Piston-type Wavemaker Research Articles

Assessment of unidirectional internal solitary wave models in a two-layer fluid system through an improved experimental method

The suitability of unidirectional internal solitary wave (ISW) models in a two-layer fluid system is evaluated using an improved experimental method. A new method for generating internal solitary waves in the laboratory tank is proposed, utilizing a piston-type wave maker in a density stratified two-layer fluid. The prescribed velocities of the pistons are determined by the mean velocities derived from the adjusted High-order Unidirectional (aHOU) model for the upper and lower layers. Experimental conditions encompass a range of wave Reynolds numbers between 1.8×105 and 2.3×105. The repeatability and amplitude control of the proposed wave-making method are verified. The unidirectional models considered in this study encompass the Korteweg–de Vries (KdV), Gardner and aHOU model. Laboratory experiments and theoretical computations have been systematically compared in this study to examine the wave characteristics inherent in typical unidirectional internal solitary wave models. It is found that Reynolds number (Re) and nonlinear parameter (α=|a|/h) serve as suitable criteria for delineating the range of applicability of unidirectional models. According to these criteria, the KdV theory is suitable for ISWs with amplitudes α<0.02 across all Reynolds number cases, while the Gardner model is valid for cases where α<0.02 and Reω>2.2×105, with α<LimG, where LimG represents the non-dimensional limiting amplitude. The aHOU model exhibits the capability to capture fully nonlinear ISWs across all Reynolds numbers considered in this study.

Experimental and numerical study on the temporal and spatial nonlinearity evolution of focused wave

Extreme waves pose a significant threat to coastal structures. Focused wave, representing the average shape of extreme waves, is frequently employed to simulate extreme waves. Despite its convenience of generating large amplitude waves, the nonlinearity evolution of focused wave during propagation has been seldom explored. It is obvious that the nonlinearity would influence the wave shape and energy structure during focusing. This study investigates the temporal and spatial nonlinearity evolution experimentally and numerically. The experiment was conducted in a wave tank using a piston-type wavemaker. The numerical model was set up with geometry adopted from physical experiment in the commercial software FLUENT. The turbulence model RNG k–ε was employed, while the volume of fraction method was used to capture the air-water interface. The high-order harmonics were extracted from the recorded wave elevations by the four-phase decomposing method. The evolution of high-order harmonics and the energy structure were obtained. It is validated that the high-order harmonics can be reasonably estimated by multiplying linear harmonics by harmonic coefficients calculated by Stokes III or V wave theory. This paper enhances the understanding of temporal and spatial nonlinearity evolution of focused wave, which would be helpful for structure design and site selection.

The hydrodynamic study of the interaction between waves and objects in permeable reef environment

This study employed a self-developed viscous numerical wave tank based on the piston type wave maker and user-defined functions of Fluent software to explore the wave shadowing effect of the transmission of waves near the underwater object and the reef. In the study, various factors such as the permeability of the reef, changes in topography, and objects motion were considered, and detailed simulation analyses were conducted for five scenarios: a solitary reef, a fixed underwater object, a forced moving underwater object, and a free moving underwater object. The results showed that this method can effectively simulate the complex flow field changes around the underwater object and the reef under different conditions, while also fully considering the strong nonlinearity and fluid viscosity effects between the wave surface and the object. Furthermore, it was found that between the free-moving object and the reef, there would be significant swirls and streamline distortion. This indicates that a portion of the incident wave energy is transformed into kinetic energy of swirls, resulting in a smoother and more uniform flow field and reducing wave forces.

