Drag and Reconfiguration of Broad Leaves in High Winds

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Abstract
Drag was measured and changes of configuration noted as a variety of leaves, leaflets, and clusters were subjected to turbulent winds of 10 and 20 m s−1. Leaves with acute bases and short petioles had the highest surface-specific drag, fluttered erratically and, most commonly, tore. Leaves with lobed bases and long petioles had lower drag, fluttered little and reconfigured into increasingly acute cones. Pinnately compound leaves had the lowest drag and formed cylinders with leaflets layered alternately. For all but individual white oak leaves, drag coefficients (based on original surface area) decreased with increasing wind speed. Single leaves of white poplar were unstable at all speeds but resisted damage even at 30 m s−1; clusters formed stable cones. These results are contrasted with the behaviour of flags in wind and are related to wind-throw in trees.