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

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.

Download Full-text

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

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4036206 ◽

2017 ◽

Vol 139 (4) ◽

Cited By ~ 15

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.

Download Full-text

Validation of Wave Propagation in Numerical Wave Tanks

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 3 ◽

10.1115/omae2005-67221 ◽

2005 ◽

Cited By ~ 1

Author(s):

Tim Bunnik ◽

Rene´ Huijsmans

Keyword(s):

Shallow Water ◽

Model Test ◽

Water Depth ◽

Linear Potential ◽

Intermediate Water ◽

Numerical Wave Tank ◽

Wave Tank ◽

Type Wave ◽

Wave Generator ◽

Numerical Wave

During the last few years there has been a strong growth in the availability and capabilities of numerical wave tanks. In order to assess the accuracy of such methods, a validation study was carried out. The study focuses on two types of numerical wave tanks: 1. A numerical wave tank based a non-linear potential flow algorithm. 2. A numerical wave tank based on a Volume of Fluid algorithm. The first algorithm uses a structured grid with triangular elements and a surface tracking technique. The second algorithm uses a structured, Cartesian grid and a surface capturing technique. Validation material is available by means of waves measured at multiple locations in two different model test basins. The first method is capable of generating waves up to the break limit. Wave absorption is therefore modeled by means of a numerical beach and not by mean of the parabolic beach that is used in the model basin. The second method is capable of modeling wave breaking. Therefore, the parabolic beach in the model test basin can be modeled and has also been included. Energy dissipation therefore takes place according to physics which are more related to the situation in the model test basin. Three types of waves are generated in the model test basin and in the numerical wave tanks. All these waves are generated on basin scale. The following waves are considered: 1. A scaled 100-year North-Sea wave (Hs = 0.24 meters, Tp = 2.0 seconds) in deep water (5 meters). 2. A scaled operational wave (Hs = 0.086 meters, Tp = 1.69 seconds) at intermediate water depth (0.86 meters) generated by a flap-type wave generator. 3. A scaled operational wave (Hs = 0.046 meters, Tp = 1.2 seconds) in shallow water (0.35 meters) generated by a piston-type wave generator. The waves are generated by means of a flap or piston-type wave generator. The motions of the wave generator in the simulations (either rotational or translational) are identical to the motions in the model test basin. Furthermore, in the simulations with intermediate water depth, the non-flat contour of the basin bottom (ramp) is accurately modeled. A comparison is made between the measured and computed wave elevation at several locations in the basin. The comparison focuses on: 1. Reflection characteristics of the model test basin and the numerical wave tanks. 2. The accuracy in the prediction of steep waves. 3. Second order effects like set-down in intermediate and shallow water depth. Furthermore, a convergence study is presented to check the grid independence of the wave tank predictions.

Download Full-text

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

Journal of Marine Science and Engineering ◽

10.3390/jmse7100375 ◽

2019 ◽

Vol 7 (10) ◽

pp. 375 ◽

Cited By ~ 2

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.

Download Full-text

Validation of Numerical Wave Tank Simulations Using REEF3D With JONSWAP Spectra in Intermediate Water Depth

Volume 1: Offshore Technology ◽

10.1115/omae2020-18298 ◽

2020 ◽

Author(s):

Csaba Pakozdi ◽

Sebastien Fouques ◽

Maxime Thys ◽

Arun Kamath ◽

Weizhi Wang ◽

...

Keyword(s):

Test Data ◽

Model Test ◽

Water Depth ◽

Model Tests ◽

Irregular Waves ◽

Wave Crest ◽

Intermediate Water ◽

Numerical Wave Tank ◽

Wave Tank ◽

Numerical Wave

Abstract As offshore wind turbines increase in size and output, the support structures are also growing. More sophisticated assessment of the hydrodynamic loads is needed, particularly for the ultimate limit state design. For higher-order phenomena related to rare steep wave events such as ringing, a better understanding of the stochastic loads is needed. As an innovative step forward to reduce the cost of extensive model tests with irregular waves, a larger number of investigations can be carried out using high-performance high-fidelity numerical simulations after an initial stochastic validation with model test data. In this paper, the open-source hydrodynamic model REEF3D::FNPF (Fully Nonlinear Potential Flow) is used to carry out three-hour long simulations with the JONSWAP spectrum in intermediate water depth conditions. Statistical properties of the free surface elevation in the numerical wave tank are validated using the available data from model tests carried out at SINTEF Ocean/NTNU. The spectral shape, significant wave height, peak period, skewness, kurtosis, and wave crest height statistics are compared. The results are analyzed and it is found that the numerical model provides reasonably good agreement with the model test data.

