Tide and surge dynamics along the Iberian Atlantic coast

The eastern Atlantic barotropic dynamics (in a region spanning from 20° N to 48° N and 34° W to 0°) are studied through numerical modelling and in situ measurements. The main source of data is the tidal gauge network REDMAR, managed by Clima Marítimo (Puertos del Estado). The numerical model employed is the HAMSOM, developed both by the Institut für Meereskunde (Hamburg University) and Clima Marítimo.In this paper, tidal and storm surge dynamics are studied for the region, focusing particularly on the nonlinear transfer of energy between the different forcings.The results of tidal simulations show good agreement between semidiurnal harmonic components and the values obtained from the tidal gauges (both coastal and pelagic) and current metres. The nonlinear transfers of energy from semidiurnal to higher order harmonics, such as M4 and M6, were mapped. Those transfers were found to be important in only two areas: The French continental shelf in the Bay of Biscay and the widest part of the African shelf, south of Cabo Bojador. The modelled diurnal constituents show larger relative differences with measurements than semidiurnal harmonics, especially in data concerning the phase.A method to isolate the nonlinear transfers of energy between tidal and atmospheric forcing during a storm surge was developed. These transfers were found to be significant in the same areas where tidal nonlinear activity was present. The effect of short period wind generated waves on sea surface elevation was also investigated. The magnitude of the spatial derivatives of radiation stress was compared with wind-induced stress. As a result of this comparison, we found the inclusion of a forcing term that depends on radiation stress in ocean model simulations at this scale and resolution to be not essential. The effect of computing wind-induced stresses, with a formulation that explicitly depends on sea state, was also explored by means of a coupled run of the HAMSOM and the spectral wave model WAM for a storm surge event in the Spanish coast. This formulation was not found to be an improvement over a classical parameterisation which only depends on wind fields.     

Fine scale distribution of nitrous oxide in marine sediments

Vertical profiles of nitrous oxide and other inorganic nitrogen compounds in the sediments in Tokyo Bay and its vicinity were measured using the whole core squeezing method. Within the bay area, vertical profiles of nitrous oxide in the interstitial waters showed one or two distinctive peaks around the depth of 2–4 cm, which corresponded to the peaks of nitrite and nitrate. In situ formation of nitrous oxide through nitrification was suggested in those sediments, which was possibly activated by the presence of benthic animals. On the other hand, at the deep sediment off Tokyo Bay the profiles of nitrous oxide, nitrite and nitrate gave a monotonous single peak, indicating less bioturbated condition. Denitrification or dissimilatory nitrate reduction might be important for the formation of nitrous oxide peaks in the latter type of sediment.     

Modelling of a wave energy converter array with a nonlinear power take-off system in the frequency domain

This paper presents a nonlinear frequency domain model and uses this to assess the performance of a wave energy converter (WEC) array with a nonlinear power take-off (PTO). In this model, the nonlinear PTO forces are approximated by a truncated Fourier series, while the dynamics of the WEC array are described by a set of linear motion equations in the frequency domain, and the hydrodynamic coefficients are obtained with the boundary element method. A single heave absorber is firstly investigated to establish the accuracy of the new model in capturing the nonlinear behaviour of the pumping system. Subsequently, simulations of a 2D array with 18 WECs and a pillar in the centre (representing the tower of a wind turbine) are carried out to understand wave interference effects. Several optimisation strategies are proposed to improve the overall performance of the WEC array. These results demonstrate a computationally effective method for accounting for nonlinear effects in large WEC arrays. The proposed approach may potentially be applied for developing control algorithms for the adaptability of a 2D array to incoming wave excitation.     

Nitrous oxide concentration and nitrification and denitrification in Zhujiang River Estuary, China

1 Introduction Estuaries have been subject to intense anthro- pogenic influences over recent decades by inputs of ni- trogen as a consequence of fertilizer usage and sewage/waste input (Howarth et al., 1996). Thus, the capacity of an estuary to buffer or alleviate additional nutrients is of importance in modifying the load of ni- trogen in the marine ecosystem. The role of microor- ganisms in regulating fluxes of nitrogen from land to sea through estuaries has received increased attention (Ni…     

辽河口海域N2O分布特征和海气通量研究

于2007年5月份(枯水期)和8月份(丰水期)采集辽河口区域大气样品及水样,应用气相色谱及静态顶空法测定大气及海水中溶解N₂O的浓度,并计算不同站位的N₂O通量。结果表明:辽河口区域水体中溶解N₂O浓度存在着明显的时空变化,5月份河口区域水体中N₂O浓度高于8月份,且河流段溶解N₂O浓度大于近海岸海域。水中溶解N₂O浓度及溶解氧浓度呈显著的负相关关系。在枯水期和丰水期,所有站位的N₂O均处于过饱和状态,N₂O气体从水体向大气排出,辽河口区域是大气中N₂O的一个排放源。     

