Numerical Simulation of Wave-Current Interaction in Laboratory Basins

Dao, My Ha (Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR)) | Lou, Jing (Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR))

OnePetro 

Abstract

In recent years, laboratory testing has become an important part of marine and offshore activities. In this field, there is a growing demand of better understanding of the wave and current interaction and its impact on offshore structures. Complementary to the physical testing, numerical simulations play an equally important role for fine-tuning of the experimental set-ups as well as further detailed investigations. However, the complexity of wave current interactions poses great challenges, not only on laboratory experiments but also on numerical models. For instance, reflected wave and current are unwanted but unavoidable problems in laboratory experiments due to the physical enclose of laboratory-scale basins. Nevertheless, these problems are often overcome in numerical models by using artificial relaxation zones or open boundary conditions which could allow almost ideal boundary conditions. That might, however, lead to discrepancies in experimental and numerical results, especially in prolonged experiments. Aiming to address some of these challenges, the present work focuses on simulations of wave and current in actual laboratory basins taking into account of main physical features. The numerical model would represent the experimental set-up as closely as possible, including wave generation by paddle motions and wave absorption by beach. Simulations of wave and current interactions validated with experimental data will be presented.

Introduction

Offshore structures such as floating platforms often have to operate in harsh conditions in the oceans. Rigorous but costly laboratory tests are required to ensure the safety of the structures under extreme operational conditions. Laboratory testing of structures under combined wave and current conditions could be carried out in sophisticated ocean basin facility such as MARIN Offshore Basin in The Netherlands (Buchner et al., 1999). The new basin facility, FloWave, in the University of Edinburgh (Ingram et al., 2014) even allows combinations of waves and currents in any direction. Parallel to the advancement of laboratory experiments, numerical simulations are becoming popular with the rapid development of supercomputers and numerical methodologies. Numerical simulations are much cheaper but able to produce full range of data in the studied domain at full scale which is impossible in laboratory experiments. Numerical simulations are, however, subject to many sources of uncertainties due to model simplification and necessary assumptions. Combinations of laboratory experiments and numerical simulations could make use the strengths while complementing the weaknesses of the individual approach. To complement a physical one, the numerical model would reflect closely the real phenomena in the physical basin, including the effects of moving paddles to the wave field, reflection on wave absorbers. A numerical wave-current basin using artificial relaxation zones or open boundary conditions could allow almost ideal boundary conditions to be generated. That might, however, lead to discrepancies in experimental and numerical results, especially in prolonged experiments. We have, therefore, chosen to explicitly model the wave generators as moving solid walls to replicate the physical movement of the wave paddles in laboratory basins. The approach was derived in Grilli (1997) for a two-dimensional potential flow numerical wave tank. By modeling physical movement of wave paddles active absorption, such as those used in FloWave, could be implemented. In addition, physical passive wave absorbers are also explicitly modeled to provide closest wave reflection conditions in the physical basins. The current generation systems are also modeled according to the setup in physical basin to model the flow field accurately.