower-amplitude coupled state that delays asymmetric shedding, whereas excessive compliance reintroduces wake distortion and transverse-load unsteadiness. The three sampled cases spanning Ca=(2.55−3.06)×10−7 retain at least 99.912% of the rigid mean drag and reduce the transverse-force standard deviation by 40.1%–42.1%. Their longitudinal drag fluctuations increase, but the added standard deviation is only 0.099%–0.160% of the rigid mean drag. At fixed Ca=3.06×10−7, mean drag exhibits an overall decrease broadly consistent with the theoretical ReD−2/3 trend but differs from the corresponding prediction by less than 7.9% over 350≤ReD≤600. The normalized lateral response and force-component trade-off vary with Reynolds number, with pronounced transverse attenuation occurring only at ReD=470 among the sampled conditions.