Numerical Investigation of Focused Waves and Their Interaction With a Vertical Cylinder Using REEF3D

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Hans Bihs ◽  
Mayilvahanan Alagan Chella ◽  
Arun Kamath ◽  
Øivind Asgeir Arntsen
Keyword(s):  
Free Surface ◽  
Stokes Equations ◽  
Free Surface Flows ◽  
Surface Elevation ◽  
Numerical Wave Tank ◽  
Free Surface Elevation ◽  
Wave Tank ◽  
Numerical Wave ◽  
Focused Wave ◽  
The Impact

For the stability of offshore structures, such as offshore wind foundations, extreme wave conditions need to be taken into account. Waves from extreme events are critical from the design perspective. In a numerical wave tank, extreme waves can be modeled using focused waves. Here, linear waves are generated from a wave spectrum. The wave crests of the generated waves coincide at a preselected location and time. Focused wave generation is implemented in the numerical wave tank module of REEF3D, which has been extensively and successfully tested for various wave hydrodynamics and wave–structure interaction problems in particular and for free surface flows in general. The open-source computational fluid dynamics (CFD) code REEF3D solves the three-dimensional Navier–Stokes equations on a staggered Cartesian grid. Higher order numerical schemes are used for time and spatial discretization. For the interface capturing, the level set method is selected. In order to test the generated waves, the time series of the free surface elevation are compared with experimental benchmark cases. The numerically simulated free surface elevation shows good agreement with experimental data. In further computations, the impact of the focused waves on a vertical circular cylinder is investigated. A breaking focused wave is simulated and the associated kinematics is investigated. Free surface flow features during the interaction of nonbreaking focused waves with a cylinder and during the breaking process of a focused wave are also investigated along with the numerically captured free surface.

Response of Beams Under the Impact of Freak Waves

2014 ◽  
Author(s):  
Zhe Hu ◽  
Wenyong Tang ◽  
Hongxiang Xue ◽  
Shaofei Ren
Keyword(s):  
Stokes Equations ◽  
Mass Conservation ◽  
Structural Safety ◽  
The Sea Of Japan ◽  
Numerical Wave Tank ◽  
Freak Waves ◽  
Wave Tank ◽  
Freak Wave ◽  
Numerical Wave ◽  
The Impact

Recently the research on freak waves has focused on the formation mechanism as well as the experimental and numerical simulation, however the study of freak waves’ action on marine structures which is often confined to numerical methods is still not much. As beams are often studied as the simplified model of plates for structural safety assessment, in this paper, the response of a beam which is hit by a 2-D freak wave is studied. The freak wave is generated in a numerical wave tank (NWT) which solves the 2-D incompressible Navier-Stokes equations. The freak wave is based on the data of real sea condition in the Sea of Japan. An efficient wave absorbing method which satisfies the mass conservation is applied in the numerical wave tank. The influence of the beam’s motion on the freak wave fluid field is also considered in this paper, as well as different boundary conditions of the beam. It is found that the natural frequency has a great impact on the response of the beam.

Analysis of Wave Interaction With Cylinders Using a 3D Numerical Wave Tank

2014 ◽  
Author(s):  
Arun Kamath ◽  
Hans Bihs ◽  
Øivind A. Arntsen
Keyword(s):  
Free Surface ◽  
Numerical Model ◽  
Flow Field ◽  
Stokes Equations ◽  
Total Force ◽  
Numerical Wave Tank ◽  
Offshore Structure ◽  
Wave Tank ◽  
Computational Performance ◽  
Numerical Wave

Evaluation of flow around a cylinder placed in waves is a challenging task due to the complex nature of the flow. A good understanding of the flow physics involved here is important as coastal and offshore structures consist of horizontal and vertical cylindrical elements. This paper explores the use of Computational Fluid Dynamics (CFD) to evaluate the flow field around cylindrical structures. A 3D numerical wave tank is employed to study the free surface and fluid velocities around a vertical cylinder placed in waves and the total force acting on the cylinder is evaluated. The numerical results are compared with experimental data. Further, a simple representation of an offshore structure modelled as multiple cylinders in proximity is also simulated in the numerical wave tank. The presence of neighbouring cylinders has an effect on the flow field. This affects the force acting on each of the cylinders in the group. The forces acting on every cylinder in the group are evaluated and the free surface elevation in the flow field is also visualised. The numerical result is compared with the result from an analytical formula. The numerical model uses the Reynolds-Averaged Navier-Stokes equations to evaluate the flow field. The convective terms are discretized using a 5th-order conservative finite difference WENO scheme. Time discretization is carried out using a 3rd-order Runge-Kutta scheme. Pressure discretization is carried out using Chorin’s projection method. The Poisson pressure equation is solved using a pre-conditioned BiCGStab algorithm. A sharp representation of the free surface is obtained using the level set method. Turbulence modeling is carried out using the k-ω model. Computational performance of the numerical model is improved by parallel processing using the MPI library.

