Experimental study of lateral dispersion in flexible aquatic canopy with emergent blade-like stems

This paper quantitatively investigates the role of flexibility of blade-like stems and, in particular, the occurrence of stem resonance on lateral dispersion in emergent aquatic canopies. Two sets of experiments are presented: single-stem and canopy tests. In the first set, the flow around single blade-like flexible model stems and the proximity to a resonant state are studied. Wake areas behind four model stems with distinct flexibilities are measured by particle image velocimetry for stem Reynolds numbers between 350 and 850. A single flexible emergent stem bends and oscillates in in-line and cross-flow directions due to periodic forcing associated with the vortex shedding. The plant motion, especially at resonance, affects the width of the wake area and, hence, the extent to which a tracer is dispersed laterally around a stem. The results show that the oscillation amplitude of a stem increases significantly as the vortex shedding frequency approaches the natural frequency of the stem in the corresponding direction. As a result, the size of the wake area is greater for the resonated stems. In the second set of the experiments, lateral dispersion in two different flexible model canopies was measured. The results show that the proximity to a resonant state is the major factor describing the variation of the lateral dispersion coefficient in the experiments for the tested Reynolds numbers and canopies. The dispersion coefficient increases as the vortex shedding frequency approaches the natural frequency of stems in either direction.