CFD Analysis of Waves Over a Submerged Cylinder in Close Proximity of the Free Surface

2014 ◽  
Author(s):  
Harald Ottens ◽  
Alessio Pistidda ◽  
Radboud van Dijk
Keyword(s):  
Inviscid Flow ◽  
Wave Force ◽  
Model Tests ◽  
Irregular Waves ◽  
Ship Motions ◽  
Wave Forces ◽  
Cfd Simulations ◽  
Submerged Body ◽  
Submerged Cylinder ◽  
Water Elevation

Diffraction programs using potential theory are a quick and effective method in calculating wave forces and ship motions. However in cases where a small layer of water is present on top of a submerged body diffraction calculations overpredict motion and wave force RAOs. This shortcoming of diffraction programs is observed after conducting model tests on a captive submerged cylinder and a free floating SSCV. Unrealistic high wave elevations were predicted by diffraction programs on top of the submerged body. In a previous study a damping lid is implemented [1], to decrease the water elevation to realistic values. In this study CFD is used to simulate the captive submerged cylinder in regular waves with different wave heights, wave periods and different submerged drafts. In addition irregular waves are used in the simulation matching the wave spectra used in the model tests. The simulations are transient and require high CPU usage, therefore the influence of numerical settings on wave propagation is investigated. Turbulent, laminar and inviscid flow are applied to evaluate which flow phenomena are important. The forces in heave and surge direction are validated with model test data of the captive cylinder. The numerical water elevation on top of the captive cylinder will be used to gain insight in the fluid flow and can be used as a guideline for the use of damping lids in diffraction programs. This paper will focus on the CFD simulations and the validation with available forces obtained by model tests of the captive submerged cylinder. It will address the use of regular and irregular waves constructing the force RAO for this non-linear phenomenon. Lessons learnt to improve the CFD simulations as well as limitations of constructing RAOs using CFD from an engineering perspective will be addressed as well.

Experimental Study of Wave Force Distribution on a Monopile Structure

2018 ◽  
Author(s):  
Malene H. Vested ◽  
Stefan Carstensen ◽  
Erik Damgaard Christensen
Keyword(s):  
Experimental Studies ◽  
Cost Effective ◽  
Wave Force ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Force Distribution ◽  
Wave Forces ◽  
Total Force ◽  
Wave Flume ◽  
Effective Manner

As the demand for offshore wind energy continues to grow, the strive to understand the wave forces acting on the substructure of the wind turbines continues. In regard to wind turbine design, it is vital to consider not only the total wave force, but also the local wave forces. Local forces are particularly important for the design of secondary structures as e.g. mooring platforms. Typically, however, experimental studies mainly concern total forces or idealized local forces. We present here a rather simple way to measure local forces along a model monopile. The study is conducted in a wave flume of 28 m in length, in which waves are generated by a piston-type wave maker at a water depth of 0.515 m and shoal onto a bed of slope 1:25. A model monopile is installed and subjected to forcing from a series of both regular and irregular waves. In the experimental set-up, the model monopile is fixed at the bottom and the top and consists of seven independent cylindrical sections. The cylindrical sections are connected by force transducers which measure local shear, and so the associated local forces may be determined. The measured local forces are compared to the force distribution given by Morisons equation combined with linear theory and Wheeler stretching, which is a force estimate commonly used in the industry. This study shows that the total force is rather well captured by Morison’s equation. The force distribution estimated from Morison’s equation, however, shows larger discrepancies from the measured forces. This encourages for further measurements. In this study, we show that it is possible to measure force distribution on a model monopile in a simple and cost-effective manner. The aim is here to demonstrate the method and we will later present a larger body of work associated with the outcome of the measurements.

Experimental Determination of Water Motions Inside a Moonpool

2017 ◽  
Author(s):  
Bastien Abeil
Keyword(s):  
Experimental Determination ◽  
Transfer Functions ◽  
Side Wall ◽  
Model Tests ◽  
Irregular Waves ◽  
Video Recordings ◽  
Non Linear ◽  
Relative Water ◽  
Water Elevation

Model tests of a drillship with a rectangular moonpool opening were conducted in regular and irregular waves from the bow and bow-quarter. Most tests were conducted at zero speed, the rest was performed with the model towed to a speed of 10 kn. From the video-recordings and transfer functions of the measured relative water elevation inside the moonpool, the typical piston and first sloshing modes are well captured, for wave frequencies that agree relatively well with relevant formulations. A few tests conducted at varying wave amplitudes show that the water elevation is non-linear by nature, while repeat tests conducted with the moonpool fitted with two layers of side wall flanges shows that these can reduce the water motions by nearly 40 %.