Focused Plunging Breaking Waves Impact on Cylinder Group in Deep Water

2021 ◽  
Author(s):  
Ting Cui ◽  
Arun Kamath ◽  
Weizhi Wang ◽  
Lihao Yuan ◽  
Duanfeng Han ◽  
...  
Keyword(s):  
Free Surface ◽  
Deep Water ◽  
Breaking Waves ◽  
Surface Elevation ◽  
Relative Distance ◽  
Structural Safety ◽  
Wave Forces ◽  
Free Surface Elevation ◽  
Single Cylinder ◽  
Numerical Wave

Abstract The correct estimation of wave loading on a cylinder in a cylinder group under different impact scenarios is essential to determine the structural safety of coastal and offshore structures. This scenario differs from the interaction of waves with a single cylinder but not a lot of studies focus on cylinder groups under different arrangements. In this study, the interaction between plunging breaking waves and cylinder groups in deep water is investigated using the two-phase flow model in REEF3D, an open-source computational fluid dynamics program. The Reynolds-averaged Navier-Stokes equation with the two equation k–Ω turbulence model is adopted to resolve the numerical wave tank, with free surface calculated using the level set method. In this study, focused waves in deep water were modeled with a fixed wave steepness method. Wave breaking occurs when the steepness of the wave crest front satisfies the breaking criteria. The model is validated by comparing the numerical wave forces and free surface elevation with measurements from experiments. The computational results show fairly good agreement with experimental data for both free surface elevation and wave forces. Four cases are simulated to investigate the interaction of breaking waves with a cylinder group with different relative distance, number of cylinders and arrangement. Results show that breaking wave forces on the upstream cylinder are smaller than on a single cylinder with a relative distance of one cylinder diameter. The wave forces on cylinders in the pile group are effected by the relative distance between cylinders. The staggered arrangement has a significant influence on the wave forces on the first and second cylinder. The interaction inside a cylinder group mostly happens between the neighbouring cylinders. These interactions are also effected by the relative distance and the numbers of the neighbouring cylinders.

Development of 3-Dimensional Fully Nonlinear Potential Flow Planar Wave Tank in Framework of OpenFOAM

2019 ◽  
Author(s):  
Zaibin Lin ◽  
Ling Qian ◽  
Wei Bai ◽  
Zhihua Ma ◽  
Hao Chen ◽  
...  
Keyword(s):  
Free Surface ◽  
Potential Flow ◽  
Wave Structure ◽  
Wave Model ◽  
Numerical Wave Tank ◽  
Wave Tank ◽  
Fully Nonlinear ◽  
3 Dimensional ◽  
Nonlinear Potential ◽  
Numerical Wave

Abstract A 3-Dimensional numerical wave tank based on the fully nonlinear potential flow theory has been developed in OpenFOAM, where the Laplace equation of velocity potential is discretized by Finite Volume Method. The water surface is tracked by the semi-Eulerian-Lagrangian method, where water particles on the free surface are allowed to move vertically only. The incident wave is generated by specifying velocity profiles at inlet boundary with a ramp function at the beginning of simulation to prevent initial transient disturbance. Additionally, an artificial damping zone is located at the end of wave tank to sufficiently absorb the outgoing waves before reaching downstream boundary. A five-point smoothing technique is applied at the free surface to eliminate the saw-tooth instability. The proposed wave model is validated against theoretical results and experimental data. The developed solver could be coupled with multiphase Navier-Stokes solvers in OpenFOAM in the future to establish an integrated versatile numerical wave tank for studying efficiently wave structure interaction problems.

scholarly journals Generation of Offshore Environments in the Numerical Wave Tank to Model Metocean Conditions Interaction with Offshore Structure Near the Free Surface

2021 ◽  
Vol 945 (1) ◽  
pp. 012018
Author(s):  
Mushtaq Ahmed ◽  
Zafarullah Nizamani ◽  
Akihiko Nakayama ◽  
Montasir Osman
Keyword(s):  
Free Surface ◽  
Offshore Structures ◽  
Operating Conditions ◽  
Wave Crest ◽  
Extreme Conditions ◽  
Numerical Wave Tank ◽  
Offshore Structure ◽  
Wave Tank ◽  
Numerical Wave ◽  
Operating Wave

