Low-speed aeroelastic interactions in tandem flags under streamlined and bluff-body wake flows

This study examines the aeroelastic interactions of triangular steel flags in tandem configuration for flow speed (U) up to 9.0 m/s, with inter-flag gap (Gf) ranging from 0.1 to 5 times the leading flag's characteristic length (L). The leading flag is subjected to both streamlined and vortex-induced flows (VIFs). Wind tunnel experiments reveal a subcritical bifurcation route to limit cycle oscillations (LCOs) under streamlined flow, while VIF alters the bifurcation to a supercritical route. First, the identical flags with a natural frequency of 3.59 Hz (first bending mode) and a flutter speed near 7.4 m/s are tested in tandem under streamlined flow. The leading flag's dynamics remains largely unchanged compared to its standalone dynamics, while the trailing flag's response is highly sensitive to its position in the wake of the leading flag. Phase dynamics, including frequency locking and bounded phase differences, emerges at different flow regimes depicting synergistic and destructive coherence. For closely spaced flags (Gf = 0.1L), trailing flag LCO amplitudes are suppressed by up to 50%, while at larger gaps, amplitudes increase by up to 1.5 times. For a non-identical trailing flag with much higher natural frequency (5.48 Hz) and flutter onset speed (U> 9.0 m/s), the flutter onset is advanced and matches that of the leading flag. The non-identical trailing flag oscillates close to its natural dynamics across a wide range of inter-flag gaps and flow speeds but exhibits small-amplitude LCOs driven by the leading flag at very low speeds. Finally, the identical tandem flags are placed downstream of a cylindrical bluff body to examine the impact of VIF conditions. Small-amplitude aperiodic oscillations are observed in the leading flag much below its critical flutter speed, which, in turn, induce a sufficiently strong wake to trigger LCOs in the trailing flag. The leading flag dynamics transitions to periodic oscillations at higher speeds, while the trailing flag exhibits strongly aperiodic oscillations, with one frequency locked to that of the leading flag. The comprehensive nonlinear aeroelastic analysis of tandem flags presented in the study can be utilized for enhancing multi-flag energy harvesting, fluid mixing, and heat transfer.