Irregular Wave Conditions Research Articles

Nonlinear wave forces on large-scale submerged tunnel element

This paper presents parametric studies of nonlinear wave forces on large-scale submerged tunnel element. A transportation project of submerged tunnel element under irregular wave conditions is taken as the research background. Three-dimensional diffraction potential and radiation potential associating with the motion of the floating body are utilized, and an efficient three dimensional Green function is implemented to solve the radiation-diffraction problem. A computational method for calculating the nonlinear wave forces of the submerged tunnel element is developed. The present method is validated by the existed methods and experimental data, and good agreements are observed for all test models. The effects of wave parameters and structural parameters on the second-order nonlinear wave forces are studied. Results from the present study can provide valuable recommendations for the weather window and downtime frequencies, the configuration of tugboats, the arrangement of sea-route, and the size and type of the design and construction of the submerged tunnel element.

Hydraulic Experiments on Transmission Coefficients for Vertical Structure under Intermediate Water Depth Condition

본 연구에서는 직립식 구조물의 ì „ë‹¬íŒŒê³ ê³„ìˆ˜ ì‚°ì •ì„ 위한 2차원 수리실험을 실시하였다. 불규칙파를 ìž ì‚¬íŒŒë¡œ ì ìš©í•˜ì˜€ìœ¼ë©° 직립식 구조물에 대한 ì „ë‹¬íŒŒê³ ê³„ìˆ˜ ì‚°ì •ì‹ì„ ì œì•ˆí•˜ê³ , Goda(1969) 등의 결과와 비교하였다. Goda(1969)의 결과는 ìž ì‚¬íŒŒë¡œ 규칙파를 ì ìš©í•œ 결과로서 불규칙파를 ì ìš©í•œ 본 실험의 결과와 차이를 보였다. Goda의 결과가 본 연구에 비하여 크게 ì „ë‹¬íŒŒê³ ê³„ìˆ˜ë¥¼ ì œì‹œí•˜ëŠ” 것을 알 수 있었다. 또한 Goda는 직립벽에 대하여 í‰ê· , 상한 및 하한에 대한 계수값을 ì œì‹œí•˜ì˜€ìœ¼ë‚˜(d = h ì¡°ê±´), 본 연구에서는 각각의 설계요소(구조물 폭, 수심, 파장 등)가 ì „ë‹¬íŒŒê³ ê³„ìˆ˜ì— 주는 영향을 계수를 통하여 ì •ëŸ‰ì ìœ¼ë¡œ ì œì‹œí•˜ì˜€ë‹¤. 또한 ì—¬ìœ ê³ ê°€ 0인 조건에 대한 에너지보존 및 ì „ë‹¬íŒŒê³ íŠ¹ì„±ì„ ì œì‹œí•˜ì˜€ë‹¤. 핵심용어: 직립식 구조물, ì „ë‹¬íŒŒê³ ê³„ìˆ˜, 2차원 수리실험, 불규칙파

LABORATORY SIMULATIONS OF WAVE ATTENUATION BY AN EMERGENT VEGETATION OF ARTIFICIAL PHRAGMITES AUSTRALIS: AN EXPERIMENTAL STUDY OF AN OPEN-CHANNEL WAVE FLUME

This paper presents a well-controlled laboratory experimental study to evaluate wave attenuation by artificial emergent plants (Phragmites australis) under different wave conditions and plant stem densities. Results showed substantial wave damping under investigated regular and irregular wave conditions and also the different rates of wave height and within canopy wave-induced flows as they travelled through the vegetated field under all tested conditions. The wave height decreased by 6%–25% at the insertion of the vegetation field and towards the downstream at a mean of 0.2 cm and 0.32 cm for regular and irregular waves respectively. The significant wave height along the vegetation field ranged from 0.89–1.76 cm and 0.8–1.28 cm with time mean height of 1.38 cm and 1.11 cm respectively for regular and irregular waves. This patterns as affected by plant density and also location from the leading edge of vegetation is investigated in the study. The wave energy attenuated by plant induced friction was predicted in terms of energy dissipation factor (fe) by Nielsen’s (1992) empirical model. Shear stress as a driving force of particle resuspension and the implication of the wave attenuation on near shore protection from erosion and sedimentation was discussed. The results and findings in this study will advance our understanding of wave attenuation by an emergent vegetation of Phragmites australis, in water system engineering like near shore and bank protection and restoration projects and also be employed for management purposes to reduce resuspension and erosion in shallow lakes.

