Nonlinear fluid damping of elastically mounted pitching wings in quiescent water

Abstract

Abstract

Force moment partitioning and scaling analysis of vortices shed by a 2D pitching wing in quiescent fluid

“…The present work extends the study of Zhu et al (2021) on cycle-averaged vortex-induced damping and examines the instantaneous vortex trajectory, strength, structure, and vorticityinduced moment of sinusoidally pitching wings in a quiescent fluid. We also aim to use the present problem as an example to address the aforementioned potential challenges of applying FMPM to experimental data.…”

“…1(a). The setup is very similar to that used in Zhu et al (2021), but without the cyber-physical control loop. We conduct all the experiments in the Brown University free-surface water tunnel (test section width × depth × length = 0.8 m × 0.6 m × 4.0 m), with a zero flow speed (U ∞ = 0 m/s) to create a quiescent environment.…”

“…Because we pitch the wing sinusoidally, the wing moves symmetrically during pitch-up and pitchdown motions. Moreover, Zhu et al (2021) have shown that the fluid damping induced by LEVs and TEVs are comparable, although there are subtle differences caused by the rounded leading edge and the sharp trailing edge of the NACA 0012 airfoil. Therefore, in this study, we choose to focus on analyzing the TEV dynamics during the pitchdown motion.…”

“…Without a free-stream flow, however, the aerodynamic damping becomes purely positive, as the pitch-induced vortices act as a source of drag (Morison et al, 1950). Zhu et al (2021) have adopted a dynamical system approach to study the nonlinear fluid damping associated with vortices shed from a cyber-physically mounted pitching wing in the absence of a freestream flow. They extracted the fluid damping coefficient using "ring-down" experiments and were able to identify a universal non-dimensional fluid damping coefficient that depends on the pitching frequency, amplitude, pivot location and sweep angle.…”

“…However, their analysis of the complex vortex dynamics was purely qualitative, and no quantitative evaluations of the vortex trajectory and strength were performed. Another limitation of Zhu et al (2021)'s work was that the dynamical system framework was only capable of resolving cycle-averaged vortex-induced damping. No direct measurements of the instantaneous force and moment associated with the shed vortices were available, although correlating this information with the corresponding vortex dynamics could provide useful insights toward a more complete understanding of the underlying flow physics.…”