Abstract Offshore structures play a vital role in the economy of offshore oil-producing countries, where mostly fixed jacket type structures are used to produce oil and gas installed in shallow water. In an offshore environment where structures are installed, there exist met ocean forces such as wind, waves, and currents. These met ocean conditions when interacting with offshore structures near the free surface, generate loads. The estimation of such loads is very much important for the proper design of these structures. The primary aim of this study is to investigate the interaction of waves with a jacket platform by generating offshore environments in the numerical wave tank (NWT). To achieve this goal, ANSYS Fluent is used for the flow analysis by using continuity and Navier Stokes equation. Results are verified and validated with the analytical work. Wave crests under operating condition generate a force of 1.3 MN which is the lowest in magnitude as compared to wave crest which produces 4.5 MN force under extreme conditions. Unlike operating wave crest, the operating wave trough generates a higher force of 1 MN than extreme conditions which account for 1.5 MN forces. Forces produced by the extreme offshore environment are 30% higher than those generated under operating conditions. It is concluded from the results that a positive force is exerted onto the structure during the water entry phase while a negative force is observed when the water leaves the structure.

A Nonlinear Numerical Wave Tank and its Applications

2013 ◽  
Author(s):  
Hui Sun ◽  
Odd M. Faltinsen
Keyword(s):  
Free Surface ◽  
Horizontal Cylinder ◽  
The Body ◽  
Numerical Wave Tank ◽  
Wave Tank ◽  
Fully Nonlinear ◽  
Reflected Waves ◽  
Numerical Wave ◽  
Nonlinear Free Surface ◽  
Numerical Beach

A two-dimensional fully nonlinear numerical wave tank is developed by using a boundary element method (BEM). The water depth can be shallow or deep. The waves are generated by simulating a piston wave maker or by specifying the input velocity at the upstream boundary. Fully nonlinear free surface conditions are satisfied in the numerical simulations. In the downstream region, a numerical beach is employed to dissipate the wave energy to avoid waves reflecting from the vertical downstream boundary. When there is a body piercing the free surface, another numerical beach is applied upstream the body to damp out only the reflected waves from the body. Two different applications are presented in this paper. The first one is to compute the pressure and velocity at any point inside the wave field. The other application is to calculate the forces on a horizontal cylinder fixed on the free surface. This second application is related to the investigation of the hydrodynamic forces on the pontoon of a fish farm. Nonlinearities are significant since the wave amplitudes can be large relative to the wavelength and the dimension of the cylinder.

Numerical Wave Tank: Simulation of Extreme Waves for the Investigation of Structural Responses

2005 ◽  
Author(s):  
Gu¨nther F. Clauss ◽  
Christian E. Schmittner ◽  
Robert Stu¨ck
Keyword(s):  
Free Surface ◽  
Potential Theory ◽  
Stokes Equations ◽  
Wave Train ◽  
Breaking Waves ◽  
Vof Method ◽  
Time Step ◽  
Wave Tank ◽  
Element Discretization ◽  
Numerical Wave

For the deterministic analysis of extreme structure behavior, the hydrodynamics of the exciting wave field, i. e. pressure and velocity fields, must be known. Whereas responses of structures, e. g. motions, can easily be obtained by model tests, the detailed characteristics of the exciting waves are often difficult to determine by measurements. Therefore, numerical wave tanks (NWT) promise to be a handy tool for providing detailed insight into wave hydrodynamics. In this paper different approaches for numerical wave tanks are introduced and used for the simulation of rogue wave sequences. The numerical wave tanks presented are characterized by the following key features: a) Potential theory with Finite Element discretization (Pot/FE); b) Reynolds-Averaged Navier-Stokes Equations (RANSE) using the Volume of Fluid (VOF) method for describing the free surface. For the NWT using the VOF method three different commercial RANSE codes (CFX, FLUENT, COMET) are applied to calculate wave propagation, whereas simulations based on potential theory are carried out with a wave simulation code developed at Technical University Berlin (WAVETUB). It is shown that the potential theory method allows a fast and accurate simulation of the propagation of nonbreaking waves. In contrast, the RANSE/VOF method allows the calculation of breaking waves but is much more time-consuming, and effects of numerical diffusion can not be neglected. To benefit from the advantages of both solvers, i. e. the calculation speed (Pot/FE-solver WAVETUB) and the capability of simulating breaking waves (RANSE/VOF-solver), the coupling of both simulation methods is introduced. Two different methods of coupling are presented: a) at a given position in the wave tank; b) at a given time step. WAVETUB is used to simulate the propagation of the wave train from the start towards the coupling position (case A) or until wave breaking is encountered (case B). Subsequently, the velocity field and the contour of the free surface is handed over as boundary (case A) or initial values (case B) to the RANSE/VOF-solver and the simulation process is continued. To validate these approaches, different types of model seas for investigating wave/structure interactions are generated in a physical wave tank and compared to the numerical simulations.

