Hydrodynamics around a circular collared cylinder system on a rigid bed

This study employs large eddy simulation to investigate the hydrodynamics around a collared cylinder system. To achieve this objective, both experiment and numerical simulations were conducted. The experimental study was performed for bare cylinder and cylinder with a collar at bed level using an acoustic Doppler velocimetry to validate the simulation results. In the numerical simulation, four cases were analyzed: an isolated cylinder (without collar), a cylinder with a collar at the bed level (z/H=0), and cylinder with collar at elevated positions of z/H= 0.25 and z/H= 0.5, where z represents the vertical position of the collar from the rigid bed and H is the flow depth. From the simulation results, various hydrodynamic parameters were analyzed, including mean velocities, streamlines, turbulent kinetic energy (TKE), Q-criterion, rotational tensor, and normalized Reynolds shear stress (RSS). Additionally, peak RSS, peak TKE, Strouhal number variation with collar elevation positions, integrated turbulent momentum fluxes (ITMFs), and vortical structures were investigated. It was observed that the collar placed at the bed level significantly modifies the flow field by reducing both streamwise and vertical velocities and weakening the streamline patterns around the cylinder. The results show clear recirculation zone forms downstream of the cylinder, while a marginal attenuation of TKE and Q-criterion, indicating reduced turbulence intensity and coherent structures. From the RSS components and ITMFs, it is observed that the cylinder with a collar at bed level suppresses near-bed turbulence, exhibiting the lowest magnitudes of streamwise lateral RSS (−u′v′)¯. Furthermore, vortical structure analysis revealed that the presence of the collar enhancing shear-layer instabilities, vortex shedding, fragments the flow into multiple layers, influencing the complexity and strength of vortical interactions around the cylinder.