Turbulent Evolution and Energetics of Lock-Release Gravity Currents

1 .Introduction Atypical case of gravity currents (Simpson,1997) that has been studied frequentlyis the lock-release gravity current .Asillustratedin Fig.1 ,twofluids of different densities are placedinarectan-gular channel ,beinginitially at rest andsepa…     

Numerical Solution of Lock-Release Gravity Current with Viscous Self-Similar Regime

Lock-release gravity currents with a viscous self-similar regime are simulated by use of the renormalization group(RNG) k - ε model for Reynolds-stress closure. Besides the turbulent regime with initially a slumping phase of a conslant current front speed and later an inviseid self-similar phase of front speed decreasing as t^-1/3(where t is the time measured from release), the viseous self-similar regime is satisfactorily reproduced with front speed decreasing as t^-4/5,consistent with well known experimental observations.     

Numerical Simulation of Hydrodynamic Behaviors of Gravity Cage in Waves

This paper aims at investigation of the dynamic properties of gravity cage exposed to waves by use of a numerical model. The numerical model is developed, based on lumped mass method to set up the equations of motion of the whole cage; meanwhile the solutions of equations are solved by the Runge-Kutta-Verner fifth-order and sixth-order method. Physical model tests have been carried out to examine the validity of the numerical model. The results by the numerical simulation agree well with the experimental data.     

Ocean Model Simulation of Southern Indian Ocean Surface Currents

The dynamic importance of the Southern Indian Ocean (SIO) lies in the fact that it connects the three major world oceans: the Pacific, Atlantic, and Indian Oceans. Modeling study has been used to understand the circulation pattern of this very important region. Simulation of SIO (10°N-60°S and 30°E-120°E) is performed with z-coordinate Ocean General Circulation Model (OGCM) viz; MOM3.0 and the results have been compared with observed ship drift data. It is found that except near coastal boundaries and in equatorial region, the simulated current reproduce most well known current pattern such as Antarctic Circumpolar Current (ACC), South Equatorial Current (SEC) etc. and bears a resemblance to that of the observed data; however the magnitude of the surface current is weaker in model than the observed data, which may be due to deficiency in the forcing field and boundary condition and problem with observed data. The annual mean wind stress curl computed over the oceanic domain reveals about ACC and its similar importance. The way in which the ocean responds to the windstress and vertically integrated transport using model output is fascinating and rather good.     

Ocean Model Simulation of Southern Indian Ocean Surface Currents

The dynamic importance of the Southern Indian Ocean (SIO) lies in the fact that it connects the three major world oceans: the Pacific, Atlantic, and Indian Oceans. Modeling study has been used to understand the circulation pattern of this very important region. Simulation of SIO (10°N–60°S and 30°E–120°E) is performed with z-coordinate Ocean General Circulation Model (OGCM) viz; MOM3.0 and the results have been compared with observed ship drift data. It is found that except near coastal boundaries and in equatorial region, the simulated current reproduce most well known current pattern such as Antarctic Circumpolar Current (ACC), South Equatorial Current (SEC) etc. and bears a resemblance to that of the observed data; however the magnitude of the surface current is weaker in model than the observed data, which may be due to deficiency in the forcing field and boundary condition and problem with observed data. The annual mean wind stress curl computed over the oceanic domain reveals about ACC and its similar importance. The way in which the ocean responds to the windstress and vertically integrated transport using model output is fascinating and rather good.     

Numerical Simulation of the Three-Dimensional Wave-Induced Currents on Unstructured Grid

By coupling the three-dimensional hydrodynamic model with the wave model, numerical simulations of the three-dimensional wave-induced current are carried out in this study. The wave model is based on the numerical solution of the modified wave action equation and eikonal equation, which can describe the wave refraction and diffraction. The hydrodynamic model is driven by the wave-induced radiation stresses and affected by the wave turbulence. The numerical implementation of the module has used the finite-volume schemes on unstructured grid, which provides great flexibility for modeling the waves and currents in the complex actual nearshore, and ensures the conservation of energy propagation. The applicability of the proposed model is evaluated in calculating the cases of wave set-up, longshore currents, undertow on a sloping beach, rip currents and meandering longshore currents on a tri-cuspate beach. The results indicate that it is necessary to introduce the depth-dependent radiation stresses into the numerical simulation of wave-induced currents, and comparisons show that the present model makes better prediction on the wave procedure as well as both horizontal and vertical structures in the wave-induced current field.     

