Dynamic aeroelastic response of a slender triangular flag behind bluff bodies of varying shapes

Abstract

This study explores the aeroelastic performance of a slender steel flag positioned in the wake of bluff bodies with different cross-sectional shapes, namely, C, inverted C (IC), and U shaped cut cylinders — with radial cavities cut at angles of 60° and 120° for each shape, and 180° for U shape. The flag is positioned at varying gap distances ($G$) from the bluff body. This distance varies from 0.5$d$ to 9$d$, where $d$ represents the diameter of the bluff body. A preliminary numerical analysis using the immersed boundary method reveals distinct wake patterns for each configuration. Subsequent wind tunnel experiments align with these findings, showing a range of instability regimes. For the “C-type” bluff body, flutter in flag occurs at gap distances between 3$d$ and 9$d$, while no limit cycle oscillations (LCOs) are observed for closer distances (0.5$d$ to 2$d$). In contrast, for the “IC-type” bluff body, the cut angle strongly influences the flag flutter dynamics, with the 60° cut cylinder exhibiting the most pronounced instability. As the cut angle increases, the instability regime narrows. The “U-type” bluff body results in asymmetric wake interaction, allowing the flag to flutter even in closer gap distances ($G$ = 0.5$d$ - 2$d$), with a subcritical bifurcation observed at $G$ = 0.5$d$, which is not observed elsewhere. The onset of the flutter is notably advanced under the wake of “U-type” bluff body as compared to other bluff body shapes at higher gap distances ($G$ = 3$d$ to 9$d$). Overall, this study underscores the combined role of the bluff body shape and the gap distance on aeroelastic behavior of the flag, offering insights for designing efficient flutter-based energy harvesters at low Reynolds numbers.