Analysis of flutter dynamics in thin flexible flags under streamlined and vortex-induced flows
Dheeraj Tripathi, Mehdi Ghommem, Abdessattar Abdelkefi, Lotfi Romdhane, George C. Bourantas
Abstract
In this study, numerical and experimental investigations are conducted to gain a thorough understanding of the aeroelastic behavior of flexible steel flags at low air speeds. Numerical simulations are conducted using a two-dimensional computational fluid dynamics solver to analyze the vortex shedding mechanism from a circular bluff body and to assess its effect on a downstream slender body placed at different gap distances. Experiments on a low-speed wind tunnel are performed to examine the flow-induced flutter dynamics of thin flexible flags. Four different flag shapes, namely, triangular, square, rectangular, and trapezoidal, are tested both with and without an upstream bluff body. Under streamlined flow conditions, flutter instability is characterized by subcritical bifurcation routes, while the presence of a cylindrical bluff body induces supercritical bifurcation scenarios. The aeroelastic response of the flag is inspected while varying the gap distance, separating it from the cylindrical bluff body from 0.5 to 15 times the cylinder's diameter. Experimental results show that the shape of the flag significantly influences its dynamic behavior, especially when it is placed in close proximity to the bluff body. The triangular flag exhibits the highest limit cycle oscillation (LCO) amplitudes, while the square flag has the lowest flutter onset speed. A regime of period-2 LCOs is observed in the triangular flag configuration, which is not seen elsewhere. For flags positioned farther away, the dynamics resembles those observed under streamlined flow, due to the vortices weakening with the increasing gap size. Finally, a comparative analysis demonstrates the flow speed regimes of high-amplitude oscillations for different flag shapes and bluff body positions, which can be explored for low speed energy harvesting applications.