Fluid damping scaling of elastically mounted pitching wings in quiescent water

Fluid damping plays an important role in shaping damped oscillations of aeroelastic systems. In this study, we experimentally characterize the nonlinear fluid damping associated with vortices shed from the rounded leading edge and the sharp trailing edge of a rigid but elastically mounted pitching wing in the absence of a free-stream flow. We simulate the dynamics of the elastic mount using a cyber-physical system. We perturb the wing and measure the fluid damping coefficient from damped oscillations over a large range of pitching frequencies, pitching amplitudes, pivot locations and leading-/trailing-edge sweep angles. A universal fluid damping scaling based on the Morison equation is proposed and validated. Within the small-amplitude limit, the scaled non-dimensional fluid damping is found to increase linearly with the pitching amplitude, with a constant slope corresponding to the unsteady drag coefficient. This slope decreases as the pitching amplitude increases, presumably because the shed vortices no longer follow the rotating wing. Flow fields obtained using particle image velocimetry (PIV) are used to explain the nonlinear behavior of the fluid damping.