Numerical Analysis of the Performance of Perforated Coastal Structures under Irregular Wave Conditions

ABSTRACT Chun, I.; Lim, H.S.; Shim, J.S., and Park, K.S., 2015. Numerical analysis of the performance of perforated coastal structures under irregular wave conditions. A numerical and experimental study was performed to investigate the wave control characteristics of general perforated harbor structures under irregular wave conditions. A numerical analysis using a boundary element method was applied to each of the regular wave components composing the irregular wave, and the results were linearly superimposed under linear wave assumptions. The nonlinear friction term at the perforated walls was first linearized using the root mean square values of horizontal velocity components obtained through an iterative calculation. Hydraulic experiments were also performed for three different transmissive structures, and the results were compared with those of the numerical analysis. The numerical and experimental values of reflection and transmission coefficients generally agreed within an accuracy of 0.1. The numer...

MARINET experiment KNSWING testing an I-Beam OWC attenuator

The research and results presented in this paper concerns experiments on wave energy conversion carried out in the one meter deep wave tank at the Hydraulic and Maritime Research Center (HMRC) at University College Cork, Ireland in 2013. The purpose is to investigate how much power an attenuator – a ship shaped wave energy converter facing the waves with its bow – can absorb along its sides in a range of regular and irregular wave conditions.The experiments were carried out in model scale 1:50 resembling the wave conditions and water depth of the Danish part of the North Sea and a 150m long wave energy converter with 20 Oscillating Water Column (OWC) chambers on each side. The damping applied to each chamber by the Power Take Off (PTO) is modeled by forcing the air through a hole with an area of about 1.3% of the chamber water surface area.The results in irregular wave conditions shows that the converter can absorb more than 2.5MW of wave power that in wave conditions with significant wave heights of 5m. A capture width ratio between 20% and 25% was measured in the most frequent wave conditions with average periods between 5 and 7s with significant wave height of 2 m. In short crested waves the energy absorption was slightly better due to the directional spreading of energy.The tests in monochromatic waves indicate capture width ratios up to 35% in waves with a steepness of 2.5%. The capture width ratio decreases somewhat in steeper waves.The experiments with the attenuator wave energy converter demonstrated its seaworthiness and ability to absorb wave energy. The results form a valuable base for the development of a numerical model of the system that will be used for further optimization and development.

Ship propulsion in waves by actively controlled flapping foils

Flapping wings located beneath or to the side of the hull of the ship are investigated as unsteady thrusters, augmenting ship propulsion in waves. The main arrangement consists of horizontal wing(s) in vertical oscillatory motion which is induced by ship heave and pitch, while rotation about the wing pivot axis is actively controlled. In this work we investigate the energy extraction by the system operating in irregular wave conditions and its performance concerning direct conversion to propulsive thrust. More specifically, we consider operation of the flapping foil in waves characterised by a spectrum, corresponding to specific sea state, taking into account the coupling between the hull and the flapping foil dynamics. The effect of the wavy free surface is accounted for through the satisfaction of the corresponding boundary conditions and the consideration of the wave velocity on the formation of the incident flow. Numerical results concerning thrust and power coefficients are presented, indicating that significant thrust can be produced under general operating conditions. The present work can be exploited for the design and optimum control of such systems extracting energy from sea waves for augmenting marine propulsion in rough seas, with simultaneous reduction of ship responses offering also dynamic stabilisation.

