Experimental and numerical investigation of the oscillation of an inverted flag

In this paper, we investigate the oscillations of an inverted flag using both experiments and computations. The term "inverted flag" refers to a cantilever-type sheet submerged in a fluid flowing from its free edge towards the fixed end. The flag-oscillations are controlled by the interaction between the destabilizing flow instabilities and the stabilizing structural stiffness. Recent studies (Kim et al. 2013) have demonstrated strong large-amplitude selfsustained oscillations of the flag that can potentially be exploited for beneficial purposes such as harvesting energy. Experiments are carried out in an open-loop wind tunnel and instantaneous velocities are measured with Planar Particle Image Velocimetry (PIV). The corresponding numerical simulations are undertaken using a recently-developed CURVIB-FE-FSI approach to simulate fluid-structure interaction with strong nonlinear deformation of thin structures (Gilmanov et al. 2015). The experimental results and computations are in good agreement. Phase averaged analyses reveal that the inverted flag undergoes oscillations due to vortex induced vibration and exhibit a periodic motion that results in large-amplitude flapping over a finite band of free-stream velocities. At higher velocities, the flag exhibits a fully-deflected mode, which is in agreement with the observations of Kim et. al. (2013). The flow and structure interactions are governed by growth and breakdown of large-scale structures at the leading-edge of the flag. The turbulent fluctuations are a maximum in the vicinity of the leading edge during the forward motion of the flag, and the fluctuations dissipate as the flag retracts.