3D SPH analysis of focused waves interacting with a floating structure

In recent years, the number of offshore floating structures has been growing and is expected to continue growing. Due to climate change, the frequency and severity of extreme waves are increasing. Numerical models can be a strategic tool in the reliable design and optimization of marine structures. However, the number of numerical parameters to be tuned plays a crucial role, as they could limit the model’s applicability and reliability. Furthermore, there are many challenges in modelling wave-structure interaction. Among these challenges, the simulation of one-dimensional tethers is of great concern when dealing with moored floating structures. In this study, a 3D numerical model is developed based on the Smoothed Particle Hydrodynamics (SPH) technique. This hydrodynamic model is coupled with a multi-body solver for the dynamic analysis of moorings and structures. In a three-dimensional numerical flume, focused ocean waves are generated with a piston-type wave-maker and propagated until impact with the floating structure. The developed model predicts the wave profile, impact forces, and structural dynamics, and is validated by comparing numerical results against experimental data.

CFD based modeling of OWC device positioned over stepped bottom

This research deals with various parameters associated with the hydrodynamics of an OWC device positioned over the stepped bottom. The problem is solved using the multiphase based CFD technique in “ANSYS Fluent software”. A “piston type wavemaker” and dynamic meshing technique are employed to produce the incident wave inside the numerical wave tank (NWT). Further, “volume of fluid” procedure is implemented to produce the free surface in the NWT. The “free surface elevation” is analyzed in various instant of time. Contour plots of the dynamic pressure, turbulent kinetic energy and turbulent intensity associated with the fluid motion are provided. Further, the variation of the vertex average total pressure and vertex average y− velocity at the orifice on the OWC device’s top wall are analyzed with respect to flowtime.

Methodology to measure the energy flux captured by a submerged U-OWC by using temperature sensors

The estimation of the power captured by a wave energy converters (WEC) device, needs to calculate the plant efficiency. In general, it is necessary to measure both the pressure and the discharge fluctuations of the fluid motion inside the plant. Unfortunately, gauges for the direct measurement of the velocity are bulky and provide punctual measures and, especially on converters having a U-duct, the presence of the velocity sensor produces a relevant disturbance on the motion field. To overcome this issue, an alternative method to evaluate the captured energy flux, using the pressure fluctuation and the air temperature inside the plenum, was proposed by [1] and [2]. However, no information about the accuracy of the temperature sensors and consequently about the errors in estimation of the energy flux were provided.&#x0D; In this work, following the procedure described by [1] and [2], we have analysed the influence of the time response of the temperature sensor in evaluating the variation of the air volume inside the chamber and, consequently the energy captured by the plant. To this aim, the submerged U-OWC, tested directly at sea in [1], has been simulated numerically. The aim of the numerical experiment is having the actual estimation of the air temperature inside the plenum and trough it, the captured energy flux. The computational domain is constituted by a wave-flume, with a piston-type wavemaker, placed at the left extremity and a submerged breakwater embedded a U-OWC plant, in the middle. The numerical 2D unsteady simulation is based on the Eulerian approach, using the commercial code Ansys Fluent v17.0, Academic Version.&#x0D; Starting from the knowledge of the pressure fluctuation at the upper opening of the vertical duct and, of both pressure and temperature variations of the air in the plenum, we have evaluated the energy flux absorbed by the plant and we have calibrated the mathematical model used in [1] and [2], using as input the time series of the pressure fluctuations at the upper opening of the vertical duct, and the variation of both temperature and pressure of the air inside the chamber. Then, using the time series of the actual air temperature, we have simulated the input of several first order temperature sensors characterized by different time constant t, and we have analysed the percentage differences in term of energy flux as a function of t.&#x0D; We have observed that the measurements of the temperature inside the plenum are strongly affected by time constant of the sensor, which produce large errors in the evaluation of the captured energy flux.&#x0D; Finally, we have proposed a method for conditioning the measure of the air temperature, obtaining an excellent estimation of the energy flux.&#x0D; &#x0D; Boccotti, P. (2003) "On a new wave energy absorber"Ocean Engineering 30(9), pp. 1191-1200.&#x0D; Arena, F., Filianoti, P. (2007),"Small-scale field experiment on a submerged breakwater for absorbing wave energy", Journal of Waterway, Port, Coastal and Ocean Engineering, 2007, 133(2), pp. 161–167.&#x0D;

Numerical study of wave-ice floe interactions and overwash by a meshfree particle method

