Effect of submerged vegetation on spatial structure of open-channel flow

Vegetation plays a vital role in natural open channels such as rivers, streams or artificial channels. In this work, a turbulent, sub-critical, open-channel flow (Fr = 0.2) with submerged rigid vegetation is numerically studied, using the ANSYS FLUENT code. The VOF method for free-surface treatment and the standard k-ε turbulence model were used for the numerical simulation of the flow. The vegetation was modeled as vertical, rigid cylinders fixed at the bottom of the channel. Regarding the arrangement of the stems, two cases were examined. In the first case, a single vegetative stem was considered o be in the center of the channel, while in the second, a series of three equidistant vegetative stems were located transversely at the center of the channel. In both cases, the height of the stems was equal to one third of the flow, based on the normal depth of the corresponding open-channel flow. The results showed that the vertical velocity profile in the submerged vegetation layer is significantly different from that in the upper non-vegetation layer, which reasonably followed the log law. Specifically, the vertical velocity profile for the submerged vegetation layer shows an ‘S’- shaped curve with its inflection point being close to the vegetation height. Furthermore, a slight drop in the free-surface where the vegetative stems are developed was observed. The structure of turbulence essentially affected in the neighborhood of the stems, leading to higher values of eddy viscosity at the near-crest area of each stem.