An experimental study of stall flutter kinematic, moment, and fluid dynamic development is presented. This study focuses upon understanding the higher frequency content observed in stall flutter limit cycle oscillations and how that content relates to specific fluid dynamic structures and states. This is accomplished using angular position, pitching moment, and particle image velocimetry measurements of a cyber-physically mounted, NACA 0018 rigid finite span wing undergoing stable limit cycle oscillations. The wing model is mounted with a single degree of freedom in pitch. A conceptual argument is introduced to predict the model's stall flutter susceptibility from the static moment response using two static equilibrium conditions. These conditions lead to a bound on the range of stiffnesses and reduced frequencies where stall flutter is expected to occur. These bounds, which are related to the attached and separated flow states, account for the fundamental frequency of oscillation. It is then shown that stable limit cycle oscillations contain multiple harmonic frequencies and it is argued that these higher harmonics represent important physics. In particular, it is found that the fifth harmonic is correlated with the dynamic stall vortex shedding frequency. Finally it is noted that pitching moment overshoot linearly scales with normalized pitch rate with a correlation coefficient R = 0.85. (C) 2019 Elsevier Ltd. All rights reserved.

Experiments were conducted on a flexible, finite-span cyber–physical wing model in the wind tunnel to study the structural kinematics for a wing undergoing stall flutter. The wing model was designed to be weak in torsion and stiff in bending to exhibit stall flutter oscillations. The physical deformation of the wing surface was mapped at 38%, 58%, 78%, and 98% span using a stereo vision motion tracking system. From these measurements, the wing motion is decomposed and shown to consist of a principally torsional (pitching) oscillation consistent with the first mode for a cantilevered beam in free vibration. A two equation empirical model of the wing motion was then developed and compared to the measured stall flutter motion.

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.

Submitted for the DFD16 Meeting of The American Physical Society Comparison of driven and simulated “free” stall flutter in a wind tunnel1 ETHAN CULLER, JOHN FARNSWORTH, Univ of Colorado Boulder, CASEY FAGLEY, JURGEN SEIDEL, United States Air Force Academy — Stall flutter and dynamic stall have received a significant amount of attention over the years. To experimentally study this problem, the body undergoing stall flutter is typically driven at a characteristic, single frequency sinusoid with a prescribed pitching amplitude and mean angle of attack offset. This approach allows for testing with repeatable kinematics, however it effectively decouples the structural motion from the aerodynamic forcing. Recent results suggest that this driven approach could misrepresent the forcing observed in a “free” stall flutter scenario. Specifically, a dynamically pitched rigid NACA 0018 wing section was tested in the wind tunnel under two modes of operation: (1) Cyber-Physical where “free” stall flutter was physically simulated through a custom motor-control system modeling a torsional spring and (2) Direct Motor-Driven Dynamic Pitch at a single frequency sinusoid representative of the cyber-physical motion. The time-resolved pitch angle and moment were directly measured and compared for each case. It was found that small deviations in the pitch angle trajectory between these two operational cases generate significantly different aerodynamic pitching moments on the wing section, with the pitching moments nearly 180o out of phase in some cases. 1This work is supported by the Air Force Office of Scientific Research through the Flow Interactions and Control Program and by the National Defense Science and Engineering Graduate Fellowship Program. John Farnsworth Univ of Colorado Boulder Date submitted: 29 Jul 2016 Electronic form version 1.4