Flap-type Wavemaker Research Articles

Flow and heat transfer characteristics of a porous-coated cylinder under simultaneous wave and current forces at a high Reynolds number

In the present study, a two-dimensional finite volume method is used to simulate the heat transfer rate of a circular cylinder coated with different structures of porous materials at Re=4800. The incompressible uniform flow is associated with deep-water waves created by a flap-type wave maker to investigate the effects of simultaneous current and wave forces on the pressure coefficients, flow characteristics, and heat transfer rate of cylinders wrapped by various porous structures (S2, S3, S4, and S5) and different Darcy numbers (Da=10, 10−1, and 10−2). According to the results, the porous layer thickness (e*) affects the outer shape of the porous coating and alters the influence of wave–current interaction on the flow characteristic. Therefore, compared to the current case, the lift and drag forces in the wave–current case increase in S2, but decrease in other porous structures. Moreover, S5, with the thickest porous coating showed the maximum heat transfer rate. Porous materials with low permeability decrease the impact of e* on heat transfer. However, the vortex-shedding patterns with thermal plumes become stronger with decreasing the Darcy number. The heat transfer rate of porous structures increases by about 42% as Da decreases from 10 to 0.1, and another 37% increase is observed at Da= 0.01, resulting in a total rise of 96% in the average Nusselt number (Nu¯). Therefore, the gap between the heat transfer rate of porous structures and the smooth cylinder (S1) increases by about 17%, 79%, and 126% at Da= 10, Da= 0.1, and Da= 0.01, respectively.

Experimental validation of the plunger-type flow model for irregular waves

In this paper, a study of irregular waves generated with a plunger-type wavemaker with the inclusion of mean flow is conducted along with an analysis of the standard plunger-type flow (PTF) model. It was determined that for high frequencies, the standard PTF model exhibits amplification which when compared to theoretical models for piston- and flap-type wavemakers is implausible. Therefore, a correction to the PTF model has been proposed for use with irregular waves that limits the value of the wave amplitude a to stroke amplitude s ratio a/s to one or less. Experimental validation of the correction is also given along with a study on the performance of the plunger system for generating the desired irregular Bretschneider wave spectrum. In this paper, multiple significant wave heights and peak frequencies are considered for no-flow and three mean flow cases. The experimental results showed that not only is the proposed correction valid for decreasing the percent error in the significant wave height but, plunger-type wavemakers are capable of accurately producing irregular spectra regardless of the inclusion of mean flow.

다기능 조파기의 조파 운동과 발생 파형

Piston type wave makers or flap type wave makers are usually adopted as a wave maker which disturbing the fluid domain with sinusoidal motion. Recently hybrid wave maker which could be operated as not only piston type and/or flap type but also swing type wave maker have been devised by utilizing the link mechanism. The wave board of hybrid wave maker has been devised to be independently controlled by the horizontal actuators on upper and lower end of the wave board. The wave board could operate as a flap type wave board when the lower hinge is in a stationary condition and the upper hinge is operated with sinusoidal motion. On the contrary, the swing type wave board could be obtained by the lower hinge is activated and the upper hinge is in a stationary condition. When both end of the wave board is activated in a synchronized condition, the wave board motion become piston motion. In addition the hybrid wave maker could enhance the piston motion with flap motion or swing motion by selecting control parameters. Various wave board motion of hybrid wave maker and relevant wave form have measured on the wave board and departed location. It is appeared that the novel hybrid wave maker could be utilized for the improvement of wave qualities in experiments.

Full-scale SPH simulations of ship-wave impact generated sea spray

Sea spray generated by ship-wave impact contributes to marine ice accretion onboard vessels. Limitations in field measurements and model experiments encourage the use of numerical simulation to understand the formation of such spray. In this paper, full-scale computational fluid dynamics (CFD) models of wave-generated sea spray are developed using a smooth particle hydrodynamics (SPH) method. A three-dimensional (3D) numerical wave tank equipped with a flap-type wave maker and a wave absorber is created to produce regular waves of various heights and steepness. A full-scale medium-size fishing vessel (MFV) is modeled to encounter incoming waves at head sea conditions at various forward speeds. Moving ship dynamics with three degree-of-freedom (3-DOF) in waves are resolved instead of mimicking a relative ship speed. The resultant spray water amount is measured using a numerical collection box and compared against field measurements and a theoretical model, where a reasonable agreement is found. The model is able to distinguish between green water and spray water. A multi-phase two-dimensional (2D) simulation is also performed that demonstrates the role of winds in the fragmentation of water sheets into droplets. The simulation results indicate energy released from a surging ship significantly contributes to the generation of spray.

