Wave-current Interaction Effects Research Articles

Fatigue load estimation of a spar-type floating offshore wind turbine considering wave-current interactions

This paper considers the effects of current and wave-current interactions in fatigue analysis of floating offshore wind turbines (FOWTs). Surface water waves experience frequency shifts and wave shape modification when traveling on underlying currents. The wave-current interactions are known to be important for the responses of offshore structures, however, they have not been considered in FOWT fatigue analysis. To include such interactions, a nonlinear mooring hydrodynamics model is presented which is able to consider the cable geometric nonlinearity, seabed contact, and the current effect. The mooring model is then coupled with a spar-type FOWT model which simulates the structural dynamics of turbine blades and tower; aerodynamics of the wind-blade interaction and wave-current actions on the spar. Analytical wave-current interaction models based on Airy’s theory considering the current effect are applied for generating the flow field. Based on a spar-type FOWT and the wave-current interaction model, numerical simulations have been performed for three cases with only waves, wave and current without and with interactions. The comparison of the structural responses shows that the current and the wave-current interaction can have significant influences on FOWT tower and cable responses. Furthermore, cable fatigue life is estimated for two particular cases when the cable tension is decreased and increased respectively due to the presence of current. It is found that if the current tends to increase the cable tension, neglecting the current and wave-current interactions leads to overestimate of the cable fatigue life.

Effects of Wave–Current Interaction on Salt Intrusion During a Typhoon Event in a Highly Stratified Estuary

Wave–current interaction (WCI) is important in modulating hydrodynamics and water mixing in estuaries, and thereby the transport of water-borne materials. However, the effects of WCI on salt transport and salt intrusion in estuaries during storm events have been rarely examined. In the present study, we use a coupled atmosphere–ocean–wave–sediment transport (COAWST) modeling system to investigate the effects of WCI on salt intrusion in the highly stratified Modaomen Estuary during Typhoon Hagupit (2008). The model is validated by the measured wave, water elevation, and surface salinity data, and several diagnostic model experiments are conducted. WCI increases the storm surge by 0.8 m at the peak surge (25% of the total surge height). The wave-breaking-induced momentum flux and the Stokes drift increase the magnitude of the landward flow by 0.3 m s−1 (30% of the total landward flow). In addition, the waves increase water mixing by 2–4 times compared with that without waves. Hence, WCI significantly increases the landward advective salt transport and decreases the steady shear transport. The net effect of the WCI is a significant increase of salt import and salt intrusion during the typhoon event. However, in the aftermath of the storm, the imported salt water is rapidly flushed out by the increased river discharge, and the estuary regains its stratification within one day.

Wave-current generated turbulence over hemisphere bottom roughness

The present paper explores the effect of wave-current interaction on the turbulence characteristics and the distribution of eddy structure over artificially crammed rough bed prepared with hemispheres. The effect of the surface wave on temporal and spatial-averaged mean velocity, intensity, Reynolds shear stress over, within cavity and above the hemispherical bed are discussed. Detailed three-dimensional time series velocity components were measured in a tilting flume using 3-D Micro-Acoustic Doppler Velocimeter (ADV) at a Reynolds number, 62 × 103. This study reports the fractional contributions of burst-sweep cycles dominating the total shear stress near hemispherical rough surface both for current only flow as well as for wave-induced cases. Wavelet analysis of the fluctuating velocity signal shows that the superimposed wave of frequency 1 Hz is capable of modulating the energy containing a range of velocity fluctuations at the mid-depth of the cavity region (formed due to the crammed arrangement of the hemispheres). As a result, the large-scale eddies (with large values of wavelet coefficients) are concentrated at a pseudo-frequency which is equal to the wave oscillating frequency. On the other hand, it is observed that the higher wave frequency (2 Hz) is incapable of modulating the eddy structures at that particular region.