Effects of Roughness and Vertical Wall Factors on Wave Overtopping in Rubble Mound Breakwaters in Busan Yacht Harbor

Coastlines are protected by breakwater structures against the erosion of sand or other materials along beaches due to wave action. This research examined the use of physical modeling to determine the effects of the tetrapod size and vertical walls of a rubble mound on the volume of wave overtopping under irregular wave conditions in coastal areas in Busan Yacht Harbor. In this analysis model, the structures were studied using irregular waves and the JONSWAP wave energy spectrum. To understand the effects of the tetrapod size and heights of the vertical wall, the study considered vertical walls of 0, 1.78, 6.83, and 9.33 cm with armor double layered material tetrapods of 8, 12, 16, and 20 tons. An extensive number of experiments covering a relatively large range of variables enabled a comprehensive discussion. First, in the presence of a short vertical wall, the water level played a key role in the overtopping discharge. In such circumstances, the values of the wave overtopping discharge decreased with increasing freeboard size. In the presence of a tall freeboard and middle, the value of the wave overtopping discharge was equally influenced by the vertical wall factor. Moreover, the tetrapod size decreased by an increase in the vertical wall factor, and relationship between them resulted in a short wall height. From an engineering point of view, considering a small water level may allow the choice of a shorter vertical wall, which would ultimately provide a more economical design.

Nonlinear Infragravity–Wave Interactions on a Gently Sloping Laboratory Beach

AbstractA high-resolution dataset of three irregular wave conditions collected on a gently sloping laboratory beach is analyzed to study nonlinear energy transfers involving infragravity frequencies. This study uses bispectral analysis to identify the dominant, nonlinear interactions and estimate energy transfers to investigate energy flows within the spectra. Energy flows are identified by dividing transfers into four types of triad interactions, with triads including one, two, or three infragravity–frequency components, and triad interactions solely between short-wave frequencies. In the shoaling zone, the energy transfers are generally from the spectral peak to its higher harmonics and to infragravity frequencies. While receiving net energy, infragravity waves participate in interactions that spread energy of the short-wave peaks to adjacent frequencies, thereby creating a broader energy spectrum. In the short-wave surf zone, infragravity–infragravity interactions develop, and close to shore, they dominate the interactions. Nonlinear energy fluxes are compared to gradients in total energy flux and are observed to balance nearly completely. Overall, energy losses at both infragravity and short-wave frequencies can largely be explained by a cascade of nonlinear energy transfers to high frequencies (say, f > 1.5 Hz) where the energy is presumably dissipated. Infragravity–infragravity interactions seem to induce higher harmonics that allow for shape transformation of the infragravity wave to asymmetric. The largest decrease in infragravity wave height occurs close to the shore, where infragravity–infragravity interactions dominate and where the infragravity wave is asymmetric, suggesting wave breaking to be the dominant mechanism of infragravity wave dissipation.

Effects of vertical wall and tetrapod weights on wave overtopping in rubble mound breakwaters under irregular wave conditions

ABSTRACT Rubble mound breakwaters protect the coastal line against severe erosion caused by wave action. This study examined the performance of different sizes and properties (i.e. height of vertical wall and tetrapod size) of rubble mound breakwaters on reducing the overtopping discharge. The physical model used in this study was derived based on an actual rubble mound in Busan Yacht Harbor. This research attempts to fill the gap in practical knowledge on the combined effect of the armor roughness and vertical wall on wave overtopping in rubble mound breakwaters. The main governing parameters used in this study were the vertical wall height, variation of the tetrapod weights, initial water level elevation, and the volume of overtopping under constant wave properties. The experimental results showed that the roughness factor differed according to the tetrapod size. Furthermore, the overtopping discharge with no vertical wall was similar to that with relatively short vertical walls (γν = 1). Therefore, the experimental results highlight the importance of the height of the vertical wall in reducing overtopping discharge. Moreover, a large tetrapod size may allow coastal engineers to choose a shorter vertical wall to save cost, while obtaining better performance.