A Numerical Study of a Focused Wave Packet Near the Surf Zone

2012 ◽  
Author(s):  
Csaba Pakozdi ◽  
Timothy E. Kendon ◽  
Carl-Trygve Stansberg
Keyword(s):  
Wave Packet ◽  
Model Test ◽  
Surf Zone ◽  
Breaking Wave ◽  
Numerical Wave Tank ◽  
Wave Tank ◽  
Numerical Reconstruction ◽  
Numerical Wave ◽  
Focused Wave ◽  
Sloping Bottom

In this paper the numerical modeling of breaking waves propagating on a gently sloping bottom in shallow water is investigated. As more and more countries look to install offshore wind farms in their coastal waters, the breaking wave impact force on wind turbine foundations has been an area of increased research. For meaningful comparisons between measurement and simulation, the numerical reconstruction of the model test breaking wave event must be fairly exact. The combination of numerical reconstruction of model tests using computational fluid dynamics has proved a valuable tool to provide insight into the physics of the breaking wave phenomenon in deep water ([1]). To refine the technique of numerically reconstructing the breaking wave in shallow water, comparisons to a series of model tests with breaking wave events near the surf zone are made. The sloping bottom is modeled with a ramp with a gradient of 2.8 degrees in a wave tank. This paper describes the numerical reproduction of a focused wave packet, for studying its shoaling and breaking. The commercial CFD tool Star-CCM+ has been used to reproduce a measured focused wave packet train in a numerical wave tank. Its RANSE physical model with VOF technique is applied for this investigation. The numerical wave generation is based on the technique presented in [1], where the measured angle of the wave makers flap and the measured free surface elevation at the flap has been used to define a transient inlet condition for the simulation. In this paper, the numerically simulated wave elevation around the breaking point is compared with the measured time series. Promising results are obtained. The numerical model reproduces, at the same location and time, the breaking events as it was observed during the model test. One conclusion from this particular case is that the time step is critical; it should be very small during the breaking events which may result in a very long simulation time. Further work is suggested to meet this challenge, as well as for more refined studies to improve the complete numerical wave tank model.

scholarly journals Investigation of Focusing Wave Properties in a Numerical Wave Tank with a Fully Nonlinear Potential Flow Model

2019 ◽  
Vol 7 (10) ◽  
pp. 375 ◽  
Author(s):  
Weizhi Wang ◽  
Arun Kamath ◽  
Csaba Pakozdi ◽  
Hans Bihs
Keyword(s):  
Wave Height ◽  
Potential Flow ◽  
Wave Steepness ◽  
Numerical Wave Tank ◽  
Wave Groups ◽  
Wave Tank ◽  
Fully Nonlinear ◽  
Nonlinear Potential ◽  
Numerical Wave ◽  
Focused Wave

Nonlinear wave interactions and superpositions among the different wave components and wave groups in a random sea sometimes produce rogue waves with extremely large wave heights that appear unexpectedly. A good understanding of the generation and evolution of such extreme wave events is of great importance for the analysis of wave forces on marine structures. A fully nonlinear potential flow (FNPF) model is proposed in the presented paper to investigate the different factors that influence the wave focusing location, focusing time and focusing wave height in a numerical wave tank. Those factors include wave steepness, spectrum bandwidth, wave generation method, focused wave spectrum, and wave spreading functions. The proposed model solves the Laplace equation together with the boundary conditions on a σ -coordinate grid using high-order discretisation schemes on a fully parallel computational framework. The model is validated against the focused wave experiments and thereafter used to obtain insights into the effects of the different factors. It is found that the wave steepness contributes to changing the location and time of focus significantly. Spectrum bandwidth and directional spreading affect the focusing wave height and profile, for example, a wider bandwidth and a wider directional spread lead to a lower focusing wave height. A Neumann boundary condition represents the nonlinearity of the wave groups better than a relaxation method for wave generation.

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