Flow Structures around Tandem Slender Cylinders in Shallow Flow

Small scale obstacles in shallow water bodies are highly influential to the sustenance of aquatic ecosystems. These obstacles are mostly emerging stems of vegetation patches that interact with the flow. The wake generated by these stems generate complex flow structures. The dynamics of these flow structures are responsible for the transport of nutrients and pollutants in rivers and streams. To acquire a complete three-dimensional description of these organized flow structures, numerical modelling of the flow field is a viable option. In the present study, the plant stems are simplified as partially submerged slender tandem cylinders in a shallow open-channel flow. A hybrid RANS-LES approach termed as Improved Delayed Detached Eddy Simulations (IDDES) is used to model the turbulence. The free-surface deformation is captured using the Volume of Fluid (VOF) multiphase model with High-Resolution Interface Capturing (HRIC) technique. The shallow open-channel flow past tandem cylinders are simulated for two different Froude numbers. The results of the simulation are validated with available experimental data. The contrast in free-surface deformations between the two Froude numbers and its subsequent influence on the flow field is presented with detailed discussions. Coherent structures in the flow field are captured using vortex identification techniques. The effect of the free surface and the vertical variability of velocity on the development and evolution of these structures are also presented with discussions.