Calculation of Wave Height Dependent Force RAO’s on Submerged Bodies in Close Proximity to the Free Surface

2013 ◽  
Author(s):  
Ivan van Winsen ◽  
Job S. Bokhorst ◽  
René H. M. Huijsmans
Keyword(s):  
Free Surface ◽  
Wave Height ◽  
High Water ◽  
Wave Frequency ◽  
Parameter Study ◽  
Submerged Body ◽  
Close Proximity ◽  
Submerged Cylinder ◽  
Water Elevations ◽  
Water Elevation

Diffraction calculations overpredict motion RAO’s and force RAO’s in cases where a small layer of water is present on top of a submerged body. This was observed after conducting model tests on a free floating SSCV Thialf and a captive submerged cylinder. A parameter study is done to get a better understanding of why diffraction calculations overpredict the forces in heave direction. From this study it was observed that unrealistically high water elevations existed on top of the cylinder causing the heave forces to be overestimated. A damping lid is therefore implemented to decrease this water elevation. On top of that, a new method is developed to be able to capture the dependency of the force RAO on the wave height. This method uses the instantaneous submergence height (the height of water on top of the submerged body) to determine the time averaged force RAO for a given wave height and wave frequency.

Numerical Simulations of Regular and Irregular Wave Forces on a Horizontal Semi-Submerged Cylinder

2017 ◽  
Author(s):  
Shengnan Liu ◽  
Muk Chen Ong ◽  
Charlotte Obhrai ◽  
Sopheak Seng
Keyword(s):  
Experimental Data ◽  
Free Surface ◽  
Numerical Simulations ◽  
Stokes Equations ◽  
Wave Length ◽  
Wave Structure ◽  
Numerical Results ◽  
Irregular Waves ◽  
Wave Forces ◽  
Submerged Cylinder

Two-dimensional (2D) numerical simulations have been performed using OpenFOAM (an open source CFD software package [1]) and waves2Foam (an OpenFOAM based add-on library for wave generations and absorption [2]) to investigate free surface waves past one fixed horizontally semi-submerged cylinder. The 2-D simulations are carried out by solving Navier-Stokes equations which are discretized based on finite volume method (FVM). Volume of Fluid (VOF) method is employed to capture the free surface in the numerical wave tank. Validation studies have been performed by comparing the numerical results of Stokes first-order wave past a semi-submerged circular cylinder with the published experimental data at different incident wave properties. The numerical results are in good agreement with the experimental data. Subsequently, regular and irregular waves past semi-submerged cylinder at different wave heights and the wave lengths are computed numerically to investigate the effect of the wave height and wave length on wave-structure interaction. The numerical results for irregular waves are compared with those induced by regular waves.

Application of a Hybrid Boussinesq-Panel Model for Motion Predictions of a Moored Sevan-Floater in Finite Water Depth

2017 ◽  
Author(s):  
Jikun You ◽  
Einar Bernt Glomnes
Keyword(s):  
Water Depth ◽  
Low Frequency ◽  
Boussinesq Equations ◽  
Wave Force ◽  
Irregular Waves ◽  
Wave Forces ◽  
Rankine Source ◽  
Panel Model ◽  
Finite Water Depth ◽  
Incident Waves

This paper presents the applications of an efficient hybrid time-domain simulation model for predicting moored Sevan-floater motions in irregular waves and finite water depth. The irregular incident waves are modeled by the extended Boussinesq equations, which can capture wave-wave interactions and the low-frequency long waves accurately in finite and shallow water depth. By imposing the incident wave kinematics on the surface of the floater, a panel model based on Rankine source method is applied for the calculation of wave forces and corresponding floater motions. The contributions from low-frequency components in incident waves as well as their diffraction effects are included in the wave force calculations. Validation of the irregular waves simulated by the present numerical model are performed against experimental data. Then, the simulated moored floater motions are compared with model test results and results based on Newman’s approximation. The general good agreements with experimental results demonstrate the present model can be used as an alternative for this problem while Newman’s approximation shows non-conservative results.