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).

An analytical and numerical study of a vertically discretized multi-paddle wavemaker for generating free surface and internal waves

A vertically discretized multi-paddle wavemaker is proposed that generates various wave types without the depth-invariant flow condition limitation associated with conventional piston type wavemakers. A first-order mathematical formulation for the paddle stroke is derived and used to selectively generate free surface waves similar to either piston type or flap type wavemakers, depending on the number of utilized paddles. The mathematical formulation is further extended to a multi-phase internal wave case. Paddle discretization performance is evaluated using OpenFOAM, a numerical model that solves the Navier-Stokes equations for two (or more) immiscible and incompressible fluids. Dynamic meshing capability is incorporated in the model. Model results show multiple discrete paddles efficiently generate deep water waves under laminar flow conditions. In intermediate wave conditions, the multi-paddle discretization is less advantageous. Internal waves and resulting free surface waves among air, oil, and water phases are simulated using only two paddles and imposing a fast mode. Model results agree with flow characteristics for both internal and locked free surface waves.

A numerical solution of the wave–body interactions for a freely floating vertical cylinder in different water depths using OpenFOAM

In the present study, a finite volume method based on Reynolds-averaged Navier–Stokes equations is developed numerically in OpenFOAM to simulate two-phase flows around a freely floating cylinder in a numerical wave tank (NWT). All numerical computations are carried out using interDyMFoam solver, manipulating dynamic mesh. The left wall of NWT oscillates as a paddle wave-maker to produce desired waves. The accuracy of the current numerical method is confirmed by evaluating the results of some published experimental benchmarks, including the generation of regular waves in NWT and also the response of two fixed and floating objects in waves. Based on the current study findings, OpenFOAM is capable of predicting these cases of wave–structure interaction with good delicacy. Additionally, the moving wall of NWT as a flap-type wave-maker can produce waves with different attributes sufficiently. In the following, the influence of regular waves on a circular cylinder, operating freely in deep water, as well as the effect of the water depth on the cylinder’s movement and forces acting on it are assessed. The results indicated that the change in the water depth has a significant effect on the response of the freely floating cylinder in waves.

Reproducing an opposing sea in an experimental wave basin based on a hindcast spectrum

An opposing sea was generated in an experimental wave basin. Here, an opposing sea is defined as a sea in which the wave spectrum has two peaks that are separated almost 180° in direction. Such an opposing sea can be observed, for example, when wind waves coexist in the opposite direction with swells. The opposing sea generation is based on an actual sea spectrum hindcast by a third-generation wave model. The experiment was carried out in an actual sea model basin (ASMB) at National Maritime Research Institute. The ASMB is fully surrounded by 382 flap-type wave makers with incident wave absorbing capacity. The estimation of the directional spectrum by the maximum likelihood method (MLM) revealed that the opposing sea was successfully generated in the basin, although an analysis of the corresponding numerical wave field indicated that the MLM inherently estimated the spectrum with wider directional spreading. The reproduction of the opposing sea in this study indicates that any deep-sea wave field with arbitrary directional spectrum can be reproduced in such a wave basin with absorbing wave makers on the periphery.

