Large-scale laboratory experiment on erosion of sand beds by moving circular vertical jets

This study investigates the topographic deformation due to the erosion of a sand bed impinged by a moving submerged turbulent round jet in a large-scale laboratory. The test conditions represent the case of discharges beneath a vessel while operating in water with a limited clearance such as a shallow navigation channel. The jet moves horizontally and discharges water vertically downward towards the bed. The distance between the jet nozzle and the bed equals six times the jet diameter so the jet flow is in the potential core region. The speed of the jet horizontal motion was varied to examine its effect on the scour profile. The characteristic lengths of the scour profile in the asymptotic state were determined by modifying the empirical formulas in Aderibigbe and Rajaratnam [1996. Erosion of loose beds by submerged circular impinging vertical turbulent jets. Journal of Hydraulic Research 34(1), 19–33]. The maximum scour depth, the scour hole radius, and the ridge height were found to be a function of the ratio of the jet exit to jet translation velocities and were modeled using a hyperbolic function. Empirical equations describing the scour profile were developed and the scour profile was found to be self-similar when normalized by appropriate length scales.     

Numerical modeling of flow and scour below a pipeline in currents: Part II. Scour simulation

A vertical two-dimensional (2D) numerical model for time dependent local scour below offshore pipelines subject to unidirectional steady flow is developed. The governing equations for the flow and sediment transport are solved by using finite difference method in a general curvilinear coordinate system. The performance of two turbulence models, the standard k–ɛ model and Smagorinsky subgrid scale (SGS) model, on modeling time dependent scour processes is examined. Both suspended load and bed load are considered in the scour model. The suspended-load model is verified against two channel sediment transport cases. The change of bed level is calculated from the continuity equation of total sediment transport. A new time marching scheme and a sand slide scheme are proposed for the scour calculation. It is found that the proposed time marching scheme and sand slide model work well for both clear-water and live-bed scour situations and the standard k–ɛ turbulence closure is more preferable than the SGS model in the 2D scour model developed in this study.     

黏性土淹没射流破土实验研究

海底水力喷射开沟技术在海底管道、光缆埋设等工程中得到广泛应用,然而黏土尤其是硬质黏性土中的水力开沟施工较为困难,为阐明其中的破土机制,开展了二维垂向淹没射流开沟物理模拟实验。通过观察射流破土的过程,发现:软黏土与硬质黏性土中冲坑发展的特点有所不同,破土机制也存在差异。射流对硬质黏性土的破坏中,压力导致的剪切破坏起到很大作用,在土力学原理上近似于地基土整体剪切破坏,据此建立了硬质黏性土射流破土模型,同时提出冲坑颈口直径在硬质黏性土冲坑形态中的重要性,并以此取代原有的冲坑直径作为一个预测指标。     

Numerical study of the Reynolds-number dependence of two-dimensional scour beneath offshore pipelines in steady currents

A numerical model for local scour below pipelines is employed to investigate the Reynolds-number dependence of the two-dimensional scour beneath offshore pipelines in steady currents. A novel wall function is proposed in calculating the suspended sediment transport rate in the model. Scour developments beneath a model pipeline and the corresponding prototype pipeline are simulated under the same undisturbed Shields parameter but different values of Reynolds number. The effects of the Reynolds number difference on the scour profile development are investigated. It is found that scour depths for prototype pipelines are about 10–15% smaller than those for model pipelines. The flow phenomena that causes this difference are discussed by means of flow visualization. The normalized time scales are found to be approximately the same. The simulated scour profiles for the model pipelines agree well with the experimental results from an independent study.     

低空急流对0212号台风"北冕"后部暴雨影响的分析和数值试验

本文首先用实测资料分析了0212号台风"北冕"造成福建大暴雨过程的环流形势和物理量场特征,指出低空偏南风急流的北抬和加强为远离风暴中心的福建东南沿海大暴雨提供了有利的水汽和动力条件.然后用中尺度模式(MM5)模拟分析了低空偏南风急流强度变化对本次暴雨过程的影响,结果表明低空急流强度变化对福建东南部的暴雨落区及强度有很大影响.     

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…     

波流耦合下桩周珊瑚砂冲刷机理研究

针对我国南海某岛礁珊瑚砂地基上的圆形桩基础,采用N-S方程k-ε模型、双向耦合方式跟踪流场中颗粒运动轨迹的方法,对桩周珊瑚砂的冲刷规律进行了求解,分析了桩体周围流体的速度场以及桩体表面剪应力场的分布规律,同时对桩周珊瑚砂冲刷坑的形成过程进行了模拟。计算结果表明,在桩体周围形成的马蹄形漩涡和桩柱后方的尾涡作用下,桩周土体出现了较为明显的冲刷现象,涡旋的释放显著地影响着珊瑚砂地基上桩基的冲刷坑形状;而且,由于珊瑚砂颗粒密度较石英砂小,水动力作用下桩周冲刷坑更容易形成,所以实际工程中需要考虑有效的防护措施。     

