Regimes of Honami Wave Motion in Flexible Canopies

The interaction of fluid flow with flexible canopies may cause a regular waving motion of the canopy members. While this phenomenon, known as honami for vegetation in wind, is widespread, little is known about the dynamic coupling which causes it. We conduct wind tunnel experiments on a simplified canopy model consisting of flexible acetate sheets. By varying the sheet thickness, height, spacing and the wind speed, we span a wide range of Cauchy and Reynolds numbers. To capture the fluid-structure dynamics, we perform high-speed tracking of the edges of the sheets as well as phase-averaged particle image velocimetry of the air flow above the moving canopy.

The experiments reveal the existence of two distinct regimes. At low wind speed, a single-sheet-resonance driven “weak” honami occurs, in which the coherence of the motion is passively coupled to the coherence of the flow. As the wind and the interactions between the sheets becomes increasingly intense, the motion transitions to a “strong” honami – reported here for the first time –which is dominated by a solid wave propagating through the canopy. We propose scaling laws for the transition to and the wave motion in the strong Honami, based on dimensional analysis and analogy with the propagation of pressure waves.