A Computational Fluid Dynamics Investigation of a Numerically Simulated Wave Tank

In this paper, a two-dimensional Numerical Wave Tank (NWT) is proposed to calculate the static pressure variation along the lower wall of an experimental wave-flume. The experimental setup was a 4.72m long wave flume with a flap-type wave-maker. The experiments were carried out at various water heights of 100mm, 80mm, and 60mm, with a motor speed of 60 rpm. The numerical simulations were completed using ANSYS™ Fluent, with two sets solutions: 1) the unsteady, three-dimensional Reynolds Averaged Navier-Stokes (URANS) equations coupled with a k-e turbulence model; 2) unsteady 3-D Euler equations. In both computations, the volume of fluid (VOF) method was used to capture the free surface and a grid independence study was completed. The unsteady Euler simulations showed the best agreement to the experimental results. Several cases were run to complete validation and verification of the numerical model, and the CFD results are in good agreement with the experiment. Thus, for small two-dimensional experimental wave flumes, the unsteady inviscid, volume of fluid method can accurately predict surface pressure distribution.

Wave generation characteristic analysis of piston and flap type wave maker with rotary-valve-control vibrator

Wave maker is one of the most important experimental equipment in marine engineering. To meet the demands of simulation of higher wave amplitude and compare the effect of piston and flap type wave generation, a new wave generation device was proposed and a new piston and flap type wave maker with a rotary-valve-control vibrator was developed. A mathematical model of the new wave maker was established and analysed by Simulink, and a series of experiments were conducted on the wave maker to analyze wave generation characteristics. The results show that the wave maker can adjust the distance of wave paddle and generate different regular waves. The bigger the axial opening size of the valve port, the larger the wave paddle amplitude and the wave amplitude; the higher the pressure, the higher the wave paddle amplitude and the wave amplitude. High frequency wave making is more efficient than the lower one, and piston type wave making is more efficient than those wave makers that generate waves by flap type.

Generation and Propagation of Nonlinear Waves in a Towing Tank

Abstract The paper presents the results of the research focused on linear and nonlinear wave generation and propagation in a deepwater towing tank equipped with a single flap-type wavemaker of variable draft. The problem of wave generation and propagation has been theoretically formulated and solved by applying an analytical method; linear and nonlinear solutions were obtained. The linear solution has been verified experimentally. The laboratory experiments confirmed that a linear model can be applied to predict the generation and propagation of water waves of low steepness. However, according to the analysis, the discrepancies between wave profiles predicted by applying the linear and nonlinear models rapidly increases with increasing wave steepness. Additionally, the secondary phenomena which occur in the towing tank, including: disintegration of wave profile, wave reflections from the beach and wave damping, were analyzed. Knowledge on the nonlinear processes and phenomena is essential for modeling the environmental conditions during tests carried out to secure the safety of the naval and offshore constructions. The theoretical formulation was derived and the solution was obtained by the Institute of Hydroengineering of the Polish Academy of Sciences IBW PAN while the experimental research was carried out in Ship Hydromechanics Division of the Ship Design and Research Centre CTO S.A.

Wave generation by flap-type wavemaker in circle arc numerical wave tanks

A circle arc numerical wave tank for wave generation in a geotechnical centrifuge is presented in this paper. Compared with traditional straight wave tank, the use of the circle arc numerical wave tank can minimize the error due to the radial gravity field which produced by the centrifugal acceleration. A variety of cases generated by flap-type wavemakers in intermediate and deep waters are simulated in the circle arc numerical wave tanks and the accuracy of the numerical results are verified by a comparison with the numerical results in traditional straight wave tanks and the results of wavemaker theory. The results show that the wavemaker theory produced by Ursell can be used both in traditional straight numerical wave tanks but also in circle arc numerical wave tanks. The slight discrepancy of the results between the circle arc wave tanks and the traditional straight wave tanks can be attributed to the nonlinear effect caused by the circle arc. However, the discrepancy is insignificant which can be neglected for a circle arc wave tank with a relative long radius.

New approach to flap-type wavemaker equation with wave breaking limit

ABSTRACTThe limitations of the classical wavemaker theory motivated the development of a new equation that can directly predict both regular and broken waves based on the flap-type wavemaker setup. This is achieved first by coupling a commonly accepted wave breaking formula with the linear wavemaker equation. Both these equations were then rewritten in terms of the paddle stroke, water depth, and frequency instead of the wave number. Additionally, the validity range for each equation was explicitly defined to predict the maximum wave height before breaking. Comparison with both classical wavemaker theory and measurements confirms the reliability and accuracy of the proposed equation.

