LES of FFA-W3 airfoil uncovers 48 flow-through-time normal-force modulation at 3.2° AoA

The study conducted spanwise-periodic wall-resolved LES on the FFA-W3 airfoil section using both Nek5000 and EllipSys at effective angles of attack 3.1–3.3°. A domain width of 0.1c proved sufficient to capture mean flow and dominant disturbances; spectral proper orthogonal decomposition isolated the Kelvin-Helmholtz wavepacket inside the laminar separation bubble while parabolized stability equations confirmed mode growth rates. EllipSys matched Nek5000 mean profiles and KH evolution but damped Tollmien-Schlichting waves due to higher numerical dissipation. Long-time EllipSys runs exposed a slow, periodic modulation of normal force at f*c/U∞=0.021, equivalent to Strouhal number 0.0012 and 7.7 blade rotations on the DTU 10-MW turbine. The oscillation is hypothesized to arise from periodic stall-reattachment cycles sustained by reverse flow on both airfoil surfaces, enabling absolute instability and bubble bursting at angles of attack lower than previously documented. These simulation timescales map directly onto maintenance-relevant load cycles; if the low-frequency mode persists at full-scale Reynolds numbers, it will contribute to cumulative fatigue not captured by steady-state or short-window RANS models used in current certification. Field validation on operating 10-MW turbines within the next 18 months could falsify or confirm the predicted period. The principal limitation remains the modest chord Reynolds number and the absence of rotational and three-dimensional blade effects; increasing Re_c by an order of magnitude while retaining wall resolution would strengthen extrapolation to utility-scale conditions.