Impact of swell on simulations using a regional atmospheric climate model

When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 °C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg⁻¹). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.     

A general model for analysis of sound radiation from orthogonally stiffened laminated composite plates

A general theoretical model is developed to investigate the sound radiation from an infinite orthogonally stiffened plate under point excitation force. The plate can be metallic or composite, and fluid loading is also considered in the research. The first order shear deformation theory is used to account for the transverse shear deformation. The motion of the equally spaced stiffeners is examined by considering their bending vibrations and torsional movements. Based on the periodic structure theory and the concepts of the equivalent dynamic flexibility of the plate, the generalized vibro-acoustic equation of the model is obtained by applying the Fourier transform method. The generalized model that can be solved numerically is validated by comparing model predictions with the existing results. Numerical calculations are performed to investigate the effects of the location of the excitation, the spacing of the stiffeners, the plate thickness, the strengthening form and the fiber orientation on the sound radiation characteristic of the orthogonaUy stiffened plate, and some practical conclusions are drawn from these parameter studies.     

Analysis and reversal of dry and hydroelastic vibration modes of stiffened plates

This work describes the theoretical modal analysis model for the stiffened bottom plate of a tank that is filled with fluid having an undisturbed free surface. In the analysis model, the effects of bending, transverse shear and rotary inertia in both the plate and the stiffener are considered. Analytical results concerning the natural frequencies of the stiffened plate correlate well with experimental modal testing (EMT) results for above the second mode. To overcome the complexities in the modal analysis of the fluid–structure interaction, the Mindlin plate theory and the potential flow theory are applied; the velocity potential is also expressed using double finite Fourier transforms. Additionally, a parametric study was also performed for eigenfrequencies of the plate with a stiffener, in terms of the ratio of the depth of the stiffener to the thickness of the plate and that of the width of the stiffener to the thickness of the plate. Analysis results revealed the phenomenon of “mode reversal”, i.e. the first and second mode shapes of the plate with a deep stiffener in contact with air and water, respectively, are reversed. An occurring condition of “mode reversal” is derived and verified by both finite element analysis and EMT.     

The new simple design equations for the ultimate compressive strength of imperfect stiffened plates

The new simple design equations for predicting the ultimate compressive strength of stiffened plates with initial imperfections in the form of welding-induced residual stresses and geometric deflections were developed in this study. A non-linear finite element method was used to investigate on 60 ANSYS elastic–plastic buckling analyses of a wide range of typical ship panel geometries. Reduction factors of the ultimate strength are produced from the results of 60 ANSYS inelastic finite element analyses. The proposed design equations have been developed based on these reduction factors. For the real ship structural stiffened plates, the most general loading case is a combination of longitudinal stress, transverse stress, shear stress and lateral pressure. The new simplified analytical method was generalized to deal with such combined load cases. The accuracy of the proposed equations was validated by the experimental results. Comparisons show that the adopted method has sufficient accuracy for practical applications in ship design.     

Hydrodynamic performance of solid and porous heave plates

Heave plates have been widely utilized in floating offshore structures as they can provide additional damping and added mass to improve the hydrodynamic response of the system. This study investigates the hydrodynamic characteristics (added mass and damping) of oscillatory solid or porous disks using model scale experiments. All experiments were conducted via forced oscillation model tests using a planar motion mechanism (PMM). The hydrodynamic coefficients of the solid or porous disk obtained from the force measurements are analysed and presented. The sensitivities of the damping and added mass coefficients to both motion amplitude and the disk porosity are examined.     

加筋板的轴压强度受加筋随机点蚀的影响研究

船舶与海洋结构在服役期间会遭受电化学和微生物侵蚀等多种腐蚀环境影响,加筋板作为此类结构的主要受力构件,其表面易产生不规则形态的点蚀,引起构件的强度退化。利用ANSYS有限元软件构建加筋腹板遭受随机点蚀损伤的加筋板有限元模型,研究带板长宽比、带板长细比、加筋长细比和腐蚀体积损失率对加筋板极限强度的影响。研究结果表明,加筋遭受同等腐蚀体积损失率下,带板长宽比变化对极限强度退化的影响很小,但带板长细比和加筋长细比变化产生的影响明显;带板长细比越大,极限强度退化越严重,而加筋长细比越大,极限强度退化反而越小。针对文中研究的加筋板,当加筋长细比为 0.2时,腐蚀体积率为14%的随机点蚀导致结构极限强度退化程度达到约14.0%。因此,加筋的随机点蚀损伤会显著削弱加筋板结构的极限强度,其影响不可忽略。     

