Effects of Plant Leaf Shape on Drag Forces Imposed by Water Flow

Plants growing in rivers and lakes experience drag forces imposed by flowing water. In order to reduce the drag, plants reconfigure and streamline themselves at multiple scales. The ability of plants to reconfigure depends on their morphological and bio-mechanical properties. The plant leaves play an important role in this process which is still to be better defined and quantified. Therefore, the objective of this study is to determine experimentally the effect of leaf shape on the drag force and its time variability. The experiments have been conducted in an open-channel flume at seven leaf Reynolds numbers ranging from 8x10 to 32x10. The study involved artificial leaves of the same surface area but with three distinctly different shapes (‘rectangular/linear’, ‘elliptic’, and ‘pinnate’). The leaves were cut from a very flexible, thin plastic sheet with a low flexural rigidity. Instantaneous drag forces have been measured using a specially designed drag measurement device synchronized with two ADVs. The results show that under all flow conditions a ‘pinnate’ leaf experiences, on average, 70% and 75% higher drag force with 64% and 68% higher drag coefficient compared to ‘elliptic’ and ‘rectangular’ leaves, respectively. Within a high Reynolds number range, there is a linear relationship between the drag force and velocity for all leaf shapes. The elliptic leaf has a better hydrodynamic shape than the other leaves as it experiences less drag force due to a streamlined shape and smaller frontal area. The rectangular leaf presented a behavior similar to the elliptic leaf. Overall, leaf shape strongly affects the drag force and its statistics through both direct shape effect and shape effect on leaf ability to reconfigure.