Eddy viscosity and tidal cycles in a shallow sea

The very pronounced spring-neap tidal cycle in the South Australian Gulfs leads to an unusually large variation in the magnitude of the tidal currents. Measurements of the currents in these Gulfs show that the non-tidal circulation depends on the strength of the tidal currents and hence on the spring-neap cycle. A simple model is produced in which the vertical eddy viscosity is a function of both wind strength and tidal currents. Results from the model agree with the observations and suggest a means whereby variations in tidal mixing may be accounted for in hydrodynamic modelling of the circulation in shallow seas.     

A Tidal Flow Model for the Gulf of Kachchh, India

A vertically integrated model has been used to study the tidal circulation and currents in the Gulf of Kachchh along the west coast of India. The model is fully nonlinear and uses a semiexplicit finite difference scheme to solve the basic hydrodynamic equations on a staggered grid. The model is forced by prescribing the tides along the open boundary of the model domain. The flow is simulated both with and without the presence of the proposed tidal barrage across the Hansthal Creek in the Gulf of Kachchh. The results show a considerable change in the behavior of the tidal flow in the presence of the barrage.     

A Tidal Flow Model for the Gulf of Kachchh,India

A vertically integrated model has been used to study the tidal circulation and currents in the Gulf of Kachchh along the west coast of India. The model is fully nonlinear and uses a semiexplicit finite difference scheme to solve the basic hydrodynamic equations on a staggered grid. The model is forced by prescribing the tides along the open boundary of the model domain. The flow is simulated both with and without the presence of the proposed tidal barrage across the Hansthal Creek in the Gulf of Kachchh. The results show a considerable change in the behavior of the tidal flow in the presence of the barrage.     

A numerical study of sea level and current responses to Hurricane Frederic using a coastal ocean model for the Gulf of Mexico

A three-dimensional, nonlinear, primitive equation ocean general circulation model is used to study the response of the Gulf of Mexico to Hurricane Frederic. The model has free surface dynamics and a second order turbulence closure scheme for the mixed layer. Realistic coastlines, bottom topography and open boundary conditions are used in the study. The model has a vertical sigma coordinate with 18 levels, and a horizontal resolution of 0.2°×0.2° for the entire Gulf. The study focuses on hurricane generated sea level, current, and coastally trapped wave (CTW) responses of the Gulf. Time series of sea levels from U.S. coastal tide gauge stations and the numerical model simulation of sea levels and currents on the shelf are used to study sea level, current and CTW responses. Both model sea levels and observations from tide gauge stations show a westward progression of the surge as a CTW response. The results of the study of sea levels and currents indicate that CTW propagate to the west with phase speeds of 7–10 m s^–1. There is also a strong nonlinear interaction between the Loop Current and hurricane induced currents. The surface current attains a maximum of 200 cm s^–1 in the eastern Gulf. The model surface elevation at several locations is compared with tide gauge data. The current meter data at three moorings are also compared with the model currents. The model simulations show good agreement with observed data for the hurricane induced coastally trapped wave, storm surge, and current distribution in the Gulf.     

Mechanistic Drifting Forecast Model for A Small Semi-Submersible Drifter Under Tide-Wind-Wave Conditions

Understanding the drifting motion of a small semi-submersible drifter is of vital importance regarding monitoring surface currents and the floating pollutants in coastal regions. This work addresses this issue by establishing a mechanistic drifting forecast model based on kinetic analysis. Taking tide-wind-wave into consideration, the forecast model is validated against in situ drifting experiment in the Radial Sand Ridges. Model results show good performance with respect to the measured drifting features, characterized by migrating back and forth twice a day with daily downwind displacements. Trajectory models are used to evaluate the influence of the individual hydrodynamic forcing. The tidal current is the fundamental dynamic condition in the Radial Sand Ridges and has the greatest impact on the drifting distance. However, it loses its leading position in the field of the daily displacement of the used drifter. The simulations reveal that different hydrodynamic forces dominate the daily displacement of the used drifter at different wind scales. The wave-induced mass transport has the greatest influence on the daily displacement at Beaufort wind scale 5-6; while wind drag contributes mostly at wind scale 2-4.     

Coupled modeling of currents and wind waves in the Kerch Strait

We present a numerical model of the dynamics of the Kerch Strait allowing one to perform the coordinated analysis of the fields of currents and wind waves. The model includes the spectral wave module and the hydrodynamic block of currents. The influence of waves on the currents is taken into account in the hydrodynamic block both via the surface and bottom tangential stresses and via the radiation stresses. In order to take into account the inverse influence of currents upon the waves, we use the fields of currents and sea level from the hydrodynamic block in the wave module. The specific features of the structure of currents and wind waves in the strait are studied for the typical wave situations. The results of the coupled and separate simulation are compared and the importance of taking into account the mechanisms of interaction between waves and currents in the analysis of the dynamic processes in the strait is demonstrated. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 3–20, September–October, 2007.     

