Wave Tank Model Research Articles

Numerical examination of wave forces on partially submerged horizontal circular cylinders near free surface

Determining wave forces on partially submerged horizontal circular cylinders near free surface is a significant issue. This study explores hydrodynamic coefficients and wave forces on horizontal circular cylinders near free surface under large-amplitude wave actions using a viscous fluid numerical wave tank model based on the finite volume method, with a particular aim to accurately predicting the slamming force. By analyzing the characteristics of the wave forces and slamming forces on the circular cylinder, the influences of wave amplitude, wave frequency and vertical position of the cylinder on the wave forces are investigated, and the relationships between the slamming forces and fluid velocity are discussed. Additionally, a modified Morison's equation for predicting the wave forces is derived considering the slamming force and the condition of the circular cylinder completely exposed to the air phase under the action of large-amplitude wave. The results of this study reveal that the wave forces on the circular cylinder are significantly influenced by wave amplitude, frequency and vertical position of the cylinder, and the influences of the cylinder position on the predicted minimum slamming velocity are remarkable. The present study demonstrates that modified Morison's equation is plausible and accurate in predicting the wave and slamming forces for submerged or floating structures.

Research On Numerical Wave Tank Based on SPH Method

Numerical wave-current flume is one of the effective methods to study waves and their effects on structures. In this paper, the Smoothed Particle Hydrodynamics (SPH) method is used to simulate and analyze the numerical wave flume. Based on the SPH method, an efficient and flexible numerical wave tank model is constructed to simulate complex water wave dynamics. Firstly, the basic principle of SPH method is summarized, and then the establishment process of numerical wave tank model is described in detail, including particle initialization, boundary condition setting and so on. Through the flume simulation under different wave conditions, the accuracy and stability of the model are verified. Finally, the interaction between wave and structure is studied. It provides a new and effective tool for the numerical simulation of wave flume, and also provides a useful reference for the related research in the fields of marine engineering and coastal protection.

CFD analysis of wave loading on a 10 MW TLP-type offshore floating wind turbine in regular waves

This paper aims to study the higher-harmonics wave-induced surge force on the CENTEC-TLP platform in regular head waves with non-breaking assumptions, using a CFD-based numerical wave tank model, which is a useful tool for the analysis of the nonlinear interaction of waves and structures. Moreover, the influence of change in the wave diffraction and the incident wave angle on the surge force subject to operational and extreme wave conditions is investigated. The numerical analysis is conducted using the CFD toolbox OpenFOAM. The link between Secondary Load Cycle and the wave scattering Type2 is apparent and it is concluded that they are all inertia-driven and that 2nd harmonics play an important role.

Smoothed particle hydrodynamics study of a heaving point absorber in various waves using wave tank and calm-water models

In this research, the influence of wave parameters on the response of a tuned point absorber was evaluated using the smoothed particle hydrodynamics (SPH) wave tank and calm-water models. In the first model, the device response was obtained under the effect of waves generated by a wavemaker. In the calm-water model, the added mass and hydrodynamic damping coefficients of the device were calculated from a short-time SPH-free decaying test. Then, using the Runge–Kutta method, the governing equation of motion was solved in MATLAB. Both models were verified by previously published experimental tests. Considering the viscous damping makes calm-water model superior to potential-flow models. Meanwhile, the computation time of this model is very lower than wave tank model. For wave steepness values below 8.5% which is more than the upper limit of wave steepness for deep-water waves, both models predicted a direct relationship between wave height and device motion. However, wave tank simulations showed that for steeper waves, the device motion was not significantly affected by the wave height. Moreover, the device response decreased as the wave period moved away from the device's natural period. The predictions of the two models had an average difference below 10%. While in linear conditions, the calm-water model predicted a slightly larger response than the wave tank model; in nonlinear cases, wave tank model predicted slightly higher device motions. This research shows that using the SPH calm water model is an efficient way to investigate wave-point absorber interactions.