Effects of wave-current interaction on storm surge in the Taiwan Strait: Insights from Typhoon Morakot

The effects of wave-current interaction on storm surge are investigated by a two-dimensional wave-current coupling model through simulations of Typhoon Morakot in the Taiwan Strait. The results show that wind wave and slope of sea floor govern wave setup modulations within the nearshore surf zone. Wave setup during Morakot can contribute up to 24% of the total storm surge with a maximum value of 0.28m. The large wave setup commonly coincides with enhanced radiation stress gradient, which is itself associated with transfer of wave momentum flux. Water levels are to leading order in modulating significant wave height inside the estuary. High water levels due to tidal change and storm surge stabilize the wind wave and decay wave breaking. Outside of the estuary, waves are mainly affected by the current-induced modification of wind energy input to the wave generation. By comparing the observed significant wave height and water level with the results from uncoupled and coupled simulations, the latter shows a better agreement with the observations. It suggests that wave-current interaction plays an important role in determining the extreme storm surge and wave height in the study area and should not be neglected in a typhoon forecast.

EFFECT OF WAVE-CURRENT INTERACTION ON WAVES AND CIRCULATION OVER GEORGES BANK DURING STORM EVENTS

The coupled spectral wave and circulation model SWAN+ADCIRC was applied to investigate the wave-current interaction during storm events over Georges Bank, a large shallow submarine bank on the eastern seaboard of North America that separates Gulf of Maine from the North Atlantic Ocean. The current over the Georges Bank displays a rotary feature over a tidal cycle. The wave-induced current is in the same order as the wind-driven current and generally in the same direction as the depth-averaged tidal current, indicating strong nonlinear wave-current interaction. The magnitude of wave-induced current reaches 0.07 m/s at low tide and 0.2 m/s at the other three tidal phases. The effect of wave-current interaction on waves at the four tidal phases is also analyzed. The role of Georges Bank in dissipating wave energy is most significant at rising mid-tide and high tide, which is close to the storm peak. At rising mid-tide, the wave height is decreased by 0.3 m to 0.5 m over the majority of the bank when the wave propagates in the same direction as the current. At falling-mid tide, the wave height is increased by 0.5 m at the southern flank and decreased by 0.5 m at the northern flank of the bank.

하구에서 파랑-흐름 상호작용이 3차원 흐름특성에 미치는 영향

하구에서 파랑-흐름 상호작용이 3차원 흐름특성에 미치는 영향

Effect of Tidal Current and Wave-Current Interaction on the Sediment Transport and Morphological Change in the Channel Water Intake

Investigation of sediment transport rate induced by tidal current and wave-current interaction are very important to explain the sediment transport mechanics, sedimentation and erosion process, and also the coastal morphological changes. The understanding of sediment transport caused by wave and current interaction in detail should be known well in order to predict it, accurately. This paper aim to investigate the characteristics of sediment transport rate due to tidal current and wave-currents interaction motion in the the channel water intake by using numerical modelling. The performance of the modelling results indicated by the Root Mean Square (RMSE) with error of 0.5 % by means validating the model water surface elevation from the fields measurement data and the modelling results. The result showed that sediment transport caused by wave-current interaction give more the amount of total sediment transport than caused by tidal current only. It can be concluded that the wave has dominant effect than tidal current to conduct sedimentation and morphological change in the canal water intake.

Numerical study to estimate the wave energy under Wave-Current Interaction in the Qingdao coast, China

The purpose of this study is to characterize how the Wave-Current Interaction (WCI) at a specific location modifies the wave energy available for conversion. In this work, a third generation spectral model MIKE21 SW (version 2014) is utilized to investigate the wave energy resources under WCI in the Qingdao coast. This model can directly calculate the wave power in terms of the wave spectrum, which improves the accuracy of the wave energy estimation, especially in the nearshore zones. Here, the models run for coupled (waves and currents) and uncoupled (only waves) modes over the whole year of 2014 with the same model setup, except the tide currents setting. The results indicate that the annual mean differences between coupled and uncoupled models are mainly occurred in the nearshore zones, varying up to 100% when the WCI effect is neglected. Further investigations reveal that those differences can be attributed to the tidal level, the strength of currents and the relative wave direction, which may cause underestimation or overestimation of the wave energy resources. This work confirmed that the WCI should be considered when performing estimation of the wave energy resources in the shallow nearshore zones with macro tidal range and current.

