NUMERICAL SIMULAITON OF THE VORTEX SHEDDING BEHAVIOR OF A HORIZONTAL CIRCULAR CYLINDER NEAR THE FREE SURFACE USING OPENFOAM

Hydrodynamic characteristics of VIV are expected to significantly change when the circular cylinder is placed close and parallel to a free surface comparing to that of deep water condition. In addition, when a hybrid method is applied, it is of significate importance to understand the feature of vorticity for both free surface and vortex shedding in the wake. A 2D circular cylinder horizontally placed beneath a free surface subject to both laminar and turbulent flow was numerically studied using OpenFOAM in this paper. The main purpose of this study is through the investigation of the influence of the submergence depth (h/D), Froude number (Fr) and Reynolds number (Re) to figure out the critical boundary for the models coupling in the hybrid method, which can provide more insights into the hybrid method with the presence of the free surface. The hybrid method is aimed to deal with the turbulent problems in a way of intermediate cost and degree of accuracy with respect to the traditional single model. The theoretical hypothesis of the hybrid model is that the viscous/turbulent effects are only significant in the limited area, e.g. near the offshore structures and breaking waves, and may be ignored in other areas. And correspondingly, different solvers can be applied for different flow properties. The sketch of an example of the hybrid model is demonstrated by Fig. 1. There are two typical approaches attempt to realize this strategy. The first one is the spatially hierarchical approach, in which the computational domain is decomposed into several subdomains: in the subdomain with significant viscous effects, the Navier-Stokes (NS) model is adopted; in other subdomains, simplified models are implemented. The second one is the model/velocity decomposition, in which a simplified model covers the entire computational domain and a complementary NS model is solved in a subdomain with significant viscous effects to correct the solution of the simplified models. This strategy improves the computational efficiency by limiting the computational domain governed by the time-consuming NS model to a considerably small area. However, in both the domain-decomposition and the model-decomposition approaches, the size of the subdomain with significant viscous/turbulent effects is critical to obtain reliable results. One may agree that the size of the subdomain shall be closely related to the spatial variation of the turbulent viscosity. The feature of turbulent viscosity associated with vortex shedding without the free surface has been studied by Li (2015). Nevertheless, there are significant differences in terms of the spatial variation of the turbulent viscosity between the situation with and without the free surface. And therefore, the main purpose for this study is to systematically examine the influence of the Froude number, the submergence depth and the Reynolds number for the flow past the circular cylinder with the presence of the free surface. And finally, to achieve more understanding of the force properties, vortex street behavior and shedding mode which can contribute to the further coupling of different solvers in the hybrid method.