Ronan Costello

Hydrodynamic modelling competition: Overview and approaches

This work describes the overall aspects of the Competition on Hydrodynamic Modelling of a Rigid Body, a competition outlined to evaluate different ways to model and simulate the motions of a rigid body in waves. The main objective is to determine a hydrodynamic model for a submerged horizontal cylinder which best predicts a recorded motion to a specific excitation in panchromatic waves. A blind study was performed by the competition participants, i.e., the simulation results were obtained without knowledge of the actual recorded motion of the cylinder. Only the geometry of the cylinder in solid model and the time series of the incoming waves were issued to the participants. The proposed approaches by the participants for modelling the rigid body and the fluid motions are based on the boundary-integral equation methods (potential flow theory) with additional viscous damping forces, where the drag terms are calculated either empirically or via the Navier-Stokes equation method. This paper describes the details about rationale for choice of the rigid body, the experimental tests, the competition criteria and an overview of all modelling approaches proposed by the competition participants.Copyright

COMPARISON OF NUMERICAL SIMULATIONS WITH EXPERIMENTAL MEASUREMENTS FOR THE RESPONSE OF A MODIFIED SUBMERGED HORIZONTAL CYLINDER MOORED IN WAVES

To facilitate commercially relevant numerical design optimization in wave energy conversion accurate and validated simulations of wave body interactions are necessary. Wave energy, more so than almost any other industry, can benefit from such numerical optimization because of the high cost and long period of design iteration in experimental and field testing. For the foreseeable future wave energy device design and optimization will continue to rely heavily on potential flow solvers. Two important prerequisites to successfully using simulations based on these codes are firstly a need to validate the simulation implementation by comparison with experiment and secondly a need to supplement the potential flow solution with experimentally (or CFD) derived coefficients for the forces that are neglected by the potential flow solver. This paper attempts to address both of these prerequisites. A comparison of numerical simulations and physical wave tank experiments on a submerged horizontal cylinder moored in waves is presented. Good agreement between numerical model and experiment is achieved. At operating points where the body response is linear a numerical model based purely on potential flow and linear mooring stiffness achieves excellent results and at operating points where the body response is non-linear a time domain model with frequency independent quadratic damping is shown to give good agreement for a wide range of wave periods and amplitudes.

A MULTI-BODY ALGORITHM FOR WAVE ENERGY CONVERTERS EMPLOYING NONLINEAR JOINT REPRESENTATION

When large relative displacements take place between the bodies in a multi-body Wave Energy Conversion system linearisation of the constraints on motion imposed by the joints between the bodies is no longer valid and a non-linear timedomain analysis is necessary. As a part of the Techno-Economic Optimisation of Wave Energy Conversion (TEOWEC) software, which has been developed at the Centre for Ocean Energy Research (COER), NUI Maynooth, we developed an algorithm for the dynamic simulation of Multi-Body Systems for Wave Energy Conversion (MBS4WEC) with fully non-linear representation of the body-to-body joints. The algorithm is based on the Jointcoordinate formulation, which provides a systematic procedure to transform the mixed differential-algebraic equations of motion in body coordinates, for open chain systems, to a minimal set of ODEs. When a closed-loop chain occurs, the same method can be adopted by removing one or more kinematic joints from each loop. Knowing the topology of the system, a path matrix is generated and together with the formulation of data structures representing the body-to-body joints, the Velocity Transformation Matrix is computed. The main advantage of this approach is a fast and automatic generation of the Velocity Transformation Matrix, which leads to a higher computational efficiency, especially for complex systems. This paper presents the equations underpinning the method together with results for simulation of two specimen floating multi-body systems. These two are a simple multi-body hinged barge and a device with a sliding internal reaction mass. In each case the results are contrasted to the results produced by a linearised analysis of the same system.

Evanescent Wave Reduction Using a Segmented Wavemaker in a Two Dimensional Wave Tank

The concept of a segmented wavemaker, in a two dimensional tank, has been investigated analytically to see if it can reduce the effect of parasitic evanescent waves in a wave tank. Evanescent waves can contaminate test areas in tanks leading to unreliable results, but are typically avoided by establishing the test area two to three times the water depth away from the wavemaker. This space requirement can be quite restrictive in terms of the necessary tank size and, with the increasing interest in off-shore renewable energy, many technology developers may not be able to afford a workspace large enough to accommodate a long tank. Previously, flexible wavemakers have been designed to tackle the problem and, in some cases have proven very effective in eliminating evanescent waves at the wavemaker’s “tuned” frequency. However, flexible wavemakers have been shown to have little benefit in modeling panchromatic seas. Discussed here is the linear potential theory of a segmented wavemaker designed to reduce the evanescent waves in a tank over a large frequency range. Each segment in the segmented wavemaker is programmed with an individual stroke, allowing the system to best approximate the horizontal displacement of the fluid over the depth of the water in a naturally occurring wave. A comparison of the influence of evanescent waves created by segmented wavemakers, piston and hinged paddle wavemakers, on the free surface elevation is presented.Copyright