How Leaves Flutter in Slow Flows and Reconfigure in Strong Flows.

Many flexible plants, fungi, and sessile animals are thought to reconfigure in strong flows to reduce the drag forces that act upon them. It is also thought that many plants flutter in slow flows to enhance exchange, cooling, and light penetration. For example, many broad leaves roll up into cone shapes that reduce flutter and drag when compared to paper cut-outs with similar shape and flexibility. Simple mathematical models of a flexible beam immersed in a two-dimensional flow will also exhibit this behavior. What is less understood is how the mechanical properties of a two-dimensional leaf in a three-dimensional flow will passively allow roll up. In slow flows, flutter is thought to enhance gas exchange and light penetration in the canopy in both terrestrial and aquatic ecosystems. In some cases, flutter in slow flows is greater for leaves than for physical models of similar shapes and flexibilities. In this project, we use computational fluid dynamics and particle image velocimetry to understand both mechanisms. The shape of the leaf, the material properties (including the venation patterns), and the shape of the reconfiguration all play a role in enhanced flutter at low speeds and reduced drag at high speeds.