Development of an unstructured-grid wave-current coupled model and its application

An unstructured grid wave-current coupled model was developed by coupling the SWAN (Simulating Waves Nearshore) wave model and ADCIRC (Advanced Circulation model) ocean model through the Model Coupling Toolkit (MCT). The developed coupled model has high spatial resolution in the coastal area and is efficient for computation. The efficiency of the newly developed SWAN+ADCIRC model was compared with that of the widely-used SWAN+ADCIRC coupled model, in which SWAN and ADCIRC are coupled directly rather than through the MCT. Results show that the directly-coupled model is more efficient when the total number of computational cores is small, but the MCT-coupled model begin to run faster than the directly-coupled model when more computational cores are used. The MCT-coupled model maintains the scalability longer and can increase the simulation efficiency more than 35% by comparing the minimum wall clock time of one day simulation in the test runs.The MCT-coupled SWAN+ADCIRC model was used to simulate the storm surge and waves during the typhoon Usagi which formed in the western Pacific on September 17, 2013 and landed at Shanwei, China. Three numerical experiments were performed to investigate the effect of wave-current interaction on the storm surge and waves. The results show that the coupled model can better simulate the storm surge and waves when considering the wave-induced radiation stress, the wave effect on the wind stress drag coefficient and the modulation of current and water level on waves. During the typhoon Usagi, the effect of wave radiation stress could result in a maximum of 0.75m increase in the extreme storm surge, and the wave induced wind stress could cause a −0.82∼0.48m change of the extreme storm surge near the coastal area. Besides, the radiation stress forced currents cannot be ignored either in the study of mass transport at coastal zones. Results of this study are useful for understanding the wave-current interaction processes and the development of the operational prediction technique for storm surge and waves.

A simplified method to estimate tidal current effects on the ocean wave power resource

Although ocean wave power can be significantly modified by tidal currents, resource assessments at wave energy sites generally ignore this effect, mainly due to the difficulties and high computational cost of developing coupled wave-tide models. Furthermore, validating the prediction of wave-current interaction effects in a coupled model is a challenging task, due to the paucity of observational data. Here, as an alternative to fully coupled numerical models, we present a simplified analytical method, based on linear wave theory, to estimate the influence of tidal currents on the wave power resource. The method estimates the resulting increase (or decrease) in wave height and wavelength for opposing (or following) currents, as well as quantifying the change in wave power. The method is validated by applying it to two energetic locations around the UK shelf – Pentland Firth and Bristol Channel – where wave/current interactions are significant, and for which field data are available. Results demonstrate a good accuracy of the simplified analytical approach, which can thus be used as an efficient tool for making rapid estimates of tidal effects on the wave power resource. Additionally, the method can be used to help better interpret numerical model results, as well as observational data.

Electromagnetic backscattering from one-dimensional drifting fractal sea surface I: Wave–current coupled model**Project supported by the National Natural Science Foundation of China (Grant No. 41276187), the Global Change Research Program of China (Grant No. 2015CB953901), the Priority Academic Development Program of Jiangsu Higher Education Institutions (PAPD), Program for the Innovation Research and Entrepreneurship Team in Jiangsu Province, China, the Canadian Program

To study the electromagnetic backscattering from a one-dimensional drifting fractal sea surface, a fractal sea surface wave–current model is derived, based on the mechanism of wave–current interactions. The numerical results show the effect of the ocean current on the wave. Wave amplitude decreases, wavelength and kurtosis of wave height increase, spectrum intensity decreases and shifts towards lower frequencies when the current occurs parallel to the direction of the ocean wave. By comparison, wave amplitude increases, wavelength and kurtosis of wave height decrease, spectrum intensity increases and shifts towards higher frequencies if the current is in the opposite direction to the direction of ocean wave. The wave–current interaction effect of the ocean current is much stronger than that of the nonlinear wave–wave interaction. The kurtosis of the nonlinear fractal ocean surface is larger than that of linear fractal ocean surface. The effect of the current on skewness of the probability distribution function is negligible. Therefore, the ocean wave spectrum is notably changed by the surface current and the change should be detectable in the electromagnetic backscattering signal.

