Session: 12-02-02 Wave-Structure Interactions II

Paper Number: 125069

125069 - Numerical Modelling of Wave Interaction With Porous Floating Breakwater 

Floating breakwater is a promising solution for protecting marine or coastal structures in relatively deep waters, but the breakwater’s width must be comparable to the wave length for effective wave reflection, which would require an unrealistically large structure for blocking long waves. Making the floating structure porous allows addition energy dissipation when water wave travels through the porous media, and therefore can reduce the structure’s size and the force on the mooring structure. However, there are currently no mature numerical simulation methods tailored to porous floating structures, which poses significant challenges in the design of porous breakwaters. This study aims to develop a solver for simulating the motion of porous floating structures based on the OpenFOAM platform, thereby offering a solution for the design of porous breakwaters.

Our model will utilize the olaFlow  toolkit within the open-source platform OpenFOAM. OlaFlow employs the finite volume method to solve the three-dimensional VARANS (Volume-Averaged Reynolds-Averaged Navier Stokes) equations. Water and air are considered as incompressible fluids, and the Volume of Fluid (VOF) method is adopted to represent the morphology of a free water surface, allowing for the simulation of wave deformation. One of the key features of olaFlow is its provision of wave generation, wave absorption boundary conditions, and wave-flow coupling methods required for various wave conditions in numerical wave tanks.

 

We have developed a 6DOF module for simulating the motion of porous floating structures, the overset mesh and dynamic mesh method were used for treating the movement of floating body. When calculating the 6 degrees of freedom (6DOF) for rigid floating bodies, it is necessary to compute the overall forces acting on the structure by integrating over the rigid body's surface. Existing force calculators in OpenFOAM cannot be applied to porous floating structures, as they lack the capability to calculate forces acting on porous media. In this study, we will follow the approach used in olaFlow to handle porous media. We generalize the forces exerted by the fluid flow on porous media as volume forces on computational grids and incorporate pressure gradient forces due to wave action. This allows us to compute the overall forces on porous media structures by integrating volume forces over the occupied volume by the porous media, enabling the subsequent solution of their motion states. Furthermore, we will introduce the reactive forces generated by the motion of porous media into the hydrodynamic solvers in olaFlow to accurately replicate their motion responses. In this study, one of the key questions is to find the empirical coefficients used in the calculation of volume forces on porous media. These coefficients are dependent on the physical properties of the porous medium, such as porosity and geometric characteristics, as well as flow field characteristics. Therefore, specific studies tailored to the particular medium and problem are required.

In order to address the dynamic response of floating breakwaters under varying loads, our solver will couple the dynamic mooring module MoorDyn, which is based on the lumped-mass Method. MoorDyn is a widely utilized mooring module integrated into the open-source software OpenFAST, commonly used for integrated wind turbine simulations. MoorDyn will be bidirectionally coupled with the 6 degrees of freedom (6DOF) solver for floating breakwaters. At each time step, mooring forces computed by MoorDyn will be transferred to the 6DOF solver as external constraint forces, while the platform motion data (obtained from the 6DOF solver) will be fed back to MoorDyn as motional boundary conditions. It is noteworthy that achieving the values for the chain stiffness parameters is crucial to the accurate computation of mooring forces under various mooring scenarios, e.g., semi-taut and taut mooring.

Furthermore, to verify the numerical model, physical experiments are also conducted to monitor the motion state of the object and calculate the effect of wave elimination. The experimental results and models predictions exhibit good consistency.

Presenting Author: Yiyong Dong Tsinghua University

Presenting Author Biography: Yiyong Dong received his B.A. degree from Tianjin University in 2022 and currently is a Ph.D. student in Tsinghua University from 2022 to now. His research interests are wave-structure interaction and computational fluid mechanics.

Authors:

Yiyong Dong Tsinghua University
Jing Yuan Tsinghua University
Weikai Tan Southeast University