High aspect ratio (L/D) riser VIV prediction using wake oscillator model

A two-dimensional (2-D) vortex-induced vibration (VIV) prediction model for high aspect ratio (L/D) riser subjected to uniform and sheared flow is studied in this paper. The nonlinear structure equations are considered. The near wake dynamics describing the fluctuating nature of vortex shedding is modeled using classical van der Pol equation. A new approach was applied to calibrate the empirical parameters in the wake oscillator model. Compared the predicted results with the experimental data and computational fluid dynamic (CFD) results. Good agreements are observed. It can be concluded that the present model can be used as simple computational tool in predicting some aspects of VIV of long flexible structures.     

A second order random wave model for predicting the power performances of a wave energy converter

The power performances of a point absorber wave energy converter(WEC) operating in a nonlinear multidirectional random sea are rigorously investigated. The absorbed power of the WEC Power-Take-Off system has been predicted by incorporating a second order random wave model into a nonlinear dynamic filter. This is a new approach, and, as the second order random wave model can be utilized to accurately simulate the nonlinear waves in an irregular sea, avoids the inaccuracies resulting from using a first order linear wave model in the simulation process. The predicted results have been systematically analyzed and compared, and the advantages of using this new approach have been convincingly substantiated.     

On-line learning control of autonomous underwater vehicles usingfeedforward neural networks

A neural-network-based learning control scheme for the motion control of autonomous underwater vehicles (AUV) is described. The scheme has a number of advantages over the classical control schemes and conventional adaptive control techniques. The dynamics of the controlled vehicle need not be fully known. The controller with the aid of a gain layer learns the dynamics and adapts fast to give the correct control action. The dynamic response and tracking performance could be accurately controlled by adjusting the network learning rate. A modified direct control scheme using multilayered neural network architecture is used in the studies with backpropagation as the learning algorithm. Results of simulation studies using nonlinear AUV dynamics are described in detail. The robustness of the control system to sudden and slow varying disturbances in the dynamics is studied and the results are presented     

Upwelling and river runoff as sources of dissolved nitrous oxide to the Alsea estuary,Oregon

Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N₂O to the atmosphere. Large and rapid increases in the concentrations of N₂O, NO₃^?, and NO₂^? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N₂O in rivers.     

柔性管卷管式铺管上卷有限元模拟

采用卷管法进行海底管道铺设过程中,管道首先通过牵引作用上卷于卷筒进行储存。管道与卷筒发生非线性接触,可能会产生复杂的塑性变形和局部屈曲。通过全尺寸柔性管力学性能试验获得柔性管轴力—应变以及弯曲—曲率等非线性力学性能关系,将试验所得的非线性材料性能参数导入建立的两种柔性管上卷ABAQUS有限元模型（梁—实体单元模型与壳和桁架—实体单元模型）,实现柔性管较大轴向抗拉刚度和较小抗弯刚度的同步模拟以及管道与卷筒的非线性接触响应特征。通过对比分析两种有限元模型数值模拟得到的管道弯矩、弯曲曲率、管道轴力、管道与卷筒的接触压强等数据,发现在管道上卷过程中管道沿副法线方向的SM3弯矩占据其弯曲变形主导地位;管道与卷筒之间的摩擦效应对于管道轴力的影响较为显著;管道与卷筒的最大接触压强主要发生在卷管过渡段区域。     

Nitrous oxide solubility in water and seawater

The solubility of nitrous oxide in pure water and seawater has been measured microgasometrically over the range 0–40°C. The data have been corrected for nonideality and are fitted to equations in temperature and salinity of the form used previously to fit the solubilities of other gases. The fitted values have a precision of 0.1% and an estimated accuracy of 0.3%. The nonideal behavior of nitrous oxide—air mixtures is discussed, and the solubility of atmospheric nitrous oxide is presented in parametric form. A similar parametric representation for the solubility of atmospheric carbon dioxide is given in the Appendix.     

A nonlinear PDE formulation for offshore vessel pipeline installation

In this paper a nonlinear dynamic PDE formulation for a pipe string suspended from a pipelay vessel to the seabed in a pipelay operation is developed. This model extends a three-dimensional beam model capable of undergoing finite extension, shearing, twist and bending, to apply for marine applications by adding the effects of restoring forces, hydrodynamic drag and seabed interaction. The model is validated against the natural catenary equation and the FEM code RIFLEX. The model is extended to include the pipelay vessel dynamics by applying a potential theory formulation of a surface vessel, suited for dynamic positioning and low speed maneuvering, as a boundary condition for the PDE. This system is found to be input-output passive and stable. Pipeline installation applications where the presented model is suited are e.g., analysis and simulation of the installation operation, operability analysis, hardware-in-the-loop (HIL) testing for vessel control systems, and automation of the pipelay operation.     