Investigation of the Spiral Wave Generation and Propagation on a Numerical Circular Wave Tank Model

A two-phase incompressible flow model in three-dimensional cylindrical coordinates is applied for oblique wave generation in a numerical circular wave tank. The governing equations are discretized by a finite volume method, and a mass source function is added to reproduce oblique waves through a spiral wave generator positioned at the center of the tank. The volume of fluid method is implemented to track the free surfaces between the air and fluid, and the zonal embedded grid system is adopted to obtain a grid-independent solution in the cylindrical coordinates. A permeable, circumferentially sloping topography (1:7), similar to a natural beach profile, is set up for investigating wave propagation and other characteristics. The simulation and physical experimental results are compared, which show a good agreement in randomly selected water surface elevation profiles and wave heights under the same wave and sloping conditions. The spiral waves are reproduced and propagated in a phenomenon similar to that observed in the physical experiment. The results also show that the wave-breaking positions differ in different wave conditions and suggest a relationship with cross-shore and longshore velocity distribution in terms of incident wave heights and wave-breaking positions in different wave periods on the same sloping topography. Furthermore, this model can be used to investigate the mechanism of longshore current generation and the influences of beach slope on the generated wave propagation in a sloping topography.

Development of a CFD-based numerical wave tank of a novel multipurpose wave energy converter

The development of new wave energy converters usually involves small-scale experiments in physical wave tanks. The jump to physical models at larger scales is an expensive and time-consuming process that can be supported by computational fluid dynamics (CFD) models. Using the CFD approach, it is possible to numerically simulate complex flows with high accuracy, once validation with experimental data has been carried out. This article describes an approach based on guidelines taken from the literature and adopted to develop and explore the capabilities of a CFD-based numerical wave tank for a novel multipurpose wave energy converter, REEFS. An incremental validation procedure, using experimental data collected with a piston-type wave tank, was adopted; the procedure began with wave-only tests which were followed by wave-structure interaction tests. Snapshots of numerical and experimental approaches were used to analyse fluid flow in the envelope of the REEFS converter. The results demonstrate that the CFD-based numerical wave tank model can adequately simulate the global wave surface profile, as well as local complex phenomena, such as the Venturi aspiration effect, that typically occur near the exterior stay vanes of the device. The results encourage the adoption of this model for future REEFS analysis.

Power output estimation of WEC with HF radar measured complex representative sea states

Wave tank model testing has been widely used to assess the performance of Wave Energy Converters (WEC) in different technology readiness levels (TRL). At early stage the use of simple wave conditions such as regular waves and parametric wave spectrum JONSWAP or Pierson-Moskowitz spectrum is acceptable. However at later stages there is a need to use site specific complex wave conditions representative of potential prototype deployment sites. In previous research, 10 different regrouping methods on HF radar measured wave spectrum were tested to find out the most representative sea states for tank testing. It has been shown that by using the K-means clustering technique, the characteristics of wave conditions can be well preserved. In order to assess the power capture performance of a typical WEC in these representative sea states, the RM3 point absorber has been simulated. By analysing how well the average power output predicted from different representative sea-state selection methods compares with the total power output prediction, it is shown that the non-directional wave spectrum K-means clustering method provides the most representative sea states and, for a point absorber, with a very accurate estimation of the total power output, which is not the case by using a traditional binning method. The importance of using the complex site-specific sea states rather than simplified parametric JONSWAP sea states to obtain the accurate total power estimation has also been shown.

Study of Liquid Viscosity Effects on Hydrodynamic Forces on an Oscillating Circular Cylinder Underwater Using OpenFOAM®

By neglecting the viscosity of fluid and rotation in flow, the theory of potential flow cannot accurately predict the hydrodynamic forces on the structures under significant viscous effects. In this study, the effects of liquid viscosity on the hydrodynamic forces on a horizontal circular cylinder underwater with a large-amplitude forced oscillation were investigated. The study used a two-dimensional two-phase flow wave tank model based on the viscous fluid theory using the OpenFOAM® package. The numerical calculations were carried out under different types of liquid (i.e., liquid with different viscosities). The liquid viscosity effects are visually shown by comparison of the various frequency components of the hydrodynamic forces on the cylinder, and the magnitude and phase relations of the viscous shear forces and the pressure forces. By analyzing the distribution characteristics of the flow fields around the circular cylinder, the viscous-effect mechanisms are revealed. It is found that the discrepancies of the contributions of viscous shear forces, and the discrepancies of the vortex effects on the phase and magnitude of the pressure forces lead to the obvious differences among the results under different liquid viscosities.