Download Full-text

Numerical Simulation and Analysis of Phase Focused Breaking and Non-Breaking Wave Impact on Fixed Offshore Platform Deck

Volume 3: Structures, Safety, and Reliability ◽

10.1115/omae2019-95193 ◽

2019 ◽

Author(s):

Rameeza Moideen ◽

Manasa Ranjan Behera ◽

Arun Kamath ◽

Hans Bihs

Keyword(s):

Impact Force ◽

Experimental Results ◽

Breaking Wave ◽

Numerical Wave Tank ◽

Wave Impact ◽

Wave Heights ◽

Numerical Wave ◽

Focused Wave ◽

The Impact ◽

Vertical Impact

Abstract Extreme wave impact due to tsunamis and storm surge create large wave heights causing destruction to coastal and offshore structures. These extreme waves are represented by focused waves in the present study and the impact on offshore deck is studied. Numerical wave tank used is modelled using open-source software REE3D, where the level set method is used to capture the air-water interface. Vertical impact force on offshore deck is computed and compared with the experimental results to validate the numerical model. Focused wave is generated by phase focusing a group of waves at a particular position and time. The nonlinearity of focused wave and its effect on the vertical impact force is quantified for different airgap and increasing wave heights. The steepness of this focused wave is increased to initiate phase focused breaking in the numerical wave tank, which is validated with experimental results of Ghadirian et al., 2016. The main purpose of this paper is to examine breaking focused wave group loads on the offshore deck and to study the impact on deck at different breaking locations. The positioning of the deck with respect to breaker location have shown that the maximum horizontal impact force due to breaking wave occurs when the plunging crest hits the deck side.

Download Full-text

Numerical Modeling of Breaking Waves Over a Reef With a Level-Set Based Numerical Wave Tank

Volume 7: CFD and VIV ◽

10.1115/omae2013-10363 ◽

2013 ◽

Author(s):

Mayilvahanan Alagan Chella ◽

Hans Bihs ◽

Arun Kamath ◽

Michael Muskulus

Keyword(s):

Level Set ◽

Wave Breaking ◽

Stokes Equations ◽

Numerical Results ◽

Navier Stokes ◽

Breaking Wave ◽

Numerical Wave Tank ◽

Wave Tank ◽

Navier Stokes Equations ◽

Numerical Wave

Wave breaking is a highly unsteady, non-linear and extremely turbulent phenomenon. During the wave breaking process, the energy of the wave system is focused close to the crest of the wave and a spatial spread of wave energy occurs. Thus, the description of such a physical phenomenon is highly complex and it requires a deep insight into the breaking wave process. The accurate assessment of breaking wave kinematics is essential for an accurate prediction of hydrodynamic loads on structures. Besides, the understanding of the transformation of waves propagating over an artificial or natural reef is important concerning the coastal processes. The numerical model used in this study is a two-phase model, which solves the flow problem for air and water simultaneously. The Navier-Stokes equations are solved on uniform Cartesian grids in two dimensions. The complex free surface is captured by the level set method. A staggered grid is used for the computation with the velocities defined at the cell edges and the pressure at the cell centres. This avoids unphysical pressure oscillations that can occur due to the coupling of pressure and velocity in the incompressible Navier-Stokes equations. The Ghost Cell Immersed Boundary Method is employed to handle the boundary conditions for complex boundaries. Turbulence modelling is carried out using the k-ω model. Discretization of the convective terms is performed using the 5th order Weighted Essentially Non-Oscillatory (WENO) scheme. In this study, a two-dimensional numerical wave tank is used to simulate waves propagating over steep slopes and wave dissipation. The main objective of the present study is to investigate the wave breaking process over a submerged reef. This is accomplished by examining the wave profile during wave breaking and the breaker indices. Also, the numerical results are compared to data from physical experiments and the numerical results exhibit reasonable agreement with experimental data.

Download Full-text