Investigating the flow structure around floating vegetation islands in an open channel

The existence of floating vegetation islands (FVIs), especially the unanchored root canopy beneath it, significantly disrupts the internal structure of fluid while increasing the complexity of the flow pattern. Therefore, this paper aims to investigate the flow structures in the presence of FVIs in an open channel. In this study, a three-dimensional (3-D) computational fluid dynamics (CFD) approach was adopted using the FLUENT software tool. The numerical model’s validation was confirmed with the help of experimental data. The parameter of varying vegetation density (n) for FVIs patches was considered in this study with almost a uniform difference of cylinders in a patch for three considered cases i.e., n = 0.075 cm⁻² (dense; Case 1), n = 0.045 cm⁻² (intermediate; Case 2), and n = 0.015 cm⁻² (sparse; Case 3). The mean flow and turbulent characteristics at various positions and cross sections are presented in detail. The findings revealed that stream-wise velocities of flow decreased significantly within the canopy region (CR) i.e., vegetation region, when compared to the alternate free region (FR) i.e., the unobstructed space between the CR and the side walls, with this effect becoming more pronounced as FVIs’ density increased. The velocity of flow significantly increased in the gap region (GR) between the channel bed and the FVIs canopy column, and the disparity in velocity results between the CR column and the GR column resulted in an S-shaped profile through FVIs. The flow through dense root canopy i.e., with a high solid volume fraction of vegetation in the patch, experienced the highest production of turbulence within the patch zone followed by a prominent large Kármán vortex street behind FVI patches. The study highlighted the impacts of FVIs in open channel flows for effective management of aquatic ecosystems.