Author(s): W. Y. Chang; George Constantinescu; W. F. Tsai
Keywords: Vegetated flows; Turbulent Wakes; Large eddy simulation; Coherent Structures
Abstract: This paper investigates the turbulent flow within and downstream a circular patch of emerged vegetation (surface-mounted porous cylinder) in an open channel using eddy-resolving numerical simulations. The presence of the plant stems induce the formation of large-scale coherent structures (e. g., the horseshoe vortex system forming around the upstream face of the porous cylinder, the eddies shed in the separated shear layers and the large-scale billow vortices shed in the wake). These types of eddies are similar to those observed in the widely studied case of flow past a solid emerged cylinder. The paper discusses the qualitative and quantitative differences between the flow and turbulence structure past a surface-mounted porous cylinder with a solid volume fraction (SVF) of 11. 5% and past a solid cylinder (SVF=100% ). The porous cylinder of diameter D contains 44 solid cylinders arranged in a regular staggered way. The diameter of the small cylinders (rigid plant stems) is 0. 045D. The simulations are conducted at a Reynolds number of 41, 000 defined with D and the bulk velocity of the uniform incoming flow. Simulation results show that the SVF of the porous cylinder is sufficiently large such that necklace vortices form in front of the porous cylinder in the instantaneous flow field. However, the necklace vortices are much weaker compared to those observed in the case of a solid cylinder. The bleeding flow through the porous cylinder is the main reason why the length of the separated shear layers and the distance at which the wake billows form increase substantially with respect to the case of a solid cylinder. Still, the large scale vortex shedding in the wake of the porous cylinder remains qualitatively similar to the one observed for solid cylinders. The amplification of the mean pressure fluctuations in the near wake is larger in the case of a porous cylinder. A region of high turbulence, driven by the interaction of the bleeding flow with the small rigid cylinders that form the patch, is observed within part of the porous cylinder.