Wave–current interaction effects on tidal stream turbine performance and loading characteristics

The transient interaction between tidal currents and the rotation of a horizontal axis turbine rotor have the potential to induce high asymmetric loadings, which are subsequently transmitted to the drive shaft and potentially high speed drive train components. To mitigate the potential for early component failure, analysis of asymmetric loading on marine turbines is fundamental to the design process. To investigate these loads a turbine mounted on a circular stanchion has been used to highlight the effects of introducing more realistic boundary conditions. Depending on their wavelength, waves can also have a significant effect on the overall design decisions and placement of devices. Thrust loading and bending moments applied to the drive shaft can be of the order of hundreds of kN and kNm respectively.Knowledge of the flow regime can allow designers to evaluate material selection for components and incorporate some deformation capability of the turbine blades to increase the power output and potentially alleviate some of the stress distribution through key structural points. The resulting data can then be used to estimate component life via fatigue prediction.This paper includes a multi-physics approach to modelling tidal energy devices and the potential for modelling to inform device condition monitoring.

Wave–current interaction in a river and wave dominant estuary: A seasonal contrast

The dynamic feature of the Modaomen Estuary (ME) in the Pearl River Delta in southern China has been the subject of extensive research. In previous studies, wave–current interaction (WCI) was often neglected due to its complexity. This study uses a coupled hydrodynamic module TELEMAC-2D and wave propagation module TOMAWAC in the TELEMAC-Mascaret modeling system to quantify the effects of WCI on the hydrodynamics in the ME. The coupled wave and current modeling system was well validated against the field measurements at selected locations. The model results show that WCI varies with the seasonal change in runoff in the ME. The effect of waves on the currents is insignificant during the wet season with a current change of no more than 0.07m/s; but, in contrast, the currents have a noticeable effect on waves. However, during the dry season, the interactions of waves and currents on each other are found to be equally significant. When wave model and current model are coupled, the velocity could increase up to a maximum of 0.30m/s and decrease up to a maximum of 0.17m/s. WCI is greatly affected by the propagation directions of wave and current in both seasons. Generally, wave height decreases and current increases for a following wave and current; wave height increases and current decreases for an opposing wave and current. The effects of waves on currents change with the tide. Changes are larger during neap tide than during spring tide, and stronger during ebb tide than during flood tide.

サブメソスケール渦・非定常海浜流オーバーラップ領域における波ー流れ相互作用の効果について

Effects of two-way wave-current interaction (WCI) on the surfzone-inner shelf circulations affected by submesoscale eddies and unsteady littoral currents are investigated with the ROMS-WEC model (Uchiyama et al., 2010) coupled tightly with a spectrum-peak wave model to account for both wave effects on currents (WEC) and current effects on waves (CEW) (e.g., Uchiyama et al., 2009). Eddy activity around the outer edge of the surfzone is enhanced when CEW is considered. The key mechanism to induce the surfzone-inner shelf dynamical interaction and enhancement of offshore eddy activity is considered to be CEW that weakens the offshore component of the littoral currents. In turn,offshore eddy activity is attenuated by WEC, demonstrating that WCI plays an essential role not only in the sufzone but also in the shelf region.

