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

Abstract

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.

The Impact of Plant Oscillation on Dispersion in Emergent Aquatic Canopies

“…Note, Equations and were used here to predict the effective canopy density, φ′ . There is also a good agreement (with R 2 = 0.91) between the model prediction and all experimental data collected by Jamali and Sehat (2020), highlighting the validity of the presented model in predicting transverse dispersion for a wide range of canopy and flow conditions. Furthermore, the dispersion coefficient increases with vibration amplitude and thus with the effective canopy density which highlights the importance of many parameters such as reduced velocity, instability parameter, drag, and lift coefficient in steady flows.…”

“…Vertical bars represent the standard deviation across all experimental data. Circle markers show Jamali and Sehat (2020) data. The regression ( R 2 ) and mean‐squared meter are 0.89 and 0.005 respectively, indicating an excellent agreement between the experimental and predicted dispersion coefficients.…”

“…Although the subject of lateral dispersion in flexible aquatic canopies has received attention recently, there is a demand for further understanding of the impact of plant oscillation and resonance on dispersion considering its potentially significant impact on material transport. Jamali and Sehat (2020) studied the impact of flow‐induced vibration and resonance on the size of wake area behind individual flexible stems. Their results revealed that, in the resonant state, the amplified stem vibration leads to a larger wake area behind a stem which can in turn impact rates of dispersion.…”

“…Flexibility is one of the most important physical factors that impact mixing in aquatic canopies (Abdolahpour et al., 2018; Jamali & Sehat, 2020; Variano et al., 2009). The interactions between a flexible canopy and the flow results in plant oscillation in transverse and stream‐wise directions (Gosselin, 2019; Sumer & Fredsoe, 2006).…”

Hydrodynamics affected by submerged vegetation with different flexibility under unidirectional flow

“…Kouwen and Li (1980) measured the modulus of elasticity of natural vegetation and recorded distributions ranging from the order of MPa to GPa. This paper selected three materials, LDPE, polyurethane, and silicon (Jamali and Sehat, 2020). Young's modulus of the three materials ranges from 0.0018 to 3 GPa, and the densities range from 1,007 to 1,180 kg/m 3 .…”