CFD Modeling of Regular and Irregular Waves Generated by Flap Type Wave Maker

The improvement of wave generation in numerical tanks represents the key factor in ocean engineering development to save time and effort in research concerned with wave energy conversion. For this purpose, this paper introduces a numerical simulation method to generate both regular and irregular waves using Flap-Type wave maker. A 2D numerical wave tank model is constructed with a numerical beach technique, the independence of the numerical beach slope is tested to reduce the wave reflections. The different governing parameters of the Flap type wave maker were studied such as periodic time dependency and length of the flap stroke. The linear wave generated was validated against the wave maker theory WMT, the numerical results agreed with WMT. The Pierson-Moskowitz model is used to generate irregular waves with different frequencies and amplitudes. The numerical model succeeded to generate irregular waves which was validated against published experimental data and with Pierson-Moskowitz spectrum model using Fourier expansion theory in the frequency domain. Useful results are presented in this paper based on the numerical simulation to understand the characteristics of the waves. This paper produces a full guide to generate both regular and irregular waves numerically using ANSYS-CFX approach to solve the 2D Unsteady Reynolds Averaged Navier-Stokes Equation (URANS).

Numerical study on the improvement design of the oscillating buoy wave energy converter

The oscillating buoy wave energy converter (OBWEC) is a popular and widely used wave energy converter (WEC) due to the simplicity of its structure and its strong adaptability. In this paper, the “Haiyuan No. 1″ OBWEC is taken as the research object, in which the buoys adopt heave mode of motion. In order to further enhance the hydrodynamic performance of the buoys, an improved scheme with longitudinal swing motion is proposed through theoretical analysis for the buoy. A 2D numerical wave tank (NWT) model based on the RANS equation is established by using CFD software. The numerical model is applied to compare and analyze the hydrodynamic performance differences of the buoy under two different motion forms: heaving and longitudinal swing, and the consistency of the numerical results is validated by the experimental data under regular waves. Finally, the irregular wave data obtained during the sea trial of the “Haiyuan No. 1″ OBWEC is used to numerically simulate the two buoys. The results show that the buoy with longitudinal swing motion is more effective in improving the hydrodynamic performance compared to the buoy with heave motion, in both regular and irregular waves. The conclusions of this paper can serve as a guide to improve the design of the hydrodynamic performance of the buoy.

Model testing of a floating wave energy converter with an internal U-shaped oscillating water column

The development of wave energy converters is centred on the combination of two factors: cost-effectiveness of energy extraction and system survivability on extreme sea conditions. Despite advances in numerical modelling, wave tank model testing still presents the most reliable option to evaluate these two factors. This paper presents an experimental study on the hydrodynamic responses of the UGEN, a wave energy converter consisting of an asymmetric floater with an internal U-shaped tank partially filled with water. The device absorbs energy through the oscillating-water-column (OWC) motion inside the U-shaped tank, induced by the wave action on the floater. The experimental testing of a bottom-moored 1:24th-scale model was performed at the COAST Laboratory, University of Plymouth, UK, considering regular and irregular wave conditions. The wave tank test measurements report six-degree-of-freedom rigid-body motions, mean drift forces, OWC motion, structural stresses and mooring loads. Results present the characterization of the energy absorption at the internal OWC induced by the floater’s sway, heave and roll. The occurrence of low-cycle auto-parametric resonance under certain wave conditions was detected and induced large roll motions, which affected power extraction and increased mooring line loads, particularly for large wave amplitudes.

Numerical simulation of water entry of a symmetric/asymmetric wedge into waves using OpenFOAM

This paper presents a dynamic overset mesh based two-dimensional Numerical Wave Tank (NWT) model to study the water entry of a wedge into water waves in the process of offshore lowering. The NWT model is developed by integrating an incompressible multiphase flow solver on the dynamic overset mesh and a wave generation library in OpenFOAM. Numerical results of water entry of a symmetric/asymmetric wedge into the still water are presented to validate the NWT model by comparing with the published data. A series of numerical simulations of water entry of a symmetrical/asymmetrical wedge into regular waves are carried out, and the pressure coefficients, total force and free surface profiles are presented. Based on the parametric study on the water entry of a wedge into waves, the influence of wave amplitude, water entry velocity, and water entry location (wave peak, wave trough, cross point with the still water level) is analyzed. The numerical solutions provide the fundamentals for the further research on the safe control of water entry of payloads during the offshore installation.

