Effects of angle of attack on the large oscillations of a thin elliptical cylinder

The effect of angle of attack on the flow-induced vibration (FIV) response of an elastically mounted thin elliptical cylinder has been investigated by measuring the structural displacement and fluid forces acting on the body in water-channel experiments. Specifically, an elliptical cylinder with a cross-sectional elliptical ratio of $\varepsilon =b/a=5$ was chosen due to the presence of a region of vibration response associated with the combined effect of vortex-induced vibration (VIV) and galloping, where large vibration amplitudes nearly eight times the cross-flow dimensions can be sustained. Here, $a$ and $b$ are the semi-minor axis (aligned with the streamwise direction) and the semi-major axis, respectively. The present experimental results demonstrated that the large vibration amplitudes (i.e. where the maximum observed value was approximately 6$b$) generally decrease with the angle of attack, resulting in substantial reductions for $\alpha \gtrsim 2^{\circ }$ (with $\alpha =3.50^{\circ }$ corresponding to a $\sim$ 60% decrease in the maximum vibration amplitude). Particle image velocimetry (PIV) measurements revealed that the dominant vortex shedding mode consists of two single opposite-signed vortices shed per body vibration cycle. The presence of additional vorticity regions that were absent in the zero angle of attack case was also observed, including crescent-shaped wake structures and secondary inline vortices. This study shows the importance of maintaining axial symmetry in such an FIV system, and that the flow incidence angle is an essential consideration for efficient energy harvesting using this elliptical geometry.