CFD Analysis of Waves Over a Submerged Cylinder in Close Proximity of the Free Surface

Abstract

Diffraction programs using potential theory are a quick and effective method in calculating wave forces and ship motions. However in cases where a small layer of water is present on top of a submerged body diffraction calculations overpredict motion and wave force RAOs. This shortcoming of diffraction programs is observed after conducting model tests on a captive submerged cylinder and a free floating SSCV. Unrealistic high wave elevations were predicted by diffraction programs on top of the submerged body. In a previous study a damping lid is implemented [1], to decrease the water elevation to realistic values. In this study CFD is used to simulate the captive submerged cylinder in regular waves with different wave heights, wave periods and different submerged drafts. In addition irregular waves are used in the simulation matching the wave spectra used in the model tests. The simulations are transient and require high CPU usage, therefore the influence of numerical settings on wave propagation is investigated. Turbulent, laminar and inviscid flow are applied to evaluate which flow phenomena are important. The forces in heave and surge direction are validated with model test data of the captive cylinder. The numerical water elevation on top of the captive cylinder will be used to gain insight in the fluid flow and can be used as a guideline for the use of damping lids in diffraction programs. This paper will focus on the CFD simulations and the validation with available forces obtained by model tests of the captive submerged cylinder. It will address the use of regular and irregular waves constructing the force RAO for this non-linear phenomenon. Lessons learnt to improve the CFD simulations as well as limitations of constructing RAOs using CFD from an engineering perspective will be addressed as well.