An Improved Time Domain Approach for Analysis of Floating Bridges Based on Dynamic Finite Element Method and State-Space Model

The floating bridge bears the dead weight and live load with buoyancy, and has wide application prospect in deep-water transportation infrastructure. The structural analysis of floating bridge is challenging due to the complicated fluid-solid coupling effects of wind and wave. In this research, a novel time domain approach combining dynamic finite element method and state-space model (SSM) is established for the refined analysis of floating bridges. The dynamic coupled effects induced by wave excitation load, radiation load and buffeting load are carefully simulated. High-precision fitted SSMs for pontoons are established to enhance the calculation efficiency of hydrodynamic radiation forces in time domain. The dispersion relation is also introduced in the analysis model to appropriately consider the phase differences of wave loads on pontoons. The proposed approach is then employed to simulate the dynamic responses of a scaled floating bridge model which has been tested under real wind and wave loads in laboratory. The numerical results are found to agree well with the test data regarding the structural responses of floating bridge under the considered environmental conditions. The proposed time domain approach is considered to be accurate and effective in simulating the structural behaviors of floating bridge under typical environmental conditions.

     

Mechanistic Drifting Forecast Model for A Small Semi-Submersible Drifter Under Tide–Wind–Wave Conditions

Understanding the drifting motion of a small semi-submersible drifter is of vital importance regarding monitoring surface currents and the floating pollutants in coastal regions. This work addresses this issue by establishing a mechanistic drifting forecast model based on kinetic analysis. Taking tide–wind–wave into consideration, the forecast model is validated against in situ drifting experiment in the Radial Sand Ridges. Model results show good performance with respect to the measured drifting features, characterized by migrating back and forth twice a day with daily downwind displacements. Trajectory models are used to evaluate the influence of the individual hydrodynamic forcing. The tidal current is the fundamental dynamic condition in the Radial Sand Ridges and has the greatest impact on the drifting distance. However, it loses its leading position in the field of the daily displacement of the used drifter. The simulations reveal that different hydrodynamic forces dominate the daily displacement of the used drifter at different wind scales. The wave-induced mass transport has the greatest influence on the daily displacement at Beaufort wind scale 5–6; while wind drag contributes mostly at wind scale 2–4.     

Salinity Variations of the Intermediate Oyashio Waters and Their Relation with the Lunar Nodal Cycle

New oceanographic observations in the period 1990–2015 revealed significant salinity variations in the Oyashio Current. In the last 26 years, the salinity of the upper layer decreased by 0.2 PSU. The most rapid changes in salinity and temperature have been observed in the last five years. The time series of salinity measurements is characterized by the high-amplitude fluctuations synchronized with the lunar nodal cycle (18.6 years); i.e., high salinity is observed in the period of strong tidal currents. Modulation of diurnal tidal currents with the K₁ and O₁ periods in the lunar nodal cycle is significant [8, 9]. The amplitude was maximal in 1988 and 2006 and minimal in 1997 and 2015. The characteristics of tidal currents in the Oyashio Current and Sea of Okhotsk are considered based on available data of drifting buoys over the Kruzenshtern and Kashevarov banks. The amplitude of salinity variations synchronized with the lunar cycle is approximately 0.1 PSU; therefore, it has made a significant contribution to the salinity decrease in recent years.     

非通航孔桥墩自适应拦截网防撞装置实船拦截试验与水动力计算

针对非通航孔桥墩,研发了一种自适应拦截网防船舶撞击装置,主要由系泊大浮体、系泊锚链和固定锚、自适应小浮筒、拦截网、恒阻力缆绳以及触发钢索所组成。阐述了该防撞装置设计原理,即偏航船舶撞击该防撞装置,小浮筒会带动拦截网自适应地从水平状态竖起展开,包裹住来撞船首,再通过相连浮体的运动阻力和恒阻力缆绳来吸收船舶动能,拦截住船舶,保护非通航孔桥墩安全。随后介绍在福建平潭海峡大桥引桥附近海域实施的实船撞击自适应拦截网防撞装置的大型试验,试验结果显示:自适应拦截网成功升起,船舶被安全拦截,从而实验证实了设计原理与设计方案的可行性和可靠性。最后,采用大型水动力分析软件AQWA对防撞装置拦截船舶过程进行数值模拟,模拟结果与实验结果基本一致,说明了数值仿真具有较好的计算精度和可靠性,能够为该防撞装置的结构设计与优化提供重要的参考。     

