Reconfiguration and the reduction of vortex-induced vibrations in broad leaves

Abstract

SUMMARYFlexible plants, fungi and sessile animals reconfigure in wind and water to reduce the drag acting upon them. In strong winds and flood waters, for example, leaves roll up into cone shapes that reduce drag compared with rigid objects of similar surface area. Less understood is how a leaf attached to a flexible leaf stalk will roll up stably in an unsteady flow. Previous mathematical and physical models have only considered the case of a flexible sheet attached to a rigid tether in steady flow. In this paper, the dynamics of the flow around the leaf of the wild ginger Hexastylis arifolia and the wild violet Viola papilionacea are described using particle image velocimetry. The flows around the leaves are compared with those of simplified physical and numerical models of flexible sheets attached to both rigid and flexible beams. In the actual leaf, a stable recirculation zone is formed within the wake of the reconfigured cone. In the physical model, a similar recirculation zone is observed within sheets constructed to roll up into cones with both rigid and flexible tethers. Numerical simulations and experiments show that flexible rectangular sheets that reconfigure into U-shapes, however, are less stable when attached to flexible tethers. In these cases, larger forces and oscillations due to strong vortex shedding are measured. These results suggest that the three-dimensional cone structure in addition to flexibility is significant to both the reduction of vortex-induced vibrations and the forces experienced by the leaf. Supplementary material available online at

Strong and weak, unsteady reconfiguration and its impact on turbulence structure within plant canopies

“…A wide range of broad leaves can fold into cones and experience 2D reconfiguration, and thus enter the −4/3 regime. 58 Many terrestrial canopies have a Vogel number between −2/3 and −4/3, 28,30 suggesting that the classes of 1D and 2D reconfiguration identified by previous researchers for flexible strips, plates, and disks can be used to describe the reconfiguration of many plant canopies. However, a greater refinement of models may be needed for more complex plant geometries, and an exploration of the impact of canopy density on reconfiguration is also needed.…”

Clap and fling mechanism with interacting porous wings in tiny insect flight

“…Parabolic flow with maximum velocity U max was driven within a computational channel and upstream of the model wing by applying an external force, f′ ext , to the fluid proportional to the difference between the desired fluid velocity and the actual fluid velocity, as described elsewhere (Miller et al, 2012). The difference between the actual and desired velocities was controlled with a 'stiffness' parameter, k ext =10k targ (where k targ is the target stiffness), such that the difference between the two velocities was always less than 0.1%.…”

Lift vs. drag based mechanisms for vertical force production in the smallest flying insects

“…Flexibility of biological structures has been demonstrated to reduce drag in a number of studies (Koehl, 1984;Vogel, 1989;Etnier and Vogel, 2000;Alben et al, 2002;Alben et al, 2004;Miller et al, 2012). To compare a flexible wing to the rigid case, we selected k beam ' ¼31.…”