Numerical Study on the Optimization of Hydrodynamic Performance of Oscillating Buoy Wave Energy Converter

Abstract The oscillating buoy wave energy converter (OBWEC) captures wave energy through the undulating movement of the buoy in the waves. In the process of capturing wave energy, the hydrodynamic performance of the buoy plays an important role. This paper designed the “Haida No. 1” OBWEC, in which the buoy adopts a form of swinging motion. In order to further improve the hydrodynamic performance of the buoy, a 2D numerical wave tank (NWT) model is established using ADINA software based on the working principle of the device. According to the motion equation of the buoy in the waves, the influence of the buoy shape, arm length, tilt angle, buoy draft, buoy width, wave height and Power Take-off (PTO) damping on the hydrodynamic performance of the buoy is studied. Finally, a series of physical experiments are performed on the device in a laboratory pool. The experimental results verify the consistency of the numerical results. The research results indicate that the energy conversion efficiency of the device can be improved by optimizing the hydrodynamic performance of the buoy. However, the absorption efficiency of a single buoy for wave energy is limited, so it is very difficult to achieve full absorption of wave energy.

WAVE FIELD GENERATED BY FINITE-SPAN HYDROFOILS OPERATING BENEATH A FREE SURFACE

The present paper focuses on the numerical investigation of the flow around the fully submerged 2D and 3D hydrofoils operating close to a free surface. Iterative boundary element method is implemented to predict the flow field. This study aims to investigate the aspect ratio effect on the free surface interactions and hydrodynamic performance of the hydrofoils under a free surface by using potential flow theory. Three different submergence depths and aspect ratios are studied in the wide range of Froude Numbers. In 3D cases, spanwise width of the numerical wave tank model is selected both equal and wider to the foil span, to observe the sidewall effects. Wave field seems to be two dimensional at low Froude numbers. On the other hand, signs of three dimensionalities are observed on the free surface structure for higher Fn, even the predicted wave elevations are very close to 2D calculations in the midsection. Increment in the Fn give a rise to the amplitude of the generated waves first, however a further increase in Fn has a lowering effect with the beginning of waves spill in the spanwise direction in the form of Kelvin waves. Free surface proximity and resultant wave field are also seeming to be linked with the lift force on the hydrofoil. As aspect ratio of the foil increase, 3D lift values are getting closer to those of 2D calculations. However, it is seen that, 3D BEM predictions of a hydrofoil under free surface effect cannot be considered two-dimensional even the aspect ratio is equal to 8.

A Finite Volume Based Fully Nonlinear Potential Flow Model for Water Wave Problems

A new Fully Nonlinear Potential Flow (FNPF) numerical model has been developed for the simulation of nonlinear water wave problems. At each time step, the mixed boundary value problem for the flow field is spatially discretised by Finite Volume Method (FVM) and the kinematic and dynamic free surface boundary conditions are defined in a semi-Eulerian-Lagrangian form, which are used to update the wave elevation and velocity potential on the free surface. In the numerical model, waves are generated through a relaxation zone and absorbed by an artificial damping zone at the inlet and outlet of the numerical wave tank (NWT), respectively. Instead of a five-point smoothing technique, a more versatile fourth-order technique is developed to eliminate the possible saw-tooth instability at the free surface. Test cases with increasing complexities, such as wave generation and absorption, 2- and 3-Dimensional wave shoaling, and wave-cylinder interaction are simulated to assess its accuracy, convergence, and robustness. For all the cases considered, satisfactory agreements of free surface elevation and wave-induced forces against the experimental measurements and other existing numerical results are achieved. The developed numerical model fully utilises the existing functionalities in OpenFOAM and has the potential to provide an effective alternative to other FNPF based models for constructing a hybrid numerical wave tank model through its coupling with the multiphase flow models in OpenFOAM.

Validation of a CFD-Based Numerical Wave Tank Model of the 1/20th Scale Wavestar Wave Energy Converter

Numerical wave tanks (NWTs) provide efficient test beds for the numerical analysis at various stages during the development of wave energy converters (WECs). To ensure the acquisition of accurate, high-fidelity data sets, validation of NWTs is a crucial step. However, using experimental data as reference during model validation, exact knowledge of all system parameters is required, which may not always be available, thus making an incremental validation inevitable. The present paper documents the numerical model validation of a 1/20 scale Wavestar WEC. The validation is performed considering different test case of increasing complexity: wave-only, wave excitation force, free decay, forced oscillation, and wave-induced motion cases. The results show acceptable agreement between the numerical and experimental data so that, under the well-known modelling constraints for mechanical friction and uncertainties in the physical model properties, the developed numerical model can be declared as validated.