Implementing convection inito Lorenz's global cycle. Part I. Gridscale averaging of the energy equations

Sub‐gridscale processes take place throughout the global atmosphere. Yet they have been neglected in traditional estimates of the global energy cycle on the ground that they can be treated as molecular heat fluxes. This view may cause quantitative underestimates of the efficiency of the global circulation of the atmosphere. In Part I of this two-part study we revisit the classical theory, beginning with the local energy equations. Similar to Lorenz we introduce a barotropic reference pressure p _r and define a generalized field equation for the integrand of available potential energy, without reference to hydrostasy. The emerging energy quantity is new in that it comprises not only the classical correlation between efficiency factor and enthalpy but also an additional potential that depends upon p _r . We then perform mass-averaging over the scale of contemporaneous global models (40‐400 km) and come up with averaged field energy equations, valid at the gridscale. Additional global and time-averaging of these removes all divergences and tendencies and yields two equations for the global energy reservoirs. The available potential energy reservoir is fed by gridscale plus sub-gridscale generation. The kinetic energy reservoir is tapped by gridscale plus sub-gridscale dissipation. Exchange between the reservoirs is carried by both gridscale and sub-gridscale conversion terms ( C ^grid, C ^sub ). Generation, conversion and dissipation fluxes are complete, as compared to the approximate quantities in the traditional formulation of the energy cycle. This approach allows to fully exploit Lorenz's original concept. The gridscale equations derived will be the basis for evaluating numerically the classical Lorenz terms plus a couple of new global conversion fluxes, notably C ^sub, to be presented in Part II of this study.     

An Oscillating Wave Energy Converter with Nonlinear Snap-Through Power-Take-Off Systems in Regular Waves

Floating oscillating bodies constitute a large class of wave energy converters, especially for offshore deployment. Usually the Power-Take-Off (PTO) system is a directly linear electric generator or a hydraulic motor that drives an electric generator. The PTO system is simplified as a linear spring and a linear damper. However the conversion is less powerful with wave periods off resonance. Thus, a nonlinear snap-through mechanism with two symmetrically oblique springs and a linear damper is applied in the PTO system. The nonlinear snap-through mechanism is characteristics of negative stiffness and double-well potential. An important nonlinear parameter is defined as the ratio of half of the horizontal distance between the two springs to the original length of both springs. Time domain method is applied to the dynamics of wave energy converter in regular waves. And the state space model is used to replace the convolution terms in the time domain equation. The results show that the energy harvested by the nonlinear PTO system is larger than that by linear system for low frequency input. While the power captured by nonlinear converters is slightly smaller than that by linear converters for high frequency input. The wave amplitude, damping coefficient of PTO systems and the nonlinear parameter affect power capture performance of nonlinear converters. The oscillation of nonlinear wave energy converters may be local or periodically inter well for certain values of the incident wave frequency and the nonlinear parameter , which is different from linear converters characteristics of sinusoidal response in regular waves.     

Self-localization of planetary wave structures in the ionosphere upon interaction with nonuniform geomagnetic field and zonal wind

The generation and further linear and nonlinear dynamics of planetary magnetized Rossby waves (MRWs) in the rotating dissipative ionosphere are studied in the presence of a zonal wind (shear flow). MRWs are caused by interaction with the spatially nonuniform geomagnetic field and are ionospheric manifestations of ordinary tropospheric Rossby waves. A simplified self-consistent set of model equations describing MRW-shear flow interaction is derived on the basis of complete equations of ionospheric magnetohydrodynamics. Based on an analysis of an exact analytical solution to the derived dynamic equations, an effective linear mechanism of MRW amplification in the interaction with nonuniform zonal wind is ascertained. It is shown that operators of linear problems are non-self-adjoint in the case of shear flows, and the corresponding eigenfunctions are nonorthogonal; therefore, the canonically modal approach is of little use when studying such flows; a so-called nonmodal mathematical analysis is required. It is ascertained that MRWs effectively get shear flow energy during the linear stage of evolution and significantly increase (by several orders of magnitude) their energy and amplitude. The necessary and sufficient condition of shear flow instability in an ionospheric medium is derived. Nonlinear self-localization begins with the development of shear instability and an increase in the amplitude, and the process ends with the self-organization of strongly localized isolated large-scale nonlinear vortex structures. Thus, a new degree of freedom and a way for perturbation evolution to occur appear in medium with shear flow. The nonlinear systems can be a pure monopole vortex, a vortex streets, or vortex chains depending of the shape of the sheared flow velocity profile. The accumulation of such vortices in the ionospheric medium can produce a strongly turbulent state.     

