Comparison of Hydrodynamics of Flow around a Single, Side by Side and Tandem Arrangements of Cylinders using oOpenFOAM in between LES and RANS

Scour happens in steady current mainly because of horseshoe vortex and the contraction of streamlines at the side face of the pier, which takes sediment away from the edge of the pile. Lot of works have been carried out for hydrodynamics in local scour, most of which is focused on experimental investigation. In this paper authors have modelled the hydrodynamics in the scour by means of numerical simulations. Here, numerical simulations of flow around a single cylinder, side by side cylinders and tandem cylinders have been carried out using openFOAM toolbox for both circular and square cylinders. The computational flow domain is taken as 26D×20D×3D where D is the diameter of the cylinder. Reynolds number, is varied from 300 to 10000 (representing laminar flow to turbulent flow). First grid convergence study is carried out. Then the simulated result of velocity and pressure distribution and streamlines is analysed. Comparison of results between circular and square cylinders are presented between Reynold Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES) simulations. Effect of gap between two cylinders is conducted for cylinders in tandem arrangement. Vortex formation, low pressure and velocity zones are discussed for each of the above cases. In RANS turbulence model k-ε equations is used and in LES turbulence model Smagorinsky model is used. RANS model mainly focuses on averaged or mean properties whereas the LES turbulence model considers filtered values with filtering functions in addition to the mean properties. This permits LES turbulence model to capture larger eddies with larger energy and smaller eddies with smaller energy. It is observed in this study that LES turbulence model has better accuracy compared to the RANS model in the flow domain. The openFoam results are validated against the existing experimental data obtained from Chen et. al (2003) and Zhou et. al (2001).