基于VOF模型的浮式消波海流机性能优化

针对浮式消波海流机样机测试时出现的问题,对其吃水深度线和叶片安装角度进行优化。根据装置结构特点,用Gambit建立了装置横截面的二维网格模型。根据浮式消波海流机实际工作环境,利用Fluent流体仿真软件,使用VOF(volume of fluid)两相流模型,分配空气相与液态水相在流域中的不同比例,来确定不同的吃水深度线。并且结合k-epsilon紊流模型建立模拟仿真环境。先对3种水线进行仿真分析,然后进行实验验证。通过分析对比装置的3种不同吃水深度线的模拟与实验结果,得到装置的最优吃水深度线为1/3水线。基于最优吃水深度线,分别对叶片的4种安装角度在相同的仿真环境中进行模拟仿真。利用仿真得到的扭矩数据,计算扭矩系数、功率系数,分析对比仿真数据以及仿真过程中扭矩趋势图得出4种叶片安装角度的最优角度是90°。吃水深度线决定了装置的消波能力,并且为叶片获能提供条件,叶片安装角度直接影响着装置的获能效率,所以最优的吃水深度线和叶片安装角度对海流机的消波能力和获能效率具有重要意义。     

Influence of Bottom Stress on the Two-layer Flow Induced by Gravity Currents in Estuaries

Baroclinic circulation in highly stratified and partially stratified estuaries is characterised by a two-layer flow: a bottom salt- water inflow and a surface brackish-water outflow. Tidal period variation of the thicknesses of a two-layer flow is observed to be associated with mixing, bottom stress and hydraulic characteristics of superposed tidal and gravity currents. Here, both analytical two-layer hydraulic equations with weak friction and a numerical model including a turbulence closure were utilised to understand the mechanism of the layer tendency within a two-layer flow under different barotropic flow conditions. It has been found that in the weak bottom friction case, a gravity current has two critical solutions at the layer thickness equal to 0·5Hand 0·292H. The layer thickness towards a particular critical solution is dependent on the sign of the bottom stress, i.e. when the bottom stress is opposite (favor) to the bottom gravity current, its layer thickness converges to 0·5H(0·292H). In the case of strong bottom stress and mixing opposing the gravity current, the solutions of the gravity current layer thickness at 0·5Hand 0·292Hwill not be valid. Both mixing and vorticity produced by bottom stress erode the halocline, and produce a high velocity core in the mid-depth, which leads to the thickness of a bottom gravity current greater than 0·5H. These internal hydraulic tendency and mixing processes, varying with time-dependent barotropic tidal current forcing, determine the tidal period variation of the gravity current structure.     

Simulation and Analysis of a Gravity Cyh''''nder''''s Stability in Short-Crested Sea

Generally, the reliability of ami -sliding and anti-overturning stability of an isolated gravity cylinder in a certain working period can be evaluated only when the statistical properties of short term stability are given first. The authors used numerical method to simulate the stability state function of a cylinder in short-crested sea, and further to get the probabilitical characteristics of the structure's stability by time domain analysis. The external loads appeared in the state functions include horizontal wave force, lift force and the respective moments, and the loads are correlated by co- spectrum. The numerical method presented in this paper can be used not only to solve short term reliability problem directly, but to calculate and analyse the long term reliability problem as well. For circular cylinders, an example of simulation and analysis is displayed in this paper.     