Application of system identification technique in efficient model test correlations for a floating power system

Both the time domain simulations based on potential flow theory and wave tank model tests are used to evaluate the motion responses of a wave energy device. In this work, system identification is applied to obtain the frequency dependent transfer functions (between motions and excitations) from a series of model test runs of a wave energy platform exposed to irregular (random) waves. This is the first application of Reverse-Multiple Input Single Output (R-MISO) to a realistically (catenary) moored system (typical characteristics of wave energy devices) comparing physical model tests with our in-house time domain simulation program with addition of a mooring model. Based on the comparisons between the transfer functions from the time domain simulations and those from the model tests, reasonable frequency dependent dampings have been directly pulled out from the test cases under random sea states. System identification derived corrections to the linear or quadratic damping in pitch significantly improved the accuracy of motion responses. In this sense, this methodology can be a powerful tool in assisting the accurate simulation and design of wave energy devices under random sea states.

THE ANALYSIS OF FLUID DYNAMICS OF WAVE POWER STATION WITH WELLS TURBIN BY CFD

Natural energy such as wind, wave and other natural vibrations is one of the high potential renewable energy sources. The Wells turbine is based on the use of bidirectional turbines, which act as axial-flow self-rectifying turbines that employs a symmetrical blade profile and rotating unidirectionally in reciprocating airflows generated by the air chamber to extract energy from vibrations. These topics have been extensively studied both numerically and experimentally such as research on the parameters of the effects of structure, angle of attack, blade shape, etc. In this paper, numerical simulation is carried out using commercially available tool Fluent for fluid dynamics analysis and focus on oscillating predictions, with particular attention to the behavior of the flow. Based on the Numerical Wave Tank (NWT) model is simulated in a two dimensional used in this model, which is constructed mainly based on the spatially averaged Navier Stokes equation with the k-ε model for simulating the turbulence and modeled with Volume of Fluid (VOF). Axial-flow turbines system and future development as well as the proposed limitations will be discussed in detail.

THE ANALYSIS OF FLUID DYNAMICS OF WAVE POWER STATION WITH WELLS TURBIN BY CFD

Natural energy such as wind, wave and other natural vibrations is one of the high potential renewable energy sources. The Wells turbine is based on the use of bidirectional turbines, which act as axial-flow self-rectifying turbines that employs a symmetrical blade profile and rotating unidirectionally in reciprocating airflows generated by the air chamber to extract energy from vibrations. These topics have been extensively studied both numerically and experimentally such as research on the parameters of the effects of structure, angle of attack, blade shape, etc. In this paper, numerical simulation is carried out using commercially available tool Fluent for fluid dynamics analysis and focus on oscillating predictions, with particular attention to the behavior of the flow. Based on the Numerical Wave Tank (NWT) model is simulated in a two dimensional used in this model, which is constructed mainly based on the spatially averaged Navier Stokes equation with the k-ε model for simulating the turbulence and modeled with Volume of Fluid (VOF). Axial-flow turbines system and future development as well as the proposed limitations will be discussed in detail.

Numerical investigation of hydrodynamic characteristics on horizontal circular cylinder induced by a forced oscillatory

This study investigates the hydrodynamic characteristics of a submerged horizontal circular cylinder under forced oscillation by using a 2-D two-phase flow numerical wave tank (NWT) model based on the viscous fluid theory. The calculations of the hydrodynamic forces on the circular cylinder under various liquid-phase viscosities were performed. By studying the relationship between the hydrodynamic forces and motion of the cylinder and analysing the flow fields around the cylinder, the mechanism of the viscous effects was identified. The study shows that differences between the results of the cases under different viscosities are on account of different contributions of the viscous shear forces to the total hydrodynamic forces and different influences of the vortex on the hydrodynamic forces. The considerable influences of fluid viscosity on the motion hysteresis of the vortex lead to a phase difference between the pressure forces on the cylinder under different viscosities. The phase advance phenomenon of the viscous shear forces can be explained by the fact that compared with the motion of the cylinder, the motion hysteresis of the water-particles in the higher viscosity fluid is more obvious.