Wave–current interaction effects on marine energy converters

Many countries have significant interests in generating electricity using waves and tidal current technologies. In energetic areas, waves and tidal currents interact for modifying the energy resource and impacting on the design conditions. Changes to the wave climate depend on the strength of the current and the relative wave direction. SWAN simulations of the wave climate around the Orkney Islands, with and without currents, show that considerable changes in the wave climate occur near sites of interest to wave and tidal energy project developers. Using circular statistics the effect of the relative angle between the waves and the current can be investigated. Local effects can lead to 150–200% increases in wave height when the waves oppose the current. These dramatic changes lead to an increase in wave power of over 100kWm−1. The complex nature of the tides in the channels also leads to large changes in wave power during the so-called slack water period. Wave amplification diagrams are proposed to provide a convenient summary of wave–current effects at a particular site and allow a statistical analysis to be made. When performing resource analysis and site selection work for marine energy projects, wave–current interaction must be considered.

Improving the assessment of wave energy resources by means of coupled wave-ocean numerical modeling

Sea waves energy represents a renewable and sustainable energy resource, that nevertheless needs to be further investigated to make it more cost-effective and economically appealing. A key step in the process of Wave Energy Converters (WEC) deployment is the energy resource assessment at a sea site either measured or obtained through numerical model analysis. In these kind of studies, some approximations are often introduced, especially in the early stages of the process, viz. waves are assumed propagating in deep waters without underneath ocean currents. These aspects are discussed and evaluated in the Adriatic Sea and its northern part (Gulf of Venice) using locally observed and modeled wave data. In particular, to account for a “state of the art” treatment of the Wave–Current Interaction (WCI) we have implemented the Simulating WAves Nearshore (SWAN) model and the Regional Ocean Modeling System (ROMS), fully coupled within the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) system. COAWST has been applied to a computational grid covering the whole Adriatic Sea and off-line nested to a high-resolution grid in the Gulf of Venice. A 15-year long wave data set collected at the oceanographic tower “Acqua Alta”, located approximately 15 km off the Venice coast, has also been analyzed with the dual purpose of providing a reference to the model estimates and to locally assess the wave energy resource. By using COAWST, we have quantified for the first time to our best knowledge the importance of the WCI effect on wave power estimation. This can vary up to 30% neglecting the current effect. Results also suggest the Gulf of Venice as a suitable testing site for WECs, since it is characterized by periods of calm (optimal for safe installation and maintenance) alternating with severe storms, whose wave energy potentials are comparable to those ordinarily encountered in the energy production sites.

Wave-tide-surge coupled simulation for typhoon Maemi

The main task of this study focuses on studying the effect of wave-current interaction on currents, storm surge and wind wave as well as effects of current induced wave refraction and current on waves by using numerical models which consider the bottom boundary layer and sea surface roughness parameter for shallow and smooth bed area around Korean Peninsula. The coupled system (unstructured-mesh SWAN wave and ADCIRC) run on the same unstructured mesh. This identical and homogeneous mesh allows the physics of wave-circulation interactions to be correctly resolved in both models. The unstructured mesh can be applied to a large domain allowing all energy from deep to shallow waters to be seamlessly followed. There is no nesting or overlapping of structured wave meshes, and no interpolation is required. In response to typhoon Maemi (2003), all model components were validated independently, and shown to provide a faithful representation of the system’s response to this storm. The waves and storm surge were allowed to develop on the continental shelf and interact with the complex nearshore environment. The resulting modeling system can be used extensively for prediction of the typhoon surge. The result show that it is important to incorporate the wave-current interaction effect into coastal area in the wave-tide-surge coupled model. At the same time, it should consider effects of depth-induced wave breaking, wind field, currents and sea surface elevation in prediction of waves. Specially, we found that: (1) wave radiation stress enhanced the current and surge elevation otherwise wave enhanced nonlinear bottom boundary layer decreased that, (2) wind wave was significantly controlled by sea surface roughness thus we cautiously took the experimental expression. The resulting modeling system can be used for hindcasting (prediction) the wave-tide-surge coupled environments at complex coastline, shallow water and fine sediment area like areas around Korean Peninsula.

Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin

Sclavo, M., Benetazzo, A., Carniel, S., Bergamasco, A., Falcieri, F.M., and Bonaldo, D., 2013. Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basinThe shallow northern Adriatic Sea (namely Gulf of Venice) serves as a model for exploring the interaction of surface gravity waves and oceanic currents and how they influence bottom sediment dispersal and bathymetry evolution. This wave-current interaction effect is investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the Regional Ocean Modeling System (ROMS), the Simulating WAves Nearshore (SWAN) model, and the CSTMS (Community Sediment Transport Modeling System) models and routines. The 2-way data transfer between wave and ocean circulation models is synchronous via MCT (Model Coupling Toolkit), with ROMS providing to SWAN the 2-D current field, free surface elevation, and bathymetry. COAWST modeling system is implemented on two computational grids at different horizontal resolution: a parent grid (with resolution of 2.0 km) covering the whole Adriatic Sea and a child grid resolving the Gulf of Venice at a resolution of 0.5 km. Simulated waves and currents are validated against in-situ observations at the CNR-ISMAR Acqua Alta oceanographic tower, located 15 km off the Venice lagoon. The analysis of wave-current interaction effect on sediment dispersal and sea bottom evolution are performed over the 2011 winter season (January-March) with particular focus on the waves generated by dominant and prevailing winds blowing on the Adriatic Sea: Bora and Sirocco. Results show that while the effects on bottom stress may vary depending on wave propagation and current direction, the effects on advective dynamics may become dominant particularly in presence of severe storms with parallel wave propagation and global circulation, which is the case of Bora storms in northern Adriatic Sea.

The irrotational progressive gravity waves propagating on uniform currents in Lagrangian analysis and experiments Part 1. Theoretical analysis

A third-order Lagrangian solutions is found directly by the full Lagrangian parametric governing equations, with the parameters of identifying each water particle which is obtained in mass conservation from taking the wavelength-averaged level of the vertical displacement of water particles along the direction of the wave propagation and is just the position of particle in the original still water, for the irrotational free surface progressive gravity waves propagating on uniform current in the three dimension. It is found that the effect of wave-current interaction is occurred in Lagrangian velocity potential, so that the pressure is not affected by the uniform current in the wave-current field. The drift velocity, Lagrangian mean level of particle and its motion period longer than the wave period which are exponentially decreasing with the depth of particle's level and the pathlines of particle and the streakline are then all presented, these are to be excluded in Eulerian solution. Furthermore, the forms of the helical pathlines of particles varying with the direction and speed of the uniform current are appeared to be turned from the wave velocity and current velocity component in co-direction to in anti-direction, even become into a closed orbit. The special case when the wave-current field is at the steady motion is also interpreted its consequential charateristics. The present solution can be reduced to that of the pure progressive waves as no current exists.

Tidal energy resource complexity in a large strait: The Karori Rip, Cook Strait

Successful extraction of tidal stream energy will require a good understanding of flow at a range of scales, including those relating to average energy, variability in energy supply and fatigue. Current and turbulence measurements from the Karori Rip area of Cook Strait, the prime focal region of open-water tidal stream electricity generation in New Zealand, are described. A key issue is that a significant portion of the energy is contained in waters deeper than normally considered for energy extraction. Here we compare shallow and deep sites. Velocity data were derived from acoustic Doppler current profiler moorings, as well as spatial surveys and show flow magnitudes reaching 3.4 m s −1 in the shallow regions. The maximum speeds in both shallow and deep sites were typically located in the upper part of the measured water column although moored acoustic Doppler current profiler (ADCP) observations showed some complexity in this regard. Benthic boundary-layers were resolved in the bottom ∼20 m of the water column. Measured turbulent kinetic energy dissipation rates ε exceeded 10 −5 m 2 s −3 and estimated maximum ε is a factor of 10 greater. This was not distributed evenly through the water column, with stratification and velocity shear clearly persisting, especially around the turn of the tide. The implications for tidal stream energy are that (i) there is sufficient energy resource in the region for a moderate sized array of turbines, (ii) the vertical variability in the flow speed suggests turbines that can operate near the surface would be more effective at accessing the resource, (iii) stratification may persist and influence the scales of turbulence and (iv) wave–current interaction effects will influence any near-surface structure as well as vessel operations.