上混合中剪切湍流和朗缪尔环流动力特征差异

Large eddy simulation(LES) is used to investigate contrasting dynamic characteristics of shear turbulence(ST)and Langmuir circulation(LC) in the surface mixed layer(SML). ST is usually induced by wind forcing in SML. LC can be driven by wave-current interaction that includes the roles of wind, wave and vortex forcing. The LES results show that LC suppresses the horizontal velocity and greatly modifies the downwind velocity profile, but increases the vertical velocity. The strong downwelling jets of LC accelerate and increase the downward transport of energy as compared to ST. The vertical eddy viscosity Km of LC is much larger than that of ST. Strong mixing induced by LC has two locations. They are located in the 2ds–3ds(Stokes depth scale) and the lower layer of the SML,respectively. Its value and position change periodically with time. In contrast, maximum Km induced by ST is located in the middle depth of the SML. The turbulent kinetic energy(TKE) generated by LC is larger than that by ST. The differences in vertical distributions of TKE and Km are evident. Therefore, the parameterization of LC cannot be solely based on TKE. For deep SML, the convection of large-scale eddies in LC plays a main role in downward transport of energy and LC can induce stronger velocity shear(S2) near the SML base. In addition, the large-scale eddies and S2 induced by LC is changing all the time, which needs to be fully considered in the parameterization of LC.     

渤海及黄海北部的风海流数值计算及余流计算

依据黄渤海实测风的资料对渤海及北黄海进行了月平均风海流数值计算。计算表明 ,1月份在西北风的作用下 ,在渤海出现 1个逆时针旋转的环流 ,在辽东湾北部及黄海北部出现 1个顺时针旋转的环流 ,渤海海峡的海流北进南出。 7月份在南风和东南风的作用下 ,风海流的变化形式与 1月份大致相反 ,海峡处呈南进北出的形式。对渤海中部某点 1年的潮流资料通过低通滤波的方法计算逐时的余流值 ,得到该点 1年内表层最大的实测余流为 31.9cm/ s,全年 90 %多的时间内表层余流小于 10 cm/ s。对辽东湾北部某点和渤海湾西南部某点数月潮流资料也进行了低通滤波 ,并将得到的逐时余流与同步风作了比较。依据该 2点风和余流的关系以及黄海北部 6个点风和余流的关系验证了风海流数值计算的结果 ,表明在这些点上实测与计算结果拟合良好     

Numerical Analysis of Gravity Coring Using Coupled Eulerian-Lagrangian Method and a New Corer

The shortcomings of gravity corers in sampling marine sediments have been observed extensively in various field tests. In order to optimize the coring, this article provides an alternative numerical way to model the gravity coring and analyze the sampling effect. Based on this analysis, a new hydraulic hammer corer is devised. A coupled Eulerian-Lagrangian method with capability of simulating the problem involving extreme deformation, penetration is used to simulate the coring process. The results show that the hydrostatic pressure and deviator stress increase and reach their peak when the pile tip is slightly above or at the level of the observation point and then drop rapidly when the pile tip slides below the observation point. In addition, the stress path indicates that the soil element sustains plastic compression before yielding and then expands until recovering to the original state. The obvious “under-sampling” phenomenon is also well-captured by the finite element model.     

Numerical Analysis on the Effects of Submerged Depth of the Grid and Direction of Incident Wave on Gravity Cage

In this paper,the numerical model of the net cage with the grid mooring system in waves is set up by the lumped mass method and rigid kinematics theory,and then the motion equations of floating system,net system,mooring system,and floaters are solved by the Runge-Kutta fifth-order method.For the verification of the numerical model,a series of physical model tests have been carried out.According to the comparisons between the simulated and experimental results,it can be found that the simulated and experimental results agree well in each condition.Then,the effects of submerged depth of grid and direction of incident wave propagation on hydrodynamic behaviors of the net cage are analyzed.According to the simulated results,it can be found that with the increase of submerged depth of grid,the forces acting on mooring lines and bridle lines increase,while the forces on grid lines decrease;the horizontal motion amplitudes of floating collar decrease obviously,while the vertical motion amplitudes of floating collar change little.When the direction of incident wave propagation changes,forces on mooting lines and motion of net cage also change accordingly.When the propagation direction of incident wave changes from 0° to 45°,forces on the main ropes and bridle ropes increase,while the forces on the grid ropes decrease.With the increasing propagation direction of incident wave,the horizontal amplitude of the forces collar decreases,while the vertical amplitude of the floating collar has little variation.     

