Wake dynamics and hydrodynamic forces of a circular cylinder beneath a free surface

This paper presents two-dimensional unsteady Reynolds-Averaged-Navier-Stokes simulations of flow past a circular cylinder beneath a free surface at a Reynolds number of 4.96 x 104. The effects of the free surface on the wake dynamics and hydrodynamics are systematically examined over a parameter space consisting of the Froude number (0.2-0.8) and the gap ratio (0.1-2.0). For high Froude numbers, it is easier to agitate the water to produce violent surface distortion compared to low Froude numbers. Three surface deformation patterns are identified and the underlying mechanisms are proposed. The wake type transition from the wake featured a jet flow, to the one-sided vortex shedding, to the free-surface modulated Karman vortex shedding, and to the pure Karman vortex shedding is discussed in detail. The strength hierarchy between the three shear layers present in the wake plays a decisive role in the transition of the wake types. An extra recirculation zone appears near the free surface due to the blocking effect of the front blunt body. The proximity to the free surface exerts a modulation effect on the temporal and spectral characteristics of hydrodynamics. Relatively large mean drag force with substantial fluctuations can be induced when (unilateral or bilateral) vortex shedding process occurs. When a jet dominates the wake, the mean drag force stays at a low level and the hydrodynamic fluctuations are suppressed. The circular cylinder is always subjected to a downward thrust which increases as the cylinder approaches the free surface. The critical range of parameter combinations is provided when the wake dynamics and hydrodynamics are not influenced by the free surface.