The interaction between regular waves and ice floes under certain wave conditions involves overwash which is characterized by waves running up over the surface of an ice floe with large deformed free surfaces. An in-house moving particle semi-implicit (IMPS) method is employed to simulate the wave-ice floe interaction, especially for the specific behavior namely overwash due to a small freeboard of sea ice and a large body motion. In this work, a series of regular waves are generated using the Lagrangian particle method based on the Stokes wave theory. Models of the floating sea ice are simplified as circular disks without and with an edge barrier and a circular disk with a central hole, which can be used to study the influences of overwash. Hydrodynamic responses of the ice floe in regular wave conditions with different wave lengths and a constant wave height are analyzed. Compared with the mesh-based methods, the Lagrangian particle-based methods have advantages in treating highly deformed free surfaces during wave overwash processes. A piston-type wave maker and a sponge layer are applied for generating waves and avoiding the wave reflection from wall boundaries, respectively. Validations are presented including the circular ice floe with a diameter of 400 mm and a thickness of 15 mm interacting with regular waves with the wave length λ/D from 1.5 to 3.5. RAOs of the original circular disk are compared with those of the circular disks with an edge barrier and with a central hole. Overwash on the ice floe can be prevented by using an edge barrier. A new type of overwash which waves flow over the upper surface and then into the hole was proposed and simulated. The present results of numerical predictions are in good agreement with the experimental data and the published numerical results using the CFD code Flow-3D.

Numerical modeling of water waves with the highly efficient and accurate Lagrangian-Eulerian stabilized collocation method (LESCM)

A novel numerical wave tank (NWT) based on the Lagrangian-Eulerian stabilized collocation method (LESCM) is constructed for modeling water waves, in which the piston-type wavemaker is employed for wave generation, and the relaxation zone technique is used for wave absorption. The LESCM is an evolution version of the particle-in-cell method. In its discretization scheme, a background mesh covering the entire tank is introduced for solving the incompressible Navier-Stokes equations by the stabilized collocation method (SCM), and the water inside is discretized by Lagrangian particles which shuttle within the Eulerian mesh. Then, the Eulerian mesh communicates with Lagrangian particles through high-order consistent reproducing kernel (RK) interpolation. Besides, the mesh is initialized at the beginning of each time step, which can avoid the reconstruction of shape functions and dramatically reduce computational efforts. Furthermore, the numerical solution of the SCM is locally and globally conserved since the SCM is a subdomain method just like the finite volume method. Moreover, the information transfer between nodes and particles satisfies the conservation property, which makes the novel NWT a fully conservative model. Several numerical examples demonstrate that the proposed NWT is highly efficient, accurate and stable, and hence can be applied to the research of coastal and offshore engineering.

Validation of multiphase water wave propagation using smoothed particle hydrodynamics

Water wave propagation is one of the classical problems in free surface flow that involved water and air. There are a lot of studies have been conducted on both experimental and numerical methods. One numerical method is computational fluid dynamics (CFD), which is divided into two major i.e. CFD mesh-based and meshfree. In this paper, smoothed particle hydrodynamics (SPH) is used to carry out a study of water wave propagation. SPH is a mesh-free CFD and Lagrangian approach that is suitable for free surface flow such as water waves. Two-phase SPH was used to reproduce long-distance propagation using a piston-type wavemaker. The wave probe was set 24.6 m in front of the wavemaker with a water depth is 0.75 m. The wave period is 1.15 s with an amplitude of the wavemaker is 33.0 mm. The simulation is carried out with a water and air phase (multiphase). The results indicate SPH is in reasonable agreement with experimental for the wave height, celerity, and amplitude.