Computational Fluid Dynamics Simulations of Regular and Irregular Waves Past a Horizontal Semi-Submerged Cylinder

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Shengnan Liu ◽  
Muk Chen Ong ◽  
Charlotte Obhrai ◽  
Sopheak Seng
Keyword(s):  
Free Surface ◽  
Numerical Simulations ◽  
Wave Height ◽  
Stokes Equations ◽  
Wave Force ◽  
Numerical Results ◽  
Irregular Waves ◽  
Wave Forces ◽  
Regular Waves ◽  
Peak Period

Two-dimensional (2D) numerical simulations have been performed to investigate both regular and irregular waves past a fixed horizontally semisubmerged circular cylinder. The 2D simulations are carried out by solving Navier–Stokes equations discretized by finite volume method. Volume of fluid (VOF) method is employed to capture the free surface in the numerical wave tank (NWT). Validation studies have been performed by comparing the numerical results of free surface waves past the cylinder with the published experimental and numerical data. The present numerical results are in good agreement with both the experimental and the other numerical results in terms of hydrodynamic forces and free surface elevation. Subsequently, the effects of the wave height and the wavelength on wave–structure interaction are investigated by conducting numerical simulations on the regular and the irregular waves past a semisubmerged cylinder at different wave heights and the wavelengths. The averaged and maximum vertical wave forces on the cylinder increase with the increasing wave height. The numerical results for the irregular waves are compared with those induced by the regular waves in terms of the maximum and averaged vertical wave forces. When the significant wave height and the spectral peak period of the irregular waves are equal to the wave height and the wave period of the regular waves, the maximum vertical wave force induced by the irregular waves is larger than that induced by the regular waves, meanwhile, the average vertical wave forces have the contrary relationship.

scholarly journals STUDY OF STATISTICAL CHARACTERISTICS OF IRREGULAR WAVE PRESSURE ON A COMPOSITE BREAKWATER

1986 ◽  
Vol 1 (20) ◽  
pp. 131
Author(s):  
Chien-Kee Chang ◽  
Ching-Her Hwang
Keyword(s):  
Pressure Distribution ◽  
Wave Height ◽  
Wave Force ◽  
Shock Pressure ◽  
Irregular Waves ◽  
Statistical Characteristics ◽  
Wave Forces ◽  
Incoming Wave ◽  
Wave Pressure ◽  
Irregular Wave

Wave pressure is the most important external force for the design of breakwater. During recent years, there has been considerable development in the technology of vertical face breakwater; however, there is no reliable method to compute wave forces induced by irregular waves. The purpose of this study is to obtain statistical characteristics of irregular wave pressure distribution from the data of model tests. The results of this study shown that vertical face breakwater under the action of irregular waves, some waves are reflected, so that the next wave breaks a critical distance resulting in a rapidly rising shock pressure on the breakwater. On the average, the wave pressure increase with incoming wave height, but the maximum wave force does not necessarily occur for the largest wave height. It can be occurred for serval larger wave group in an appropiate phase composition. The irregular wave pressure distribution on the breakwater is quite uniform; the ratio of tested and calculated wave pressures decreases with the reduction of relative crest height of breakwater. Coda formula can predict the total horizontal force of the upper part of breakwater quite well except exetreme shock pressure occurred by non-breaking waves. Wave forces calculated by Miche-Rundgren and Nagai wave force formula are about 10% cummulated exceeding percentage of wave force obtained from model test.

Prediction of Ship Response Statistics in Extreme Seas Using Model Test Data and Numerical Simulations Based on the Rankine Panel Method

2013 ◽  
Author(s):  
Bingjie Guo ◽  
Elzbieta M. Bitner-Gregersen ◽  
Hui Sun ◽  
Jens Bloch Helmers
Keyword(s):  
Numerical Simulations ◽  
Linear Codes ◽  
Bending Moment ◽  
Rogue Waves ◽  
Model Tests ◽  
Panel Method ◽  
Irregular Waves ◽  
Ship Motions ◽  
University Of Berlin ◽  
Non Linear

Earlier investigations have indicated that proper prediction of non-linear response due to non-linear waves is important for ship safety in extreme seas. Nonlinearities may increase significantly ship response in steep sea-states. The topic has not been sufficiently investigated yet, particularly when rogue waves are considered. A question remains whether the existing linear codes can predict nonlinear responses with a satisfactory accuracy and how large the deviations from linear predictions are. To indicate it, response statistics have been studied based on the model tests carried out in the Spanish basin CEHIPAR and the sea-keeping tank of the Technical University of Berlin (TUB), and compared with the results derived from numerical simulations using the DNV code WASIM. It is a potential code for wave-ship interaction based on 3D Panel method, which can perform both linear and nonlinear simulations. The numerical simulations with WASIM and the model tests including extreme and rogue waves have been performed on 3 different ship types: Chemical tanker, LNG tanker and a Cruise ship. The analysis includes both regular and irregular waves. Ship motions and bending moments have been studied. The effect of water depth on ship responses is also investigated. The study indicates that nonlinearities may have significant impact on extreme motions and bending moment induced by strongly nonlinear waves. Uncertainties related to the results are also discussed.