一种基于状态空间模型的浮式海上升压站平台动力响应计算方法研究

浮式海上升压站的动力响应分析是其设计阶段的重要内容,对浮式升压站进行结构优化进而改进其水动力性能意义重大。提出一种基于状态空间模型的浮式海上升压站平台动力响应算法,该方法通过频域拟合的方法计算延迟函数频响函数有理分式的系数,得到延迟函数的极值和留数,进而构建延迟函数的状态空间模型,通过状态空间模型代替Cummins方程中的卷积项,从而计算浮式海上升压站的动力响应。采用日本福岛示范项目的浮式升压站模型对方法进行验证,结果表明计算得到的动力响应与商业软件SESAM计算结果吻合较好,说明方法的有效性。     

Tidal transport and dispersal of marine toxic microalgae in a shallow, temperate coastal lagoon

A numerical model was used to predict toxic microalgal transport and dispersion in Ria de Aveiro in Portugal. A previously developed Lagrangean particle tracking model coupled to a calibrated two-dimensional hydrodynamic model of Ria de Aveiro was used. Microalgae were regarded as passive particles and the methodology used allowed the determination of their trajectories, as induced by the tidal currents predicted by the hydrodynamic model. The model assumes Ria de Aveiro as vertically homogeneous and does not take into account the vertical distribution patterns of microalgae. Simulations were carried out during extreme spring and neap tides, with microalgal released at the mouth of the lagoon at the local flood. The maximum and minimum areas affected during the occurrence of toxic microalgal blooms were estimated to evaluate the suitability of the distribution of the sampling stations included in the local monitoring program. It was found that the tidal currents greatly determine the microalgal horizontal distribution and dispersal in the lagoon. The results confirmed that the locations of water and bivalve shellfish sampling stations, postulated by INIAP/IPIMAR, in the context of the local harmful algal bloom (HAB) program, were appropriate, although some possible refinements were identified.     

Centrifuge modelling of an on-bottom pipeline under equivalent wave and current loading

Pipelines are the main element in transporting hydrocarbons from their extraction sites to on-shore or floating facilities, with preference now given to pipelines laid directly on the seabed due to their fast and economic installation. However, these pipelines are exposed and must be stable under all environmental conditions, and therefore, their design for on-bottom stability is of critical importance. Although accurate prediction of the pipe–soil interaction behaviour under hydrodynamic loads from waves and currents is of major concern, limited physical testing of pipes subjected to these cyclic loading conditions has occurred. Tests have concentrated on simpler load combinations in order to develop pipe–soil friction factors or the key parameters in plasticity models that described pipe–soil behaviour. In this paper, results from geotechnical centrifuge experiments of a model pipe on calcareous sand soil collected from offshore on the North West Shelf of Australia are presented. A sophisticated load control scheme allowed complex paths characteristic of hydrodynamic loads to be applied during the testing. Furthermore, pipe testing could be extended to relatively large horizontal movements of up to 5 pipe diameter. The results of the centrifuge testing programme provide improved understanding of the pipe–soil interaction under complex hydrodynamic load paths. They have also been used to assess a state-of-the-art plasticity model describing pipe–soil interaction on calcareous sands.     

超强台风“威马逊”作用下红河三角洲海域水动力环境变化的数值研究

201409号超强台风"威马逊"于2014年7月从北部湾北部过境,对红河三角洲近岸海域的水动力环境产生重要影响。本文基于Delft3D建立三维潮、流、浪耦合数值模型,对红河三角洲水位、海流及波浪对台风的响应变化进行模拟。结果表明:威马逊台风期间,红河三角洲海域风速增大约6倍,风向由偏南风转为偏北风,表底层流速均受影响,其中表层变化较大,表现为北分量流速明显增大,流向变为偏南向,与风向主分量变化有关;波高增大为正常海况的9倍,时间变化与风速一致。本研究获得了从北部湾北部海域过境的台风影响下红河三角洲海域水动力环境的响应变化特征,对该海域的物质输运研究及海洋工程建设有重要意义。     

Dynamic response simulation of a heavy cargo suspended by a floating crane based on multibody system dynamics

In this paper, the dynamic response simulation of heavy cargo suspended by a floating crane is performed. The dynamic equations of the motions of the floating crane and the heavy cargo must be considered by the coupled equations because the floating crane and the heavy cargo are connected by wire ropes and provide force and a moment for each other. Hence, the dynamic equations of motion are set up for considering the 6-degrees-of-freedom floating crane and the 6-degrees-of-freedom cargo based on multibody system dynamics. The nonlinear terms in the equations of motion are considered. In addition, the nonlinear hydrostatic force, the linear hydrodynamic force, the wire rope force, and the mooring force are considered as the external forces. Finally, we estimate the motion of the floating crane and the heavy cargo and also calculate the tension of the wire rope between the two.     

