Large Eddy Simulation of Flow Past Free Surface Piercing Circular Cylinders

Flows past a free surface piercing cylinder are studied numerically by large eddy simulation at Froude numbers up to $FrD=3.0$ and Reynolds numbers up to $ReD=1×105$⁠. A two-phase volume of fluid technique is employed to simulate the air-water flow and a flux corrected transport algorithm for transport of the interface. The effect of the free surface on the vortex structure in the near wake is investigated in detail together with the loadings on the cylinder at various Reynolds and Froude numbers. The computational results show that the free surface inhibits the vortex generation in the near wake, and as a result, reduces the vorticity and vortex shedding. At higher Froude numbers, this effect is stronger and vortex structures exhibit a 3D feature. However, the free surface effect is attenuated as Reynolds number increases. The time-averaged drag force on the unit height of a cylinder is shown to vary along the cylinder and the variation depends largely on Froude number. For flows at $ReD=2.7×104$⁠, a negative pressure zone is developed in both the air and water regions near the free surface leading to a significant increase of drag force on the cylinder in the vicinity of the free surface at about $FrD=2.0$⁠. The mean value of the overall drag force on the cylinder increases with Reynolds number and decreases with Froude number but the reduction is very small for $FrD=1.6–2.0$⁠. The dominant Strouhal number of the lift oscillation decreases with Reynolds number but increases with Froude number.