轴线倾斜海底管道冲刷速率试验研究

通过物理模型试验研究水流作用下轴线倾斜海底管道的三维局部冲刷问题。利用超声波探头监测管道下部冲刷沿管轴线方向的扩展过程,分析海底管道三维局部冲刷的动态发展机理。由模型沙的冲蚀试验,建立沙床面剪切应力与泥沙表观侵蚀速率之间的关系式,并引入经验公式对沙床面剪切应力放大系数、泥沙表观侵蚀速率以及远场床面剪切应力之间的关系进行表达。由倾斜管道模型试验,在分析冲刷扩展位置随时间变化数据的基础上,结合上述经验公式以及沙床面剪切应力放大系数与管道埋深的关系,建立轴线倾斜海底管道冲刷扩展速率的预测公式。     

Lateral loading of a pile using strain wedge model and its application under scouring

The scour hole around a pile will reduce the capacity of a laterally loaded pile. The strain wedge model is capable to derive a p–y curve for the analysis of a lateral loaded pile on a nonlinear Winkler foundation. To improve and extend the ability of the strain wedge method, a modified strain wedge (MSW) method is developed, in which a nonlinear lateral deflection of the pile is assumed to describe the varied soil strain distribution in the passive wedge. And then by treating the soil weight involved in the strain wedge as a vertical load at the bottom of the scour hole, an equivalent wedge depth is obtained to consider the effect of scour hole dimensions on the response of laterally loaded piles in sand. The validity of the MSW model is proved by comparisons with a centrifuge test without scour. And its applicability in the problem of a pile with scour is performed by a comparison with a model test and a FE analysis. The analysis shows the pile displacement at the pile head with scour can be obtained by multiplying the corresponding deflection without scour with an amplification factor related to scour depth at large load level.     

南黄海辐射沙脊群潮汐水道的悬沙输运特征

根据南黄海辐射沙脊群定点站位的流速和浊度数据,利用通量分解方法,分析了潮汐水道的悬沙输运特征和输运机制。结果表明,辐射沙脊群海域潮汐水道中潮流为往复流,悬沙浓度较高,属于强潮流控制的悬沙浓度相对较高的陆架浅海环境;悬沙输运主要受欧拉余流和潮汐捕捉效应控制,再悬浮的沉积物通过平流作用进行输运。其中,陈家坞槽和西洋水道的悬沙输运以潮汐捕捉效应占优,沉积物向潮汐水道外输运,处于冲刷状态;苦水洋水道以欧拉余流输运为主,水道内的再悬浮的沉积物在强潮流作用下向陆输运,主要堆积在蒋家沙和西洋西侧岸滩等浅滩和潮间带上。由此可见,在陆源物质供应减少背景下,南黄海辐射沙脊群内部的物质充当了新的物源,短期内能够维持近岸潮滩和沙脊的持续增长。     

黄河水下三角洲沉积物输运及海底冲淤研究

根据黄河水下三角洲的特点,建立了一个垂直平均二维沉积物输运数学模型,结合潮流和沉积物资料,模拟研究了沉积物输运机制和海底冲淤演变过程。模拟结果表明,垂直河口射流的潮流决定了本区沉积物净输运的总体格局,风应力对悬浮泥沙和推移质泥沙运动也起到重要作用。潮流底应力和活动层厚度分布表明,黄河口门和埕岛油田附近海域是潮流底应力和活动层厚度高值区,为沉积物活跃区,海底稳定性弱,易于侵蚀再搬运。冲淤计算表明,埕岛油田附近海域为冲刷中心区;现在河口水下三角洲在断流时为冲刷区,正常行河时则转为淤积区。     

基于Flow-3D的抗拖网海床基冲刷数值模拟

基于Flow-3D软件,建立抗拖网海床基沙土基床冲刷的三维数学模型,控制水流流速和泥沙粒径,对不同条件下抗拖网海床基周围沙土基床的冲刷进行数值模拟,并分析了抗拖网海床基最大冲刷深度随这两种因素改变的趋势。结果表明,抗拖网海床基沙土基床冲刷坑深度随海流流速增大而增加,在设定其他条件不改变的情况下,冲刷坑的加深速度随着流速增大会逐渐减缓,最终达到冲淤平衡。同时,由于海底环境复杂多变,导致模型与实际情况有一定的差异,还需在后续的研究中进一步进行优化。     

用VOF方法模拟横流下窄缝紊动射流

采用标准双方程k-ε紊流模型和VOF方法建立二维数值模型,模拟有横流情况下底部排污问题.将回流区的分析结果与试验进行了对比,吻合较好.分别将速度和浓度轨迹线拟合成简单的表达式并与试验拟合作了比较.通过对速度和浓度场的分析得到一些有益的结论.     