Numerical simulations using momentum source wave-maker applied to RANS equation model

For Navier-Stokes equation model using the VOF scheme, Lin and Liu (Lin, P. and Liu, P.L.-F. (1999). Internal wave-maker for Navier-Stokes equations models. J. Waterw. Port Coast. Ocean Eng., 125 (4), 207–215.) developed an internal wave-maker method for which a mass source function of the continuity equation was used to generate target wave trains. Using this internal wave-maker method, various numerical experiments have been conducted without any problems due to waves reflected by a wave-maker. In this study, an internal wave-maker method using a momentum source function was proposed. Various numerical simulations in two and three dimensions were performed using the momentum source wave-maker applied to the RANS equation model in a CFD code, FLUENT. To verify their applicability in 2 dimensions, the computational results obtained using the momentum source wave-maker in a channel of constant depth were compared with the results obtained by using the mass source wave-maker and with the analytical solutions. And the results of the present numerical simulations of hydraulic experiments, which represent nonlinear waves on a submerged shoal and breaking waves on a plane beach, were compared with measurements. The comparisons showed good agreements between them. To see their applicability into 3-dimensional cases, the present results in a basin of constant depth were compared with the analytical solutions, and they agreed well with each other. In addition, vertical variation of longshore current was presented by using the 3-dimensional simulation results.     

Impact of narrow plates on broader liquid bodies

Analytical leading-order solutions are given for liquid bodies that hit a rigid plate in impulsive incompressible impact. The liquid bodies are broader than the impacting plate which causes a mixed boundary condition. The initial velocity field, the corresponding pressure impulse field, and the associated virtual masses are calculated. The following liquid body geometries are studied: 2D rectangles hit symmetrically by the plate, including a case with impermeable sidewalls, asymmetric hit of a 2D rectangle, and symmetric hit of a 3D rectangular box, and of a circular cylinder. A plausible estimate for liquid impact is that a marine structure can never be struck by a greater force impulse than the momentum of incoming liquid perpendicular to its own cross section. By comparing the virtual mass with the total projected fluid mass above the plate for the different liquid body geometries, it is found that the estimate holds for all the computed cases.     

Prediction of maximum wave-induced liquefaction in porous seabed using multi-artificial neural network model

In the last few decades, considerable efforts have been devoted to the phenomenon of wave-induced liquefactions, because it is one of the most important factors for analysing the seabed and designing marine structures. Although numerous studies of wave-induced liquefaction have been carried out, comparatively little is known about the impact of liquefaction on marine structures. Furthermore, most previous researches have focused on complicated mathematical theories and some laboratory work. In the present study, a data dependent approach for the prediction of the wave-induced liquefaction depth in a porous seabed is proposed, based on a multi-artificial neural network (MANN) method. Numerical results indicate that the MANN model can provide an accurate prediction of the wave-induced maximum liquefaction depth with 10% of the original database. This study demonstrates the capacity of the proposed MANN model and provides coastal engineers with another effective tool to analyse the stability of the marine sediment.     

SWAN模型中不同风拖曳力系数对风浪模拟的影响

本文以荷兰哈灵水道海域为实验区域,通过敏感性实验,研究了在14 m/s、31.5 m/s和50 m/s（分别代表一般大风、强热带风暴和强台风的极端条件）定常风速下SWAN模型中不同风拖曳力系数对风浪模拟的影响程度。结果表明,对于近岸浅水区域（水深小于20 m）,风拖曳力系数计算方案的选择对有效波高影响较小,而且当风速增加到一定程度后,波浪破碎成为影响波高值的主要因素;对于深水区域（水深大于30 m）,一般大风条件下风拖曳力系数计算方案的选择对有效波高影响仍然较小,随着风速的继续增大,风拖曳力系数计算方案的选择对有效波高的影响逐渐显著。对于平均周期,风拖曳力系数计算方案的选择和风速的改变对其影响均较小,而由水深变浅导致的波浪破碎对其影响较为显著。根据敏感性实验结果,本文对SWAN模型中风拖曳力系数计算方案的选择做出如下建议:计算近岸浅水区域风浪场或深水区域一般大风条件风浪场时,其风拖曳力系数可以直接采用模型默认选项;而对于深水区域更大风速条件,可首先采用模型默认选项试算,然后结合当地海域实测波浪资料进行修正。     

Finite element analysis of ship structures using a new stiffened plate element

A new stiffened plate element is developed for the three-dimensional finite element analysis of ship structures. The plate element can accommodate any number of arbitrarily oriented stiffeners and obviates the use of mesh lines along the stiffeners. The new element provides a very economic global analysis of the complete ship structure with fewer elements and without any loss of accuracy. The global analysis of a rectangular box shaped vessel is carried out with the present element and compared with the general-purpose finite element software NISA. An Offshore Tug/Supply Vessel is analysed for crest at perpendiculars.     

