Why inverted flags flap: An experimental study

Mohammad Tavallaeinejad (1) , Michael Païdoussis (1) , Manuel Flores Salinas (2) , Mathias Legrand (3) , Mojtaba Kheiri (2) , Ruxandra Mihaela Botez (2)

1 Department of Mechanical Engineering [Montréal]
2 Concordia University [Montreal]
3 Structural Dynamics and Vibration Laboratory [Montréal]

The experiments described in this paper aim to examine the global dynamics of inverted flags and to explore the impact of periodic vortex shedding from the leading and trailing edges thereon. The effect of vortex shedding from both leading and trailing edges was investigated. It is shown that suppression of the leading and trailing edge vortices, and also inhibition of the interaction between the two counter-rotating vortices (if they exist), resulted in relatively small quantitative changes in the critical flow velocity, amplitude and frequency; but, the overall dynamics of the system remain intact. More importantly, the large-amplitude flapping persisted for all flags tested in the experiments. Force measurements provide some insights into the relationship between vortex shedding and large-amplitude flapping; a difference between the dominant frequency of the lift and that of flapping was observed for some cases. Moreover, for heavier inverted flags, additional peaks appear in the frequency spectrum of the lift signal, with amplitudes comparable to that matching the dominant frequency of flapping. The experimental results suggest that fluidelastic instability is the underlying mechanism for the flapping motion of heavy inverted flags. The near-identical qualitative behaviour of normal inverted flags and serrated ones with a splitter plate at the trailing edge suggests that the global (or qualitative) dynamics of heavy inverted flags is independent of unsteady vortex shedding from the leading and trailing edges; i.e., periodic vortex shedding is not the cause but an effect of large-amplitude flapping.

Mots clés: fluid-structure interaction; inverted flags; large-amplitude flapping; fluidelastic instability; flutter

Domaines: Mécanique des structures [physics.class-ph]

Mohammad Tavallaeinejad, Michael Païdoussis, Manuel Flores Salinas, Mathias Legrand, Mojtaba Kheiri, et al.. Why inverted flags flap: An experimental study. Second International Symposium on Flutter and its Application (ISFA2020), May 2020, Paris, France. ⟨hal-02482212⟩