黄河口二维潮波泥沙有限元数学模型及应用(Ⅰ)——模型及其验证

分析了建立黄河河口泥沙有限元模型的必要性,并结合黄河河口实际情况,建立了黄河河口二维潮流和泥沙数学模型。从数学模型模拟计算得到的潮位、流速和冲淤验证结果可以看出,由模型模拟的潮汐和潮流的等海洋动力特性以及泥沙冲淤和海底变形都与实测资料一致,表明建立的河口泥沙二维有限元数学模型可进行黄河河口的潮汐、潮流和泥沙运动及海底冲淤演变的模拟与预测。     

Numerical study on three-dimensional waves produced by a bottom jet

The eruption of an underwater volcano can initiate the disturbances of the sea surface and subsequently generate a group of outward-propagating tsunamis. The theme of this study is to introduce a three-dimensional (3D) fully nonlinear wave model for the simulation of wave motions induced by a bottom jet. A boundary-fitted coordinate system is utilized to conveniently adjust grids according to the transient moving free surface. The governing Laplace equation of the velocity potential is solved by an implicit finite-difference scheme while a mixed explicit/implicit iteration procedure is applied to solve the free-surface boundary conditions. In addition, a set of generalized Boussinesq equations are solved for comparison with the fully nonlinear model. Good agreements in comparisons with the existing numerical and analytical solutions are achieved for cases investigated. Waves induced by three types of bottom jets: namely (1) sudden eruption, (2) initial transient, and (3) periodic transient are discussed in this paper. For the case of sudden erupted jet, a system of 3D outgoing waves as the cylindrical wave pattern are presented and discussed. For the initial transient types, it shows the transition in the incipient stage has a great influence on the initial rising of the water surface and the induced leading waves. Furthermore, an interesting up-down phenomenon in the center of disturbed free surface due to the type of periodic jet is revealed.     

Measured and Predicted Burial of Cylinders During the Indian Rocks Beach Experiment

Burial of instrumented mine-like cylinders as a result of wave-induced scour was measured during experiments conducted in shallow water (15-16 m) with fine-sand (133-mum) and coarse-sand (566-mum) sediments off Indian rocks beach (IRB), FL. scour pits developed around the instrumented cylinders in the fine-sand site when significant waveheights exceeded 2 m, causing the cylinders to pitch, then roll into the developing scour pits, often changing heading to align parallel with the wave crest. Final cylinder burial was nearly 40 cm (about 70%-80% mine diameter) relative to the sediment-water interface, but only 20%-50% relative to surface area covered. The difference was caused by the lack of complete infilling of scour pits. Little development of scour pits and burial was noted on the coarse-sand site and the cylinders buried to only 20%-40% of the cylinder diameter below the sediment surface. Burial results, although variable, are in general agreement with the wave-induced scour model developed by Trembanis et al. (2007) for the fine sand, but not for the coarse sand where measured burial was much less than predicted.     

海底管道周围局部冲刷数值模拟分析

通过建立一个垂直二维紊流模型来模拟管道周围流场,建立泥沙冲刷模型来模拟海床底面变化。其中,冲刷模型同时包括悬移质输沙和推移质输沙。海床底面变化通过计算底面泥沙总输运平衡关系得到。并进行多组冲刷数值实验,分析泥沙冲刷过程,总结出一套冲刷平衡深度经验公式。     

应用切割单元法对海底管道局部冲刷数值模拟

采用SIMPLEC算法的有限体积法,求解非定常流动的N-S方程,采用k-ω紊流模型,通过模拟均匀无粘性推移质的冲刷和淤积,得到海底管道由搁置在海床上的状态变为悬跨状态这一过程的管道周围局部海床冲刷情况,建立了海底管道局部冲刷的二维数值模型。海底管道从搁置在底床上到冲刷悬空的过程中,管道周围的空间产生了拓扑变化,这给采用贴体网格并在计算过程中进行网格重构的传统方法带来了很大困难,而采用切割单元法,把物体轮廓从静止的背景直角坐标结构化网格中切割出去,计算过程中不需要传统意义的网格重构过程。数值模型预测的海底管道周围局部冲刷结果与Mao的物理模型实验实测结果及Liang和Cheng等的数值模拟结果符合较好,验证了模型的准确性。     

Longshore sediment transport on non-planar beaches

For a concave-up $\text{2}\text{3}$ power Bruun beach profile, the following two energetics-based sediment transport models are developed: (1) a Bagnold-type model and (2) a combined wave-current stress model. The stress model is calibrated with the Bagnold model using observed transport rates on planar beaches. The sediment transport profiles for the two models are in agreement within the surf zone for the planar beach case; but the stress model is also applied seaward of the breaker line where the Bagnold model is not. A mean swash transport of sand is predicted by the Bagnold model for a $\text{1}\text{2}$ power least-squares approximation to total depth including setup/setdown on a Bruun beach profile. The total longshore transport of sand is determined for each transport model as a function of the turbulent lateral mixing strength. The total sand transport is found to be less on a concave-up beach profile than for the corresponding planar beach case.