Kameswara Vepa

Fully coupled time domain modelling of 3D floating bodies and mooring systems in regular and irregular sea states

Research on floating bodies like Wave Energy Converters (WECs) and Laser Imaging Detection And Ranging (LIDAR) systems has recently known a large growth. To study the minute details of the working model, it is important to study the effect of interactions between the waves, floating bodies and the mooring systems that are controlling the motion of the floating body. To achieve a more realistic numerical model in the time domain, a number of programs are linked together. The idea is to use the strength of each individual program for better results and also reduce the computational time. This paper provides a solution in the direction of using a fully coupled time domain coupling code that controls the data flow between a fluid solver, a structural solver, and a kinematic system simulator. The fluid solver uses the Smoothed Particle Hydrodynamics (SPH) method for calculating the wave forces and responses to the forces exerted by the mooring system and the floating body. The SPH method is found to be good at simulating the gravity driven free surface flows which include both regular, irregular and breaking waves. Based on the type of material used for the floating bodies and the mooring system, the structural solver simulates the response of the structural parts to the oncoming wave loads and the loads due to the mechanical system within the floating body. The structural solver uses the well established Finite Element (FE) Method for calculating the loads on the structural parts of the whole system. The structural code is capable of simulating any complex shaped body and also material failure. The material model can be either rigid, elastic or plastic. It is also capable of modelling composite material models. The kinematic system simulator calculates the internal mechanical functioning of the floating body based on the motion of the outer structure. All the codes are extensively tested individually for their accuracy in performing the simulations and then coupled. Two- and three-dimensional fully coupled models are studied for calculation times and accuracy of results, and scaling is tested through parallelization on a large HPC cluster. The time step size of the whole model can be controlled by the user. Calculation times and memory requirements vary largely based on the factors like: domain size, SPH particle size, material model used for the floating body and the mooring system, complexity of the mechanical system inside the floating body.

Comparative Study of Slamming Loads on Cylindrical Structures

Wave impact or slamming is a phenomenon characterized by large local pressures (10 bar or more) for very short durations (order of milliseconds). Slamming loads can cause severe damage to the structure [1]. Different numerical approximation methods are available for simulating the fluid structure interaction problems. Traditional mesh techniques use nodes and elements for approximating the continuum equations whereas particle methods like smoothed particle hydrodynamics (SPH) approximates the continuum equations using the kernel approximation technique and hence can be used for a wide range of fluid dynamics problems [2]. Since composite materials are finding increased application in the ship building industry because of their low weight and high strength properties, it is important to understand the effect of slamming loads on composite structures [3]. Normally, composite structures are made quasi-rigid to resist slamming loads, but inducing some deformability helps in reducing the incident pressures and at the same time reduces the overall weight of the structure and in turn the material cost. On the other side, inducing deformability effects the durability of the structure. In this paper, the effect of slamming on two-dimensional cylindrical structures is studied using three solvers i.e., 1) SPH solver, 2) Explicit solver and 3) Implicit solver. In the case of SPH solver, water is modelled using SPH particles and cylinder is modelled using finite elements (FE), in this case shell elements. A coupling between the SPH and FE solvers is made to simulate the fluid-structure interactions. Contact is modelled using the contact algorithms. In the case of the explicit solver, water is modelled using hexahedron or brick elements with one element in the thickness direction since symmetry is applicable along the thickness of the cylinder. Shell elements are used for modelling the cylinder and contact is handled using node to surface contact algorithm. In the case of the implicit solver, water is represented by pure two-dimensional elements. Quadratic elements are used to represent the continuum around the cylinder and triangular elements are used to represent the far off field and also to control the mesh movement. Line elements are used to represent the cylinder in this case. Two models are tested in all the three solvers: 1) rigid cylinder and 2) deformable cylinder. A comparative study of these three solvers shows that the implicit solver needed more calculation time compared to other solvers. The SPH method required less particles than the number of nodes in the other two methods to converge on the peak pressure. All three solvers show reduction of peak pressure in case of the deformable cylinder.