A Comparative Study on Generation and Propagation of Nonlinear Waves in Shallow Waters

This study is concerned with the generation and propagation of strongly nonlinear waves in shallow water. A numerical wave flume is developed where nonlinear waves of solitary and cnoidal types are generated by use of the Level I Green-Naghdi (GN) equations by a piston-type wavemaker. Waves generated by the GN theory enter the domain where the fluid motion is governed by the Navier–Stokes equations to achieve the highest accuracy for wave propagation. The computations are performed in two dimensions, and by an open source computational fluid dynamics package, namely OpenFoam. Comparisons are made between the characteristics of the waves generated in this wave tank and by use of the GN equations and the waves generated by Boussinesq equations, Laitone’s 1st and 2nd order equations, and KdV equations. We also consider a numerical wave tank where waves generated by the GN equations enter a domain in which the fluid motion is governed by the GN equations. Discussion is provided on the limitations and applicability of the GN equations in generating accurate, nonlinear, shallow-water waves. The results, including surface elevation, velocity field, and wave celerity, are compared with laboratory experiments and other theories. It is found that the nonlinear waves generated by the GN equations are highly stable and in close agreement with laboratory measurements.

A high-order finite difference method with immersed-boundary treatment for fully-nonlinear wave–structure interaction

In order to predict nonlinear wave loading on marine structures, a fully nonlinear higher-order finite difference based potential flow solver with all boundary conditions treated by an immersed boundary method has been developed in this paper. The solver adopts high-order finite difference schemes for the spatial derivatives and a 4th order Runge–Kutta method for time stepping. Test cases of forced oscillation of a cylinder in an infinite fluid domain are first studied, which reveal the advantage of the acceleration potential method in terms of wave load computation. Then a wave generation problem using a piston type wave maker is tested. Special attention is paid to the intersection point between the free surface and the body surface, and a scheme which best meets the accuracy and stability requirements is suggested from several proposals. A novel hyperviscosity filter, which works for both uniform and non-uniform grids, is introduced to stabilize the time-domain solution of the wave maker problem. Finally, a forced heaving cylinder on the free surface is considered, and the nonlinear wave loads on the cylinder are analyzed and compared to benchmark results.

Prediction of buffeting responses of the thin plate under joint action of wave and wind using LSTM and transfer learning

To predicate the buffeting responses of long-span sea-crossing bridges under joint action of wave and wind, a thin plate was taken as the example, and a piston-type wave maker was adopted to simulate the second order Stokes wave in a computational domain. Through the large eddy simulation (LES) and dynamic mesh methods, time-histories of wind field and buffeting response of the thin plate under joint action of wave and wind were obtained. Then, the buffeting responses of the thin plate were predicted using the conventional long short-term memory (LSTM), convolutional LSTM (Conv LSTM) and LSTM with attention mechanism (LSTM-AM) neural network models, respectively. Furthermore, the transfer learning (TL) method was introduced, the method to solve a large amount of source task data was established, and a combined neural network model of TL-Conv LSTM-AM was proposed to predict the buffeting responses of the thin plate. The results show that the prediction accuracies of Conv LSTM and LSTM-AM models are both higher than the conventional LSTM model based on the evaluation indexes of determination coefficient (R2) and root mean square error (RMSE). The prediction accuracies of the buffeting responses can be improved significantly when the above LSTM neural network models are combined with the TL method. Also, there are still some deviations between the predicated time-histories and target ones. For the proposed combined model of TL-Conv LSTM-AM, the values of R2 in three directions of the buffering responses are all larger than 0.996 and 0.983 in the high and low turbulent flow fields, respectively. Therefore, the proposed TL-Conv LSTM-AM combined model has the highest prediction accuracy among them. Similar prediction accuracy can be achieved for longer time duration of the testing data set using the proposed combined model of TL-Conv LSTM-AM. The above phenomenon proves that the effectiveness of this proposed combined model in predicting the buffeting responses of the thin plate under joint action of wave and wind.