Study of interaction between wave-current and the horizontal cylinder located near the free surface

In this paper, the authors experimentally and numerically study the interaction between wave-current and the horizontal cylinder near the free surface. The experiments are conducted in the Circulating Water Channel with varying axis depths of cylinder. Considering the same wave height and different wave lengths, two regular waves are generated combined with current using a flap-type wave maker. Also, the numerical model based on the RANS equations is solved by the finite volume method, in which the RNG k−ε model is adopted to simulate the turbulence while the VOF method is used to capture the free surface. In this study, the free surface deformation due to wave reflection and blockage is investigated firstly. Then, the typical features of the wave-current force on the cylinder with various axis depths are studied. The peak values of the force are also discussed by comparison with those calculated by the modified Morison’s equation. Besides, the vorticity field around the fully submerged cylinder is discussed in detail. It is found that the wave-current force value is affected by both wave reflection and wave blockage under lower cylinder submergence. To be detailed, the force value increases due to wave reflection while decreases because of wave blockage. With regard to the partially submerged cylinder, wave reflection plays a dominant role compared with wave blockage. Therefore, the measured force value is larger than the theoretical one by modified Morison’s equation. However, for the fully submerged cylinder, its wave blockage is more notable in contrast with dramatically reduced wave reflection, resulting in a lower measured force value compared with theoretical one.

A laboratory investigation concerning the superharmonic free wave suppression in shallow and intermediate water conditions

This paper concerns laboratory wavemaking in shallow and intermediate water conditions. A comparison is made between two wave generation techniques, a first based on controlling the wavemaker displacement, and a second based on controlling the wavemaker force. Nonlinear wave generation in position control is well understood, and many laboratories rely on established second-order or Stream-function inputs. In deep water, using flap-type wavemakers, a force-control approach based on a linear demand signal was recently shown to offer benefits in terms of wave quality. The shallow water operation of such force-control strategies is less certain, which motivates the present study.To investigate the influence of the water depth on this type of control, a range of generation scenarios is considered, including small amplitude and large amplitude regular waves. Adopting both supporting calculations and experimental evidence, the work demonstrates that first-order force-based wave generation in shallow water suffers from similar limitations as first-order position control. This principally concerns the contamination of the testing area due to unwanted free waves, where the present focus is placed on the superharmonic range.The main advance of the work lies in the solutions it offers to overcome this free wave contamination. A number of nonlinear wave solutions upon which force-based generation can be based are discussed, and a suitable methodology is proposed and validated for each case. The developed methodology allows for high quality wave generation, whilst maintaining the benefit of active wave absorption. The work is timely in the sense that is responds to two recent developments. First, the majority of wavemaking facilities commissioned over the past two decades are computer controlled, and active absorption has become commonplace. The work presented offers solutions highly relevant to such installations. Second, developments particularly in offshore wind, have seen many new structures placed in relatively shallow-water depth. It is essential that the model testing of such structures adequately accounts for the issues and solutions presented herein.

Design of a Numerical Wave Tank and Wave Flume for Low Steepness Waves in Deep and Intermediate Water

Understanding the dynamics of wave energy converters and vortical structure of breaking waves are challenging hydrodynamic problems relevant to coastal engineering. These issues can be addressed at the laboratory scale through a specialized class of devices known as experimental wave flumes and numerical wave tanks (NWTs). This paper presents the process of design and development of a NWT and a wave flume that would eventually be applied for analyzing the hydrodynamics of breaking waves. The wave characteristics to be simulated have been selected from literature based on measurements carried out in the Gulf of Kutch, Gujarat. The NWT is based on a volume-of-fluid (VOF) formulation of the Navier-Stokes equations on a staggered grid using the mass-source function technique for wave generation. The flume is an acrylic tank equipped with a flap type wavemaker having dimensions adopted from the NWT. It is demonstrated in this work that the NWT and flume accurately generate non-linear wave trains in both intermediate as well as deep water with steepness H=H/λ∼0.04.

