Hydrodynamics of a single cylinder in free-surface flow

Large-eddy simulations (LES) of flow past a submerged circular cylinder close to a bottom flow are performed to investigate the impact of proximity to the free surface, varying the submergence depth in the range of 0.025–0.105 m, corresponding to Froude numbers (Fr) between 0.26 and 0.53 and at a constant Reynolds number of 13 333. The results of the LES with the level-set method to capture the air–water interface are well validated with the experimental data for time-averaged flow quantities, accurately capturing the upward flow motion that emerged along the lower side of the cylinder due to the near-wall effect. At low Froude numbers (Fr ≤ 0.31), the von Kármán vortex street downstream of the cylinder is barely disturbed by the free surface, whereas for higher Fr the vortex shedding pattern becomes irregular due to proximity to the free-surface, with a rapid loss of coherence of the large-scale vortices. The spatial distribution of the turbulent kinetic energy indicated that the near wake of the cylinder exhibited zones of high turbulence for all cases, with the longitudinal extent of these zones increasing with lower submergence levels. At Fr > 0.31, the proximity of the free surface to the cylinder causes a significant increase in drag and lift coefficients, with a mean negative lift force indicating the flow exerts a downward thrust force on the cylinder. As submergence decreases, the Strouhal number increases to be in a 0.30–0.45 range. Overall, at Fr > 0.31, the free-surface proximity to the cylinder has a significant influence on the near-wake characteristics, requiring the modeling of water–surface deformation to accurately capture the flow dynamics.