Identification of damage on ship's cross stiffened plate panels using vibration response

This paper describes a scheme for identifying damage on a cross stiffened plate of a tanker ship model. Two damage locations were investigated. One was located at the intersection between the longitudinal and the bulkhead. The other was located at the intersection between the longitudinal and the web frame. The amplitude of a function containing the natural frequency, the damping ratio and the response was used as an indicator for damage. The function was obtained from a simulation using a neural network technique which inputs were the model's response. Simulation results obtained using response from numerical and experimental investigations are discussed. It was observed that even when the natural frequencies and the damping ratios did not show any definable trend due to the presence of damages, the amplitudes of the function showed consistent trends as the damage became more severe.     

On the effective breadth of stiffened platings

A stiffened plate is one of the principal structural components of the hull. During the design phase it is useful to study this structure considering it as a system consisting of monodimensional elements. Therefore, it is appropriate to identify the plate breadth that collaborates with the stiffener in the simplified calculation.The aim of this work is to investigate the effective breadth, regarding both stiffness and strength. The analysis is performed comparing various numerical solutions supplied by a finite-element program with solutions based on the beam theory. Analytic expressions and graphs, derived from the results of the parametric investigation, are proposed to design this kind of structure.     

Experimental and numerical analysis of container stack dynamics using a scaled model test

This paper describes an approach to simulate a seven-tier stack consisting of scaled model of a 20 ft ISO freight container and its linking connectors, denominated twist locks, subjected to dynamical load induced by its base. The physical (dimensions, mass, and moments of inertia) and structural (longitudinal, transversal and torsional stiffness) characteristics of the scaled models were decided based on two dimensionless numbers: ratios between gravity force and inertia force, and elastic force divided by inertia force, through experimental and numerical analysis. A series of experiments with controlled parameters were performed using a shaking table test to understand the effects of each variable in the container stack dynamics and present enough data to validate the numerical model. The results of this study indicate that the numerical model built is a promising tool for further study. Moreover, the model is able to predict conditions close to real situations faced by container stacks while storage on a ship's deck.     

基于海气耦合模型的渤海两种类型风暴潮数值模拟与对比分析

渤海一年四季都易受到由温带风暴和热带气旋所致风暴潮的影响。为了缓解风暴潮灾害对海岸地区人员生命财产的影响,十分有必要了解大型风暴潮的发生过程和机制。目前大部分研究主要局限于单一的温带风暴潮或台风风暴潮。本文利用所构建的海气耦合数值模型研究了发生于渤海的两种类型的风暴潮,对发生在渤海的2次典型强风暴潮过程进行了模拟。由WRF模型模拟得到的风场强度和最低海平面气压与实测数据吻合较好,由ROMS模型模拟得到的风暴潮期间水位变化过程与潮位站观测结果也吻合较好。对两种类型风暴潮期间的风场结钩、海面风应力、海洋表面平均流场以及水位分布进行了分析对比,并将耦合模型结果与非耦合模型结果进行了对比。研究表明,渤海两种类型风暴潮期间的风场结钩、海面风应力、海洋表面平均流场以及水位分布等均存在巨大差异。渤海风暴潮的强度主要由海洋表面的驱动力所决定,但同时也受海岸地形地貌的影响。     

High-resolution simulations of a macrotidal estuary using SUNTANS

The parallel, finite-volume, unstructured-grid SUNTANS model has been employed to study the interaction of the tides with complex bathymetry in the macrotidal Snohomish River estuary. The unstructured grid resolves the large-scale, O(10 km) tidal dynamics of the estuary while employing 8 m grid-resolution at a specific region of interest in the vicinity of a confluence of two channels and extensive intertidal mudflats to understand detailed local intratidal flow processes. After calibrating tidal forcing parameters to enforce a match between free surface and depth-averaged velocities at several locations throughout the domain, we analyze the complex dynamics of the confluence and show that the exposure of the intertidal mudflats during low tide induces a complex flow reversal. When coupled with the longitudinal salinity gradient, this flow reversal results in a highly variable salinity field, which has profound implications for local mixing, stratification and the occurrence of fine-scale flow structures. This complex flow is then used as a testbed from which to describe several challenges associated with high resolution modeling of macrotidal estuaries, including specification of high resolution bathymetry, specification of the bottom stress, computation of the nonhydrostatic pressure, accurate advection of momentum, and the influence of the freshwater inflow. The results indicate that with high resolution comes the added difficulty of requiring more accurate specification of boundary conditions. In particular, the bottom bathymetry plays the most important role in achieving accurate predictions when high resolution is employed.     

