12th International Conference on CFD in Oil & Gas, Metallurgical and Process Industries SINTEF, Trondheim, NORWAY May 30th - June 1st, 2017 CFD 2017 INFLUENCE OF THE UPSTREAM CYLINDER AND WAVE BREAKING POINT ON THE BREAKING WAVE FORCES ON THE DOWNSTREAM CYLINDER Arun KAMATH 1* , Mayilvahanan ALAGAN CHELLA 1† , Hans BIHS 1‡ , Øivind A. ARNTSEN 1§ 1 NTNU Department of Civil and Transport Engineering, 7491 Trondheim, NORWAY * E-mail: arun.kamath@ntnu.no † E-mail: acm@ntnu.no ‡ E-mail: hans.bihs@ntnu.no § E-mail: oivind.arntsen@ntnu.no ABSTRACT The interaction of breaking waves with marine structures involves complex free surface deformation and instantaneous loading on the structural members. A typical offshore platform or a coastal struc- ture consists of several vertical and horizontal members exposed to breaking wave action. The breaking wave hydrodynamics and the effect of neighbouring cylinders on multiple cylinders placed in near vicinity is important due to force amplification or reduction resulting from interaction between the cylinders. The kinematics of breaking waves and the hydrodynamics of breaking wave inter- action with a single vertical cylinder have been studied in detail in current literature. Studies have established that the location of a cylinder with respect to the wave breaking point has a major influ- ence on the breaking wave forces on the cylinder. These studies have to be extended to investigate the hydrodynamics of cylinders placed close to each other to understand the modifications in the force regime due to the presence of neighbouring cylinders under a breaking wave regime. In this paper, the open-source Computational Fluid Dynamics (CFD) model REEF3D is used to simulate breaking wave interac- tion with a pair of tandem cylinders. The focus of the study is on the location of the wave breaking point with respect to the upstream cylinder and the consequences for the downstream cylinder. The free surface features associated with the incident breaking wave and the evolution of the free surface after interaction with the upstream cylinder are investigated. The overturning wave crest and the asso- ciated free surface deformation have a major influence on the wave that is then incident on the downstream cylinder. The development of a downstream jet behind the upstream cylinder leads to the nega- tion of the shadowing effect on the downstream cylinder. This can lead to an unexpected higher force on the downstream cylinder. The evolution of this downstream jet and the extent of this phenomenon changes the character of the otherwise shadow region behind the upstream cylinder. A detailed understanding of this phenomenon can provide new insights into the wave hydrodynamics related to multiple cylinders placed in close vicinity under a breaking wave regime. The knowledge regarding force amplification or reduction on downstream cylinders will aid in designing a safer and reliable substructure for marine installations. Keywords: CFD, hydrodynamics, breaking wave, wave force, tandem cylinders . NOMENCLATURE Greek Symbols Γ Relaxation function, [] ρ Fluid density, [ kg / m 3 ] ν Kinematic viscosity, [ m 2 / s] ν t Eddy viscosity, [ m 2 / s] ω Specific turbulent dissipation rate, [ 1 / s] Ω Surface of object, [m 2 ] φ(  x, t ) Level set function, [m] η Free surface elevation, [m] τ viscous shear stress tensor, [ N / m 2 ] Latin Symbols d still water level, [m]. p Pressure, [Pa]. g Acceleration due to gravity, [ m / s 2 ]. D Cylinder diameter, [m]. F Total force, [N]. H Wave height, [m]. S centre to centre separation distance between the cylin- ders, [m]. T Wave period, [s]. U time-averaged velocity, [ m / s]. Sub/superscripts i Index i. j Index j. INTRODUCTION Simulating the propagation and interaction of breaking waves produced by reducing water depth presents challenges due to the complex physical processes involved, with highly non-linear interactions and rapid variations in the free sur- face. Several numerical investigations have attempted to model wave breaking over plane slopes such as Lin and Liu (1998); Zhao et al. (2004); ALAGAN CHELLA et al. (2015a). With the help of these studies, detailed informa- tion about breaking wave characteristics and the geometric properties of breaking waves under different incident condi- tions and bottom slope have been obtained. The empirical coefficients used for the evaluation of breaking wave forces in other structural models and design considerations are de- termined using the breaking wave parameters quantified by these studies. With the advances in computational modelling and with the establishment of CFD models that can repre- sent the breaking process in a satisfactory manner, break- ing wave forces on structures can be calculated. In current literature, Bredmose and Jacobsen (2010) present breaking