Experimental study of wave-driven impact of sea ice floes on a circular cylinder

The impact of isolated sea ice floes with offshore structures is a significant environmental hazard for Arctic offshore operations. Most attention to date has focused on the impact of glacial icebergs, and very large drifting ice floes with offshore structures. There appears to be a lack of data in the case of isolated, wave-forced floe–structure interactions. To address this an experimental investigation was conducted to identify impact characteristics of floes of various shapes and sizes with a single circular cylinder. A wide selection of regular and irregular wave conditions were examined and the floe kinematics and impact characteristics determined. In regular waves, the results showed floe kinematic heave and surge responses was unaffected by the presence of the structure at distances of x/D~≥10. At x/D≤10 a slight increase in heave response was observed. In the same region, surge was markedly reduced regardless of whether there was an eventual impact. Drift velocity appeared to be the main control on whether the floe would impact (if drift velocity was high enough) or become trapped in the lee of the cylinder and be deflected to one side before impact. 3D analysis of impacts showed that two broad types of impact occurred; a relatively head-on impact and a more side-on impact. Head-on impacts were dominated by linear kinetic energy, while side on and secondary impacts exhibited increased rotational kinetic energy. In irregular waves, the impacts were found to occur at any point in the wave cycle, whereas the impacts in regular waves tended to occur at the point of maximum surge near the crest of the wave. The influence of λ/Lc and H/λ on impact occurrence and characteristics was investigated. While H/λ did not have a significant effect on impact occurrence, it appeared that the lower the λ the greater the chance of impact. The influence of Lc and shape in irregular waves indicated larger Lc and cross-sectional area of the floe relative to the cylinder increased impact occurrence.

Memory of past random wave conditions in submarine groundwater discharge

Submarine groundwater discharge (SGD) is an integral part of the hydrological cycle and represents an important aspect of land-ocean interactions. We used a numerical model to simulate flow and salt transport in a nearshore groundwater aquifer under varying wave conditions based on yearlong random wave data sets, including storm surge events. The results showed significant flow asymmetry with rapid response of influxes and retarded response of effluxes across the seabed to the irregular wave conditions. While a storm surge immediately intensified seawater influx to the aquifer, the subsequent return of intruded seawater to the sea, as part of an increased SGD, was gradual. Using functional data analysis, we revealed and quantified retarded, cumulative effects of past wave conditions on SGD including the fresh groundwater and recirculating seawater discharge components. The retardation was characterized well by a gamma distribution function regardless of wave conditions. The relationships between discharge rates and wave parameters were quantifiable by a regression model in a functional form independent of the actual irregular wave conditions. This statistical model provides a useful method for analyzing and predicting SGD from nearshore unconfined aquifers affected by random waves. Key Points Cumulative effects of antecedent wave conditions on SGD Long memory of past wave conditions back to over a hundred days found in the SGD Wave effects characterized well by a gamma distribution function

Investigations on the porous media equations and resistance coefficients for coastal structures

This paper considers the flow in porous media that occurs in coastal and offshore engineering problems. Over the past decades numerous formulations of flow equations for porous media have been presented. The present work re-examines the porous media equations of the most recent form and corrects some shortcomings which were identified. The applied type of porosity models relies on empirical resistance coefficients which often need to be measured or calibrated. Only few examples of calibration for numerical models which are present in the literature often applied the same experimental results. In this study new calibration cases were introduced to the calibration procedure in order to achieve a better understanding of the variation of the resistance coefficients. Hereby the coefficients were determined with a better description over the entire parameter space for the resistance coefficients than previously found in the literature. Constant values for the resistance coefficients for a broad range of flow conditions were recommended based on the new calibrations. The model was validated for the main physical processes that occur in wave–structure interaction in coastal structures including three-dimensional wave–structure interaction, run-up, run-down and pressure damping, regular and irregular wave conditions and evaluation of overtopping. Simple two and three dimensional uniform caisson structures and breakwater layouts were investigated. The model was implemented in the open source CFD library OpenFOAM® and has been made publicly available to the engineering community as part of the wave generation framework waves2Foam.