Dynamic modeling and motion simulation for a winged hybrid-driven underwater glider

PETREL,a winged hybrid-driven underwater glider is a novel and practical marine survey platform which combines the features of legacy underwater glider and conventional AUV (autonomous underwater vehicle).It can be treated as a multi-rigid-body system with a floating base and a particular hydrodynamic profile.In this paper,theorems on linear and angular momentum are used to establish the dynamic equations of motion of each rigid body and the effect of translational and rotational motion of internal masses on the attitude control are taken into consideration.In addition,due to the unique external shape with fixed wings and deflectable rudders and the dual-drive operation in thrust and glide modes,the approaches of building dynamic model of conventional AUV and hydrodynamic model of submarine are introduced,and the tailored dynamic equations of the hybrid glider are formulated.Moreover,the behaviors of motion in glide and thrust operation are analyzed based on the simulation and the feasibility of the dynamic model is validated by data from lake field trials.     

混合模块大型浮式结构系统耦合动力响应分析

针对具有天然岛礁庇护或人工庇护的温和海洋环境,提出了一种混合模块大型浮式结构系统,即水动力性能更优的半潜式模块作为内侧主模块,消波效果更优的箱式模块作为外侧浮式防波模块和波浪能发电模块.波浪能装置利用外侧箱式模块与内侧半潜式模块的相对纵摇运动进行发电.考虑模块间多体水动力耦合效应和连接器机械耦合效应,基于ANSYS-AQWA程序重点研究了典型海况下混合5模块串联浮式结构系统的动力响应特征.结果表明,外侧箱式模块和波浪能发电装置能有效减弱内侧半潜式主模块运动响应、连接器动力响应和系泊缆绳张力,并且提供一定的能源供给.所得研究成果可为模块化超大型浮式结构系统的防波—发电集成系统设计提供参考.     

Sediment facies and pathways of sand transport about a large deep water headland,Cape Rodney,New Zealand

Cape Rodney is a large headland that protrudes 3–4 km into deep water in the Hauraki Gulf and separates the Mangawhai‐Pakiri and Omaha littoral cells. Detailed swath mapping of seabed sediments around Cape Rodney was carried out using by side‐scan sonar and ground‐truthed by SCUBA, grab sampling, and video. Despite the barrier imposed by the headland two pathways of sand transport around the headland, separated by the topographic high of Leigh Reef, have been identified. One lies close to the headland, where sand from the beach and nearshore of the Mangawhai‐Pakiri embayment is driven by waves and currents along a 500‐m‐wide pathway in c. 20–25 m depth around the headland to the vicinity of Leigh Harbour. The other lies in 50 m water‐depth seawards of Leigh Reef. Here fine sand, sourced from the nearshore of the Mangawhai‐Pakiri embayment and driven offshore from the tip of the headland, is transported back and forth by tidal currents in 50 m water depth on the floor of the Jellicoe Channel. The sand bodies along both these pathways are thin and so sand leakage from the Mangawhai‐Pakiri embayment is thought to be small. Transport at these depths is dependent on both tide and wave generated currents and episodic occurring during storm events. The sediment facies associated with little sand transport about a headland in deep water is one of thin and discontinuous and patchy sand cover between rocky areas and over coarser megarippled substrate. Ocean swell, tidally driven phase eddies that spin up on both sides of the headland, and bathymetry all play a role in shaping those facies.     

New model to determine forces at on-bottom slender pipelines

The present paper proposes a numerical model to determine horizontal and vertical components of the hydrodynamic forces on a slender submarine pipeline lying at the sea bed and exposed to non-linear waves plus a current. The new model is an extension of the Wake II type model, originally proposed for sinusoidal waves (Soedigdo et al., 1999) and for combined sinusoidal waves and currents (Sabag et al., 2000), to the case of periodic or random waves, even with a superimposed current. The Wake II type model takes into account the wake effects on the kinematic field and the time variation of drag and lift hydrodynamic coefficients. The proposed extension is based on an evolutional analysis carried out for each half period of the free stream horizontal velocity at the pipeline. An analytical expression of the wake velocity is developed starting from the Navier–Stokes and the boundary layer equations. The time variation of the drag and lift hydrodynamic coefficients is obtained using a Gaussian integration of the start-up function. A reduced scale laboratory investigation in a large wave flume has been conducted in order to calibrate the empirical parameters involved in the proposed model. Different wave and current conditions have been considered and measurements of free stream horizontal velocities and dynamic pressures on a bottom-mounted pipeline have been conducted. The comparison between experimental and numerical hydrodynamic forces shows the accuracy of the new model in evaluating the time variation of peaks and phase shifts of the horizontal and vertical wave and current induced forces.     

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 0.07 m/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 0.30 m/s and decrease up to a maximum of 0.17 m/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.