2차원 조파수조에서의 파 생성 특성 조사

This paper investigates the characteristics of waves generated by a flap-type wave maker in a two-dimensional wave channel. Measurements are carried out for various water depths, wave heights, periods, and lengths capacitance-type wave height gages. The experimental results are shown to satisfy the dispersion relation of the linear wave theory. For waves with a small height and long period, the wave profiles agree well with those of the linear wave theory. However, as the wave height and period become higher and shorter, respectively, it is shown that the wave profiles measured in the present experiments are different from the linear wave profiles, and the measured wave heights are smaller than the target wave heights, which may be due to the non-linearity of the waves. As the wave progresses toward the channel end, the wave height gradually decreases. This reduction in the wave height along the wave channel is explained by the wave energy dissipation due to the friction of the side walls of the channel. The performance of the wave absorber in the channel is found to be acceptable from the results of the wave reflection tests.

Fully nonlinear viscous wave generation in numerical wave tanks

A numerical method for simulating the complete physics of the fully nonlinear viscous wave generation phenomenon is presented. To accomplish this objective, the motion of a solid body representing the wave generating mechanism is modeled. In this paper, both the piston-type and flap-type wavemakers are simulated and the results of the model are compared with those of the experiments and analytics. The unsteady, two dimensional Navier–Stokes equations are solved in conjunction with the volume-of-fluid method for treating the free surface. A wide range of waves from linear to nonlinear generated by piston and flap-type wavemakers in intermediate and deep water cases are studied in this paper. The accuracy of the numerical results is verified by a comparison with the results of the wavemaker theory, the available experimental data in the literature, and the experiments preformed in this study. For the cases with small wave steepness, the numerical results agree well with the theoretical and experimental results for both the piston and flap-type wavemakers. However, for cases with large wave steepness, the numerical and experimental wave heights are slightly lower than the analytics. In both the piston and flap-type wavemakers, the numerical results are in good agreement with the measurements.

Numerical simulation of linear water waves and wave–structure interaction

One of the main stages in the design of wave energy converters (WEC's) is the numerical modelling of a given converter. In this paper, the numerical simulation of both linear deep water waves and linear waves for the finite depth case are explored using computational fluid dynamics (CFD), to aid in this design stage. The CFD software package described in this paper is the commercial finite volume package ANSYS CFX (Release 12.1). The results of parametric studies, which were performed in order to optimise the CFD model, are detailed and a guide to creating a model that produces the desired waves is presented. The model was validated in two ways: (a) the wave created was compared to wavemaker theory (WMT) and (b) the water particle velocity and elevation of the wave was compared to linear, Airy, wave theory (LWT) for deep water waves. It was also found that wave generation in ANSYS CFX using a flap-type wavemaker was restricted to a low normalised wavenumber, k0h. In order to increase this restriction, the hinge of the wavemaker was raised and, with this alteration, it is possible to generate deep water linear waves. A case study of a real world application of wave–structure interaction, employing this methodology, is also explored.

Second-order wave maker theory using force-feedback control. Part II: An experimental verification of regular wave generation

This paper provides an experimental verification of the new wave maker theory outlined by Spinneken and Swan [2009. Second-order wave maker theory using forcefeedback control. Part I. A new theory for regular wave generation. Ocean Engineering, in press, doi: 10.1016/j.oceaneng.2009.01.019]. This theory concerns the generation of regular waves by a flap-type wave maker using force-feedback control, providing the first quantitative evidence of the inherent advantages of this latter approach. When the wave maker is controlled by a first-order force command signal, comparisons between the theory and experimental observations confirm two key points: (i) The first-order behaviour is crucial for the absorption characteristics of the machine. (ii) The second-order behaviour leads to a spurious, or unwanted, freely propagating second harmonic that is substantially smaller in amplitude when compared to an identical wave paddle operating with first-order position control. Both aspects of this work, effective absorption and reduced second-order spurious wave generation, are investigated over a broad range of wave frequencies and shown to be widely applicable. Furthermore, the theory also provides a force command signal correct to second order. This is introduced in a separate set of experiments and shown to provide further improvement in the quality of the wave generation.