Aeroelastic instabilities and three-dimensional vortex dynamics of pitching swept wings

We experimentally study the aeroelastic instabilities and three-dimensional vortex dynamics of pitching swept wings, with the sweep angle ranging from 0 to 25 degrees. We show that the aeroelastic instability boundary changes non-monotonically with the sweep angle, due to the non-monotonic power transfer between the ambient fluid and the elastic mount. An optimal sweep angle (10 degrees) is observed to promote flow-induced oscillations. Force and moment measurements show that the power transfer of the aeroelastic system is governed by the magnitude and phase of the unsteady aerodynamic moment. The wing sweep is found to have no effect on the unsteady lift and drag, indicating that it regulates the aerodynamic moment by changing the moment arm. Three-dimensional flow structures measured by multi-layer stereoscopic particle image velocimetry are analyzed to explain the differences in the moment generation for different swept wings. Finally, we employ a physics-based force and moment partitioning method to quantitatively correlate the three-dimensional leading-edge and tip vortex dynamics with the resultant unsteady aerodynamic moment, which further governs the power transfer and thus the stability of the aeroelastic system.