Nonlinear effects from wave-induced maximum vertical bending moment on a flexible ultra-large containership model in severe head and oblique seas

Vertical bending moment (VBM) is of crucial importance in ensuring the survival of vessels in rough seas. With regard to conventional vessels, wave-induced maximum VBM is normally considered to be experienced in head seas. It is conservative to determine the extreme VBM based on either numerical simulations or model tests in long-crested head seas. Extensive model tests have been conducted in head seas with focus on the nonlinear vertical responses in severe seas, and the measured results were compared with numerical calculations for validation. Unexpected phenomena, however, were observed during the model tests of an ultra-large containership. The maximum sagging and hogging VBMs were encountered in oblique seas. Furthermore, the significant wave height used in oblique seas was even smaller than that used in head seas. The nonlinear vertical load effects in oblique seas require further investigations for this particular vessel. Limited experimental results in oblique seas have been reported, in which the lateral responses were always more concerned than the vertical responses. Up to now, rare systematic comparisons of the nonlinear vertical responses between head and oblique seas have been published, especially when the hydroelastic effects are also accounted for. A 13000-TEU ultra-large containership model, which was designed by Hyundai Heavy Industries (HHI), has been tested in the towing tank and the ocean basin at the Marintek center in Trondheim. The experimental results in regular waves are first compared between head and oblique seas. The statistical characteristics of the VBM amidships under nineteen irregular wave conditions are then investigated. Next, the extreme hogging and sagging VBMs are compared under different wave conditions with focus on the extreme hogging VBMs. At the end of the paper, the uncertainties in the experiments are discussed.

A numerical study on coupled sloshing and ship motions of a liquefied natural gas carrier in regular and irregular waves

This article presents a numerical study of the coupled ship motions and tank sloshing of a liquefied natural gas carrier. To solve this coupled problem, we have developed a time-domain numerical method. The method combines a ship motion solver based on strip theory and a computational fluid dynamics sloshing solver. The method has been applied to a liquefied natural gas carrier model to investigate the coupling effect in regular waves. Simulations for four different loading conditions were carried out, and the results of ship motion response amplitude operators and free surface movement amplitudes were presented in frequency domain. The coupling effect on ship motions and sloshing has been discussed. Based on the assumption of linearity of ship motions and wave, the authors used the ship motion response amplitude operators (with coupling effect) obtained in regular wave conditions to generate ship motion time histories in irregular waves. Sloshing in tanks was then simulated by using the ship motion time histories in irregular wave conditions. The calculated impact pressure time histories were analyzed by using the exceedance probability function.

Predicting wave‐induced ripple equilibrium geometry

A comprehensive database of existing (since 1954) field and laboratory measurements of ripple geometry is compiled and combined with newly collected field data to examine the performance of ripple equilibrium predictors. Reanalysis of this enlarged ripple geometry data set reveals that ripples formed from monochromatic waves scale differently than ripples formed from random waves for many existing ripple predictors. Our analysis indicates that ripple wavelengths from the two data sets collapse into a single scaling when the semiorbital excursion and sediment grain diameter are used as normalizing factors. Ripple steepness remains relatively constant for both regular and irregular wave conditions, and it only slightly increases for shorter ripple wavelengths. These findings allowed for the development of a new equilibrium ripple predictor suitable for application in a wide range of wave and sediment conditions.

Shelf 지형에서 불규칙파의 쇄파실험 및 수치해석

In this study, wave breakings over a shelf region are investigated under irregular wave conditions through laboratory experiments in a wave flume. Numerical simulations based on the Boussinesq-type equations are also conducted. The characteristics of breaking waves such as significant wave height, crest and trough heights, the mean water level and the stable wave height are obtained by analyzing laboratory measurements in detail. Obtained results are compared with those of the Boussinesq-type equations model. A very reasonable agreements is observed. The broken waves over a horizontal bottom asymptotically approach a stable wave height(). In this study, the relative stable wave height is found as for irregular wave.

