Diederik Van Nuffel

Pressure measurement on the surface of a rigid cylindrical body during slamming wave impact

Among all kinds of loads that floating and fixed marine constructions experience, water wave slamming can be considered as one of the most critical. To prevent naval constructions from failing due to slamming impact, slamming loads should be carefully investigated. Besides analytical and numerical calculations, experimental data is of crucial importance. Slamming loads can be measured by performing pressure measurements on the surface of the object during impact. Previous publications showed that precise and correct measurements are very difficult to perform, especially for slamming events with small deadrise angles. Large scatter mostly characterizes these measurements. This research focuses on improving the accuracy and reproducibility of the pressure recordings. Therefore, slamming drop tests are performed on a rigid cylindrical body. Most attention is paid to the bottom of the cylinder where the deadrise angle is 0 degrees.

Experimental study on the impact loads acting on a horizontal rigid cylinder during vertical water entry

This paper experimentally studies the local and global loads acting on a rigid cylinder subjected to water wave slamming. Local loads are hereby expressed in terms of pressure on the cylindrical surface while global loads are investigated in terms of force acting on the complete cylinder. Global impact loads may be better suited for use in design processes. An experimental setup to perform vertical drop experiments to approximate wave slamming is presented and the necessary measuring equipment is described. The experimental results are firstly discussed in the time domain to understand what exactly is happening during the water entry and in what stage the maximum loads occur. The measurements learn that the time scale of the pressure and the force histories is considerably different. Secondly, the attention is focused on the peak values of the time plots. These impact pressures and impact forces are represented as function of the impact velocity. The pressure is hereby given for different positions along the circumference of the cylindrical surface. The experiments show that the impact pressure and force increase very fast with growing impact velocity, indicating that large loads accompany waves with large velocities. Wave slamming is thus an important design criterion for all kind of cylindrical structures when exposed to harsh sea conditions.

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.