An experimental and numerical study into the effect of submerged vegetation on the generation of turbulent flow structures

Vegetation within river channels has a profound influence on the functioning of fluvial systems and can significantly affect: i) flow resistance, which influences water conveyance and therefore potentially increase flooding; ii) sediment transport rates; and iii) biological activity. Vegetation also generates turbulence that drives both mixing and diffusion processes with strong velocity gradients generated around and above submerged macrophytes. Research has shown that even with sparse vegetation, the production of turbulence from stem wakes can far exceed that produced through bed shear alone. Therefore, an understanding on the processes and controls of vortex shedding remains a critical need for understanding how vegetation leads to energy losses in rivers. Here we report on a series of flume experiments that initially use Polyterafluorthylene tubing as a surrogate for vegetation and the move second set of experiments that use Vallisneria spiralis (Tape Grass). Flow measurements were taken using standard Particle Image Velocimetry and endoscopic Particle Image Velocimetry which permitted millimetre scale flow measurements at 100 Hz temporal resolution within and above the plant canopies. This data was used for the necessary boundary conditions and a validation data set for development of a novel Computational fluid Dynamics scheme where a dynamic Mass Flux Scaling Algorithm (MFSA) is used to represent vegetation. Here, each stalk is considered as a stack of cylinders and movement is predicted by a dynamic flexible cantilever beam equation applied to the stack. Both the laboratory and numerical results demonstrate vortex shedding from the top of the canopy, which is a precursor to wake instability. Moreover, the energy transfers that occur within, and just above, the canopy are shown to involve a limited free mixing layer that produces a range of complex coherent flow structures that include transverse and secondary vortices, in the form of rolls and ribs, and shear layers that are dominated by Kelvin-Helmholtz type vortices.