Coupled Mooring Analysis and Large Scale Model Tests on a Deepwater Calm Buoy in Mild Wave Conditions

2002 ◽  
Author(s):  
T. H. J. Bunnik ◽  
G. de Boer ◽  
J. L. Cozijn ◽  
J. van der Cammen ◽  
E. van Haaften ◽  
...  
Keyword(s):  
Model Tests ◽  
Irregular Waves ◽  
Scale Model ◽  
Mooring Line ◽  
Wave Forces ◽  
Mooring System ◽  
Mooring Lines ◽  
Pitch Moment ◽  
Numerical Tool ◽  
Decay Tests

This paper describes a series of model tests aimed at gaining insight in the tension variations in the export risers and mooring lines of a CALM buoy. The test result were therefore not only analysed carefully, but were also used as input and to validate a numerical tool that computes the coupled motions of the buoy and its mooring system. The tests were carried out at a model scale of 1 to 20. Captive tests in regular and irregular waves were carried out to investigate non-linearities in the wave forces on the buoy for example from the presence of the skirt. Decay tests were carried out to determine the damping of the buoy’s motions and to obtain the natural periods. Finally, tests in irregular waves were carried out. The dynamics of the mooring system and the resulting damping have a significant effect on the buoy’s motions. A numerical tool has been developed that combines the wave-frequency buoy motions with the dynamical behaviour of the mooring system. The motions of the buoy are computed with a linearised equation of motion. The non-linear motions of the mooring system are computed simultaneously and interact with the buoy’s motions. In this paper, a comparison is shown between the measurements and the simulations. Firstly, the wave forces obtained with a linear diffraction computation with a simplified skirt are compared with the measured wave forces. Secondly, the numerical modelling of the mooring system is checked by comparing line tensions when the buoy moves with the motion as measured in an irregular wave test. Thirdly, the decay tests are simulated to investigate the correctness of the applied viscous damping values. Finally, simulations of a test in irregular waves are shown to validate the entire integrated concept. The results show that: 1. The wave-exciting surge and heave forces can be predicted well with linear diffraction theory. However, differences between the measured and computed pitch moment are found, caused by a simplified modelling of the skirt and the shortcomings of the diffraction model. 2. To predict the tension variations in the mooring lines and risers (and estimate fatigue) it is essential that mooring line dynamics are taken into account. 3. The heave motions of the buoy are predicted well. 4. The surge motions of the buoy are predicted reasonably well. 5. The pitch motions are wrongly predicted.

Large-Scale Model Tests With Wave Loading on Offshore Platform Deck Elements

2002 ◽  
Author(s):  
Martin J. Sterndorff
Keyword(s):  
Large Scale ◽  
Model Tests ◽  
Offshore Platform ◽  
Irregular Waves ◽  
Scale Model ◽  
Wave Forces ◽  
Wave Loading ◽  
Large Wave ◽  
Wave Channel ◽  
The Individual

The present paper concerns a detailed large-scale experimental study of wave loading on offshore platform decks. A series of model tests with wave loading on different types of deck elements have been performed in the large wave channel (GWK) at Forschungszentrum Kiiste in Hannover, Germany. The following types of deck elements have been considered: tubular elements, plate profiles, and HEB beam profiles. The tests have been performed with individual elements and arrays of elements. Tests have also been performed with an array of beam elements covered with deck plating. A large range of different wave types, air gaps, and inundation’s have been tested. Regular waves with wave height ranging from 1.4 m to 1.8 m, irregular waves and wave packages with crest heights ranging from 0.9 m to 1.6 m have been tested. During the tests the following parameters were measured: wave elevations, deck element inundation’s, wave kinematics profile, and wave forces on the individual deck elements. The model test results will be analysed to provide hydrodynamic load coefficients to a wave-in-deck load programme based on the concept of change of fluid momentum. The results will also be used to verify a CFD code based on the Volume of Fluid method.

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