Large-eddy simulations for a free-surface turbulent flow past vertical cylinders

摘要

Large-eddy simulations (LES's) for free-surface turbulent flows are conducted by solving the three-dimensional (3D) spatial-filtered Navier-Stokes (NS) equations with the Galerkin-weighted residual minimization. The turbulent closure is based on the Renormalization Group (RNG) theory. The solution procedure employs a projection method that uncouples the pressure from the velocity through enforcement of mass conservation. Fully nonlinear free-surface boundary conditions (BC's) are adopted. The complex processes of boundary layer separations, vortex shedding and turbulence transition beneath the free surface, particularly the wake-wave interactions are investigated. Calibration is made by a uniform flow past a vertical circular cylinder. The computed results correlate fairly well with the available experimental data in the time-averaged sense and indicate that the presence of a free-surface reduces the von Karman instability locally near the free surface. Presumably, this is because the flow instability associated with large-scale flow structures and induced by small-scale ones decreases under such a free-moving boundary, and the kinetic energy is partially transferred to the potential energy in order to follow the wave motion. This inference is further sustained by the variations of the computed hydrodynamic forces on rigid body surfaces.