LES of flow past a porous cylinder

The flow past porous cylinders is important for many applications in river engineering, notably for the case in which a relatively circular patch of vegetation is present in an open channel. The effects of porosity and vegetation characteristics (e.g., mean diameter of plant stems, mean distance between the stems, flexibility) on the total drag added to the flow by the vegetation patch are of great importance to estimate bulk properties characterizing flow resistance in natural waterways. In many previous investigations, rigid circular cylinders were substituted for individual plant stems to study the fluid force on the porous cylinder and the flow structure within and downstream of the porous cylinder. We conduct fully three-dimensional Large Eddy Simulation of flow past a porous circular cylinder with a solid volume fraction (SVF) of 20%. The porous cylinder of diameter D contains 89 smaller circular cylinders arranged in a regular staggered way. The diameter of the small cylinders (plant stems) is 0.04D. The simulations are conducted at a Reynolds number of 10,000 defined with D and the bulk velocity of the uniform incoming flow. Strong wake-cylinder interactions are observed within the porous cylinder. Despite some differences in the dynamics of the separated shear layers of the porous cylinder and the streamwise (bleeding) flow penetrating through the porous cylinder in between the shear layers, the large-scale shedding is qualitatively similar to the one observed in the non-porous case (SVF=100%). However, important quantitative differences with the case of a non-porous cylinder are observed in the eddy content of the separated shear layers and within the near-wake region. In particular, the wake billows form at larger distances from the back of the porous cylinder.