Tidal dynamics in the strong tidal current environment of the Uldolmok waterway, southwestern tip off the Korean peninsula

Uldolmok waterway, located between an island off the southwestern tip of Korean peninsula and mainland, is famous for its strong tidal current that has a maximum current of about 6.0m/s. A series of field observations along with numerical modeling have been carried out in order to understand the tidal dynamics in terms of the force balance along the whole waterway and the energy balance in the narrowest part of the waterway. First, analysis of the ADCP current and the tide level variation data reveals that the tidal dynamics along the total waterway (channel) is balanced dominantly between the pressure gradient and linear bottom frictional forces, with the phase lag of sea level difference for the semi-diurnal constituents leading the current phase about by 10°. Secondly, the result of the numerical modeling reveals that the tidal energy flux vector flows toward the narrowest section, indicating that there should be related nonlinear processes. Through the numerical model experiment with multi-components, the convergence of (M₂ + S₂) tidal energy flux of 6.68 × 10⁷ Joule/s in the narrow area of the Uldolmok waterway is explained mainly by the energy consumption of 73% through the nonlinear generation of shallow water components and by the bottom frictional energy dissipation of 27%. This reveals that the remarkably strong nonlinear process dominates in the narrowest section of the Uldolmok waterway, compared with other areas, such as Yellow and East China Seas where the total M2 energy flux through the open boundary is balanced in terms of the bottom dissipation (Kang et al. 2003; Choi 1980).     

Dynamic modeling of cable towed body using nodal position finite element method

This paper analyses nonlinear dynamics of cable towed body system. The cable has been modeled and analyzed using a new nodal position finite element method, which calculates the position of the cable directly instead of the displacement by the existing finite element method. The newly derived nodal position finite element method eliminates the need of decoupling the rigid body motion from the total motion, where numerical errors arise in the existing nonlinear finite element method, and the limitation of small rotation in each time step in the existing nonlinear finite element method. The towed body is modeled as a rigid body with six degrees of freedom while the tow ship motion is treated as a moving boundary to the system. A special procedure has been developed to couple the cable element with the towed body. The current approach can be used as design tool for achieving improved directional stability, maneuverability, safety and control characteristics with the cable towed body. The analysis results show the elegance and robustness of the proposed approach by comparing with the sea trial data.     

A pseudo-coupled analytic fluid-structure interaction method for underwater implosion of cylindrical shells

Underwater implosion, the rapid collapse of a structure caused by hydrostatic pressure, is a fully coupled, highly dynamic and nonlinear fluid-structure interaction (FSI) problem. The primary motivation behind studying implosion is the short-duration, high-pressure pulse generated in the surrounding water. This paper presents a simplified analytic method to estimate the energy in the pressure pulse, based on potential flow theory. The method accounts for the varying fluid pressure and accompanying FSI. The focus is on long, thin, unstiffened metallic cylindrical shells that collapse in mode 2. The implosion pulse energy is shown to be equal to the maximum system kinetic energy developed during collapse. The kinetic energy is calculated using an energy balance approach and analytic solutions for plastic energy dissipation and energy required to compress the internal air. The time-varying fluid pressure, and subsequently the work done by the fluid on the cylinder, is found using a novel explicit time-stepping methodology. The result is a pseudo-coupled analytic solution for the fluid pressure time history and implosion pulse energy. Solutions for pulse energy agree with RANS numerical simulations within 5%.     

Head-wave parametric rolling of a surface combatant

Complementary CFD, towing tank EFD, and nonlinear dynamics approach study of parametric roll for the ONR Tumblehome surface combatant both with and without bilge keels is presented. The investigations without bilge keels include a wide range of conditions. CFD closely agrees with EFD for resistance, sinkage, and trim except for Fr>0.5 which may be due to free surface and/or turbulence modeling. CFD shows fairly close agreement with EFD for forward-speed roll decay in calm water, although damping is over/under predicted for largest/smaller GM. Most importantly CFD shows remarkably close agreement with EFD for forward-speed parametric roll in head waves for GM=0.038 and 0.033 m, although CFD predicts larger instability zones at high and low Fr, respectively. The CFD and EFD results are analyzed with consideration ship motion theory and compared with Mathieu equation and nonlinear dynamics approaches. Nonlinear dynamics approaches are in qualitative agreement with CFD and EFD. The CFD and nonlinear dynamics approach results were blind in that the actual EFD radius of gyration k_xx was not known a priori.