Spanwise Variation of Stall Flutter on a Flexible NACA 0018 Finite SpanWing

Abstract

Experiments were conducted on a flexible, finite-span cyber-physical wing model to study the structural kinematics and flow field development around a wing undergoing stall flutter. The wing model was designed to allow for twisting deformations, while minimizing bending deflections across the span. The physical deformation of the wing surface was mapped using a stereo vision motion tracking system. Specifically, the structural kinematics were captured and analyzed at 38%, 58%, 78%, and 98% span. From these measurements, the wing motion is shown to consist of a principally torsional (pitching) oscillation primarily composed of the first mode for a cantilevered beam in vibration. The periodic development of the flow field was isolated from phase-averaged stereoscopic particle image velocimetry (SPIV) measurements. The SPIV imaging planes were captured at various spanwise positions along the wing, traversing from 58% to 92% span, to quantify the three-dimensionality of the flow field. At large twist angles (αtip > 20) a localized region of flow separation was observed, with its peak centered at 75% span. The region of separation was seen to initiate at the trailing edge and grew progressively in strength and spatial extent as the local wing pitch angle increased. However at the largest pitch angles, a leading edge vortex was not observed shedding from the wing surface unlike similar work focusing on stall flutter of rigid body wings.