169 / 2024-02-26 10:09:37

Numerical modelling of capture of floating seeds by single cylinder through capillary force

riparian vegetation,seeds capture,capillary force,random vortex method,Maxey-Riley equation

Vegetation community in the fluvial system plays a key role in shaping river morphology, and it also has an important influence on the riparian ecosystem. The transportation and retention of vegetation seeds are two important factors that control the distribution of riparian vegetation within a river. Riparian vegetation seeds can be easily transported by river flow, and seeds in the river can be captured by riparian vegetation patches. After being captured by vegetation patch, seeds have a chance to germinate and start the succession loop. Therefore, it is important to understand the process of seed capture within vegetation patch. Seeds in vegetation patch can be captured by multiple mechanisms and one of them is the capillary force, while turbulence generated by vegetation stems reduces the possibility of capture. In this study, we focus on the possibility of floating seeds being captured by a single vegetation stem. Previous flume tests show that the possibility of seed being captured by a single vegetation stem is related to flow velocity and seed characteristics, and the possibility follows a gamma distribution. However, physical flume tests were time-consuming to obtain the capture possibility. To tackle this problem, we developed a one-way coupled framework including a flow field solver based on the Random Vortex Method (RVM) and a seed transport solver based on the Maxey-Riley equation. We modified the Maxey-Riley equation to include the capillary force acting on floating seeds. In the simulation, the vegetation stem was represented by a single circular cylinder (with diameter ), and seeds (with diameter ) were released from the upstream side of the cylinder. Through thousands of seed release tests, we found that a seed was difficult to escape from the stem once it was captured. This observation, attributed to the lack of stem vibration, was consistent with early flume experiments. When the ratio  was greater than 4, the calculated capture possibility agreed well with the experiment results and results calculated from semi-empirical formula. With the decrease of the diameter ratio, the capture possibility calculated from the numerical model became less accurate since the small particle assumption of the Maxey-Riley formula was not satisfied. Our results show that the Maxey-Riley equation can be applied to the seed capture problem. However, to build a more general framework that can model seed capture when dc/dp is small, further research is needed.