Under the potential flow assumptions the so called geometric non linear effects are

1. Fluid Structure Interactions Research Group Unstructured MEL modelling of 3D ship Hydrodynamics Alberto C. Chapchap – acc1e09@soton.ac.uk - Ship Science Supervisor – Professor Pandeli Temarel Description and Goal Under the potential flow assumptions the so called geometric non linear effects are intimately associated with the instantaneous wetted surface variations, higher order hydrodynamic actions and non linear free surface boundary conditions (kinematic and dynamic). In order to address these issues, and their influence on wave-induced motions and loads, the Mixed Eulerian Lagrangian scheme is investigated and the feasibility of a modified version of it, using Level Set Theory and distance functions to represent the geometric domain, is currently being tested. The main features, per time step, can be summarized as follows: 3D Potential Flow model in time domain for FSI (Mixed Eulerian Lagrangian) 1- Each time step the flow is described by the solution of the Boundary Value Problem (BVP) using the rankine source as Green’s function. 2-Free surface potential and position are then updated in a Lagrangian fashion this is written as: Boundary Data Loop Initial Boundary Value Problem (re) meshing Partial Results – Radiation Problem of a forced hemisphere Update Free Surface position and potential value Figure 2 :Crude mesh consisting of 3217 triangles used to perform forced heave oscillations of the hemisphere Update Boundary Conditions and Domain Geometry Applications • Seakeeping analysis in time domain to study the effects of geometric non linear boundary conditions on wave-induced motions • Non Linear hydroelasticity analysis ( effects of the geometric non linear free surface boundary conditions on the radiation potential of flexible floating structures are of particular interest) • Linear / Non linear wave generation Time series of heave force Hydrodynamic Coefficients Partial Results for a wigley hull performing forced heave oscilations Figure 3 :Heave hydrodynamic force of a heaving hemisphere time series for different normalized frequencies (kRs) using linear free surface boudary conditions , compared with the analytical results from Hulme (1982) Figure 4 : Hydrodynamic coefficients obtained from the time series by means of a Fourier transform as fucntion of kRs. Future Work • Replace the linearised form of the kinematic and dynamic free surface boundary conditions for their exact versions, in order to take into account to the so called non linear geometric potential flow effects. • Extend the methodology to hydroelasticity and to free floating bodies in the presence of incident waves Figure 1: Heave hydrodynamic force of a Wigley hull undergoing forced oscillations without forward speed. Acknowledgement This project is supported by funds from Lloyd's Register, Strategic Research Group. FSI Away Day 2012