Analysis of a Two-Body Floating Wave Energy Converter With Particular Focus on the Effects of Power Take-Off and Mooring Systems on Energy Capture

The present paper summarizes analyses of a two-body floating wave energy converter (WEC) to determine the mooring tension and the effect of the mooring system on energy capture. Also, the effect of the power take-off (PTO) is assessed. An axisymmetric Wavebob-type WEC is chosen as the object of investigation. However, the PTO system is modeled in a simplified manner as ideal linear damping and spring terms that couple the motions of the two bodies. The analysis is performed using SIMO, which is a time domain simulation tool that accommodates the simulation of multibody systems with hydrodynamic interactions. In SIMO, docking cone features between the two bodies allow movement as per actual operation, and fenders are applied to represent end stops. Six alternative mooring configurations are applied to investigate the effect of mooring on power capture. Mooring analysis is performed to determine the necessary capacity of mooring lines for each configuration to carry the tension due to the WEC motion in extreme conditions. Hydrodynamic loads are determined using WAMIT. We assumed that the WEC will be operated to capture wave power at the Yeu site in France. The analysis is performed for several regular and irregular wave conditions according to wave data available for that site. Simulations are performed to study the effect of the PTO system, end stops settings and several mooring configurations on power capture.

Numerical simulation of wave overtopping at coastal dikes and low-crested structures by means of a shock-capturing Boussinesq model

In this paper, a shock-capturing numerical model, based on the combined solution of Boussinesq and nonlinear shallow water equations is applied to the simulation of wave runup, overtopping and wave train propagation over impermeable, emerged and low-crested, structures. In order to improve the performances of the scheme at wet–dry interfaces, a numerical treatment is introduced to provide well-balancing of advective fluxes and source terms. One- and two-dimensional test cases are presented to validate the performances of the model under both regular and irregular wave conditions.

동영상을 이용한 부유구조물 모형의 변위 관측

움직이는 한 개의 카메라 동영상으로부터 개체의 3차원 위치를 추출할 수 있다고 알려져 있다. 이로부터 캠코더 측정시스템을 이용하여 부유체 모형에 대한 영상기반 모니터링을 수행하였다. 규칙파 및 비규칙파 실험조건에서의 디지털 캠코더 동영상으로부터 프레임 영상을 추출하고, 특징점을 정합하여, 상대적인 3차원 좌표를 획득하였다. 수정된 SURF 기반 정합의 영상 변환 정확도와 규칙파에서 부유체 모델의 영상기반 변위 관측 정확도를 평가하였다. 규칙파의 경우 조파기의 설정값은 3.0sec이고, 영상기반 변위에 의한 주기는 2.993sec이었다. 기계적 오차를 고려할 때 이 두 값은 유사한 결과로 여겨진다. 시각적으로도 X Y Z축으로의 1차원 투영결과나 3차원 공간에서의 결과에서 규칙파의 형상을 볼 수 있었다. 결과적으로 30fps의 일반 디지털 캠코더 동영상을 이용하여 근실시간으로 위치변동을 계산할 수 있었다. It is well known that a single moving camera video is capable of extracting the 3-dimensional position of an object. With this in mind, current research performed image-based monitoring to establish a floating structure model using a camcorder system. Following this, the present study extracted frame images from digital camcorder video clips and matched the interest points to obtain relative 3D coordinates for both regular and irregular wave conditions. Then, the researchers evaluated the transformation accuracy of the modified SURF-based matching and image-based displacement estimation of the floating structure model in regular wave condition. For the regular wave condition, the wave generator's setting value was 3.0 sec and the cycle of the image-based displacement result was 2.993 sec. Taking into account mechanical error, these values can be considered as very similar. In terms of visual inspection, the researchers observed the shape of a regular wave in the 3-dimensional and 1-dimensional figures through the projection on X Y Z axis. In conclusion, it was possible to calculate the displacement of a floating structure module in near real-time using an average digital camcorder with 30fps video.

A Simple and Effective Real-Time Controller for Wave Energy Converters

A novel strategy for the real-time control of oscillating wave energy converters (WECs) is proposed. The controller tunes the oscillation of the system such that it is always in phase with the wave excitation force and the amplitude of the oscillation is within given constraints. Based on a nonstationary, harmonic approximation of the wave excitation force, the controller is easily tuned in real-time for performance and constraints handling, through one single parameter of direct physical meaning. The effectiveness of the proposed solution is assessed for a heaving system in one degree of freedom, in a variety of irregular (simulated and real) wave conditions. A performance close to reactive control and to model predictive control is achieved. Additional benefits in terms of simplicity and robustness are obtained.