Numerical Study on Hydrodynamic Force and Wave Induced Vortex Dynamics around Cylindrical Pile

Three-dimensional numerical simulation of regular waves passing over cylindrical monopile was conducted to investigate the hydrodynamic forces and vortex dynamics. The rectangular wave flume and monopile were modeled with a solver; available in the open-source CFD toolkit OpenFOAM®. This solver applies the Reynolds-Averaged Navier-Stokes (RANS) equations with the volume of fluid technic (VOF) for tracking free surface. To validate numerical model, results were compared to experimental data, and admissible agreement was seen. Computations were done for four cases with different wave characteristics consequently for different Keulegan- Carpenter (KC) numbers. In-line force acting on pile was studied and the results indicated that the total in-line force was influenced as the KC number varied. The vortex dynamics around the pile was also investigated by means of the Q criterion and vorticity field. Furthermore, variation of bed shear stress around the pile was investigated; it was concluded that the bed shear stress was influenced by KC number which is the result of existence of horseshoe vortices. The bed shear stress near the pile was negative due to existing of the horseshoe vortex. It began when KC exceeds 6; while by increasing KC up to 20, the magnitude of negative values of bed shear stress near the pile increase which implied the horseshoe vortices were completely formed.