A flexible body interacting with a fluid flow deforms in a complex and frequently unforeseen manner. Understanding and predicting the morphologies of such flexible thin structures under hydrodynamic loadings is central in fluid mechanics. Herein, a simple physical system that enables the systematic investigation of a driven elastic rod at a high Reynolds number is proposed. A straight rod with a spherical sponge attached at one end and rotationally driven at the other is considered. In this setup, the air drag virtually concentrates at the tip of the rod. This enables controlling and quantifying the role of the aerodynamic effects by specifying the parameters of only the sponge. A shape transition, whereby a whirling rod transforms abruptly into a vertically straight, rapidly rotating configuration at a certain critical frequency, is observed. The scaling law for the rod behavior is established by combining experiment, numerical simulation, and analytical theory, which is verified by our experimental and numerical data. Further, a simple analytical model based on forced oscillation with nonlinear damping is presented to capture the physics underlying the observed transition.

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• Elastic deformation
• Flow-structure interactions