网箱形状在海流中变化特性的模型试验

据田内相似准则设计了一模型网箱,通过试验测量其不同装配、不同流速、不同配重下,网箱表征容积（网箱横截面的投影面积）的变化。结果表明。流速增加使网箱有效容积受到很大损失;增加配重对于改善网箱的变形是有限度的,在网箱底部加装底框可在一定程度上改善网箱的耐流性能,而作者开发的网箱耐流锚泊技术,可以在较大的流速情况下很好地改善网箱的变形,减少网箱的容积损失,提高网箱抗流性能。     

In-Situ Test and Numerical Analysis of Bore Pressure on Sheet-Pile Groin

1 .Introduction As a conventional hydraulic structure for beach protection and accretion promotion,the groin hasbeen widelyapplied.Thetraditional rock-fill groinis subjectto destructionandthe cost of maintenanceis rather high. As an improvement of traditi…     

Large eddy simulation of turbulence in ocean surface boundary layer at Zhangzi Island offshore

This study uses a large eddy simulation （LES） model to investigate the turbulence processes in the ocean surface boundary layer at Zhangzi Island offshore. Field measurements at Zhangzi Island （39°N, 122°E） during July 2009 are used to drive the LES model. The LES results capture a clear diurnal cycle in the oceanic turbulence boundary layer. The process of the heat penetration and heat distribution characteristics are analyzed through the heat flux results from the LES and their differences between two diurnal cycles are discussed as well. Energy balance and other dynamics are investigated which show that the tide-induced shear production is the main source of the turbulence energy that balanced dissipation. Momentum flux near the surface shows better agreement with atmospheric data computed by the eddy correlation method than those computed by bulk formula.     

Mechanism of local scour around submarine pipelines based on numerical simulation of turbulence model

1 IntroductionSubmarine pipelines are important ocean engi-neeringequipm ents, especiallyfortheoiland gasin-dustries. M anyresearchershavebeendedicatedtotheproblem of local scour around the submarinepipelines, butmostoftheirwork arelaboratory testsand foc…     

湍流数值模拟中封闭模式应用的局限性

通过对琼州海峡的潮流场特征进行数值模拟，指出了选择不同的特征混合长度表达式对数值模拟结果的影响，表明了基于特征混合长度理论的流封闭模式在近海湍流数值模拟中应用的局限性。     

Numerical Simulation of Wave Forces on Seabed Pipelines

A three-step finite element method(FEM)together with Large Eddy Simulation(LES)is ap-plied to incompressible turbulent flow around seabed pipelines at relatively high Reynolds numbers.Bothtwo-dimensional and three-dimensional numerical simulation is carried out to determine thethree-dimensional effect.The results of numerical simulation agree quite well with the wave forces actingon pipeline models measured in physical model test.     

A Study of Eddy Viscosity Coefficient in Numerical Tidal Simulation

Based on the fluid motion equations, the physical meaning of eddy viscosity coefficient and the rationality of the Boussinesq hypothesis are discussed in this paper. The effect of the coefficient on numerical stability is analyzed briefly. A semi-enclosed rectangular sea area, with an orthogonal spur dike, is applied in a 2-D numerical model to study the effect of horizontal eddy viscosity coefficient (AH). The computed result shows that AH has little influence on the tidal level and averaged flow velocity, but has obvious influence on the intensity and the range of return flow around near the spur dike. Correspondingly, a wind-driven current pool and an annular current are applied in a 3-D numerical model respectively to study the effect of vertical eddy viscosity coefficient (Av). The computed result shows that the absolute value of Av is inversely proportional to that of horizontal velocity, and the vertical gradient value of Av determines the vertical distribution of horizontal velocity. The distrib