Diagnosis of currents in the Brazil test area from temperature and salinity observations using a complete non-linear model

The diagnosis of current (diagnostic and adaptation calculations) with application of a complete non-linear model for the test area under study during Cruise 41 of R/VAkademik Vermadsky is reported. The model-based current velocities are compared with the observed ones. Hydrochemical and hydro-optical data were invoked to conduct an indirect analysis of the water circulation in the area under study.Translated by V. Puchkin.     

Numerical model simulations of continental shelf flows off northern California

The three-dimensional circulation on the continental shelf off northern California in the wind events and shelf transport (WEST) experiment region during summer 2001 is studied using the primitive equation regional ocean modeling system (ROMS). The simulations are performed with realistic topography and initial stratification in a limited-area domain with a high-resolution grid. Forcing consists of measured wind-stress and heat flux values obtained from a WEST surface buoy. The general response shows a southward coastal upwelling jet of up to and a weakening or reversal of currents inshore of the jet when upwelling winds relax. Model results are compared to WEST moored velocity and temperature measurements at five locations, to CODAR surface current observations between Pt. Reyes and Bodega Bay, and to hydrographic measurements along shipboard survey lines. The model performs reasonably well, with the highest depth-averaged velocity correlation (0.81) at the inshore mooring (40 m water depth) and lowest correlation (0.68) at the mid-depth mooring (90 m depth). The model shows generally stronger velocities than those observed, especially at the inshore moorings, and a lack in complete reversal of southward velocities observed when upwelling winds relax. The comparison of surface velocities with CODAR measurements shows good agreement of the mean and the dominant mode of variability. The hydrography compares closely at the southern and northern edges of the survey region (correlation coefficients between 0.90 and 0.97), with weaker correlations at the three interior survey lines (correlation coefficients between 0.44 and 0.76). Mean model fields over the summer upwelling period show slight coastal jet separation off Pt. Arena and significant separation off Pt. Reyes. The cape regions also experience relatively strong bottom velocities and nonlinearity in the surface flow. Across-shelf velocity sections examined along the shelf reveal a double jet structure that appears just north of Bodega Bay and shows the offshore jet strengthening to the south. We examine the dynamics during an upwelling and subsequent relaxation event in May 2001 in which the WEST measurements show evidence of a strong flow response. The alongshelf variability in the upwelling and relaxation response introduced by Pt. Reyes is evident. Analysis of term balances from the depth-averaged momentum equations helps to clarify the event dynamics in different regions over the shelf. A clear pattern in the nonlinear advection term is due to the spatial acceleration of the southward jet around the capes of Pt. Arena and Pt. Reyes during upwelling. Results from a three-dimensional Lagrangian analysis of water parcel displacement show significant southward displacement in the coastal jet region, including a strong signal from the double jet. Alongshelf variability in parcel displacements and upwelling source waters due to the presence of Pt. Arena and Pt. Reyes is also apparent from the Lagrangian fields. A cyclonic eddy-like recirculation feature offshore of Pt. Arena prior to the upwelling event causes large patches of onshore-displaced parcels. Additionally, across-shelf variability in the response of water parcels along the D line includes decreased vertical displacement and increased alongshelf displacement in the offshore direction.     

Implementation of state-dependent plasticity model in strain wedge model for laterally loaded piles in sand

ABSTRACT

The strain wedge model effectively performs nonlinear analyses of the lateral response of piles by using a nonlinear stress-strain relationship to describe soil behavior in the strain wedge. In this study, a state-dependent plasticity model has been implemented in the strain wedge model to calculate the stress-strain relationship for sand in the strain wedge. To complement this implementation, the effect of dilatancy on the shear strain is considered in the strain wedge. A full-scale test and a 45 g centrifuge model test on laterally loaded piles are used to validate the proposed method. The results show that the deflections and moments predicted by the proposed method accord well with those measured from full-scale and centrifugal model pile tests. Moreover, the combination of the state-dependent plasticity model and the strain wedge model allows for analyzing the lateral response of single piles using a unique set of model parameters for different relative densities of sands. In addtion, the stress-strain response in the strain wedge, not the dilatancy, dominates the soil resistance in the strain wedge and thus the lateral response of piles.