A twin wavemaker model for liquid sloshing in a rectangular tank

Sloshing of a liquid in a rectangular container under horizontal excitations is modelled using a wave basin analogue within the framework of a potential flow theory. An initial boundary-value problem is formulated replacing fixed walls of the tank with two orthogonal twin piston-type wavemaker paddle pairs. The proposed equivalent mechanical system may be applied to a class of problems covering irrotational flow of an inviscid and incompressible fluid inside partially-filled rectangular containers exposed to translatory oscillations. An analytical solution to the problem is provided under the assumption of a small-amplitude monochromatic forcing for a classical linear wavemaker in a closed flume and two- and three-dimensional twin wavemaker problems. The model is extended to cover an arbitrary horizontal motion using a semi-analytical approach based on a difference time-marching schemes applied to linearised evolution equations in a spectral domain. This relatively simple conceptual model may be used to design a novel experimental framework for studying water wave mechanics at smaller scales relying on elongated sloshing tanks substituting typical wave flumes.

A numerical experiment on a new piston-type wavemaker: Shallow water approximation

In this paper, we present a numerical experiment on a new piston-type wavemaker using a recently proposed piston-type wavemaker theory. The theory was primarily derived from the classical Boussinesq equation, based on the Pseudo-parameter Iteration Method (PIM). We first use the new theory to observe low amplitude propagating solitary waves and cnoidal waves, whereby we see the workability and validity of the theory. We further succeed in obtaining the graph of a mean power characteristic, in the range of 0.4 ≤kh≤ 1.0, of the new piston-type wavemaker from the new theory.

Numerical Generation of Solitary Wave and Its Propagation Characteristics in a Step-Type Flume

This work concerns the numerical generation of stable solitary waves by using a piston-type wave maker and the propagation characteristics of a solitary wave in a step-type flume. The numerical generation of solitary waves was performed by solving N-S (Navier–Stokes) equations on the open source CFD (computational fluid dynamics) platform OpenFOAM. To this end, a new module of dynamic boundary conditions was programmed and can be applied to prescribe the horizontal linear motion of a paddle. Two kinds of paddle motions, based on both the first-order and ninth-order solutions of solitary waves, were first determined. The time history of paddle motion was restored in a file, which was then used as an input for the virtual wave maker. The solitary wave in water with a constant depth was generated by both numerical simulation and experiment in the wave flume installed with a piston wave maker. The results show that the amplitudes of trailing waves based on the first-order solution are larger than those based on the ninth-order solution and that wave height based on the first-order solution decays more quickly. The numerical wave profiles are in good agreement with the experimental ones. The propagation characteristics of a solitary wave in a step-type flume was numerically investigated as well. It was found that a part of the solitary wave is reflected when the solitary wave passes the step due to blockage effects, and the forward main wave collapses quickly when it enters shallow water. This work presents a very successful numerical study of stable solitary wave generation and reveals the phenomena when a solitary wave propagates in a step-type flume.

Comparison of laboratory wave generation techniques on response of a large monopile in irregular sea

As the offshore wind industry moves toward larger monopile turbines, model testing and validation of hydrodynamic load models become more important for new designs of turbines. Wave generation is an important aspect of hydrodynamic model testing. When generating irregular waves with a piston-type wavemaker, first order wavemaker theory is commonly used. This leads to generating spurious free waves in the tank. Using second order wavemaker theory reduces the generation of these spurious waves. In this study, the two wave generation techniques have been used in the measurement of the dynamic responses of a monopile with (full-scale) natural period of 5 sec. The effect of superharmonic spurious waves on the response statistics was minor. A marginal improvement in experimental repeatability of the second mode response in large wave events was observed by using second order wavemaker theory.

Conceptual Design of a Vibration Test System Based on a Wave Generator Channel for Lab-Scale Offshore Wind Turbine Jacket Foundations

Structural health monitoring (SHM) systems are designed to continually monitor the health of structures (e.g., civil, aeronautic) by using the information collected through a distributed sensor network. However, performing tests on real structures, such as wind turbines, implies high logistic and operational costs. Therefore, there is a need for a vibration test system to evaluate designs at smaller scales in a laboratory setting in order to collect data and devise predictive maintenance strategies. In this work, the proposed vibration test system is based on a lab-scale wind turbine jacket foundation related primarily to an offshore environment. The test system comprises a scaled wave generator channel, a desktop application (WTtest) to control the channel simulations, and a data acquisition system (DAQ) to collect the information from the sensors connected to the structure. Various equipment such as accelerometers, electrodynamic shaker, and DAQ device are selected as per the design methodology. Regarding the mechanical part, each component of the channel is designed to be like the wave absorber, the mechanical multiplier, the piston-type wavemaker, and the wave generator channel. For this purpose, the finite element method is used in static and fatigue analysis to evaluate the stresses and deformations; this helps determine whether the system will work safely. Moreover, the vibration test system applies to other jacket structures as well, giving it greater utility and applicability in different research fields. In sum, the proposed system is compact and has three well-defined components that work synchronously to develop the experimental simulations.

Modeling of solitary wave interaction with emerged porous breakwater using PLIC-VOF method

Rubble mound breakwaters are porous structures providing a calm basin for ship berthing, loading, and offloading in ports. In this paper, a numerical model was developed to study solitary wave-porous breakwaters interaction using the VOF method. The flow was assumed to be incompressible and viscous; therefore, Navier-Stokes and continuity equations were extended as the governing equations. In the study domain, waves were generated and propagated via a piston type wave maker in the left lateral boundary, while in the right end side, an open boundary was implemented. The model was validated by comparing its results with existing data for wave interaction with porous media. A parametric study was then conducted on wave-porous breakwater interaction. Effects of the important parameters such as porosity, drag and inertia coefficients of the medium and submergence and emergence of the breakwater on the wave damping and reflection were evaluated. The results demonstrate that the developed model is a powerful tool for this complicated wave structure interaction modeling.

A simplified method and numerical simulation for wedge-shaped plunger wavemaker

The wave can be obtained by the horizontal movement of the piston-plate or flap-plate, and also could be obtained by the vertical movement of the wedge. In this paper, Firstly, the implicit equations of the relationship between the wedge vertical motion trajectory and the targeted wave including linear wave, stokes-fifth-order wave, solitary wave and irregular wave, are derived. Secondly, the RNG k-ε turbulence model and VOF method are combined to solve the RANS equations for wave-making of targeted wave by wedge-shaped plunger wavemaker using CFD software Fluent. Thirdly, the numerical results are compared with the analytical solutions systematically, also the characteristics of the wave surface, velocity field, water particle trajectories, as well as their evolution and propagation are elaborately explored. Meanwhile, the effect of the initial entry depth and the wedge angle on the wave-making quality was discussed, and the differences in the force and power between wedge-shaped plunger wavemaker and piston-type wavemaker are compared in detail. The velocity field and water particle trajectories present a comprehensive understanding of wave generation and propagation. The results indicate that the proposed simplified method and numerical simulation have high accuracy for linear wave, weakly-nonlinear stokes-fifth-order wave, solitary wave and irregular wave.

Design optimization of a novel vertical augmentation channel housing a cross-flow turbine and performance evaluation as a wave energy converter

A novel vertical augmentation channel housing a direct-drive cross-flow turbine, with nozzles on both the sides of the turbine, was designed and an optimized configuration was obtained. The geometries of the guide nozzle and the front nozzle were optimized under steady flow conditions. The performance of the cross-flow turbine was analyzed using commercial computational fluid dynamics (CFD) code ANSYS-CFX. The optimized design was then evaluated as a wave energy converter both experimentally and computationally. The waves in the numerical wave tank (NWT) were generated using a piston type wave-maker. The optimized design gave a maximum output power of 13.2 W and an efficiency of 48.31% at a wave height of 0.2 m and wave period of 2.75 s for a rotational speed of 35 rpm. The difference between numerical and experimental efficiencies was within 3.5%. In addition to this, particle image velocimetry was used to study the flow characteristics in the augmentation channel and the turbine. The results show that the CFD code captures the flow in the augmentation channel and around the turbine accurately. The optimized design, which occupies less space than other wave energy converters, can be used to efficiently harness energy from the waves.