STUDY ON BOTTOM STRESS UNDER CONDITIONS OF STORM SURGE AND TIDE

Sea bottom stress is conventionally assumed to be directly proportional to the square of the verticallyaveraged velocity,and the drag coefficient to be dependent on the speed and direction of the wind on the sea surface,the depth and dimension of the sea,the period of the tide and so on. In this paper a three-dimensional numerical model is used to discuss the relation the dragcoefficient and the above-mentioned factors.It can be shown from calculation that the relation, is valid,that the drag coefficient is a constant in a major part of a sea as thought conventionally,andthat there is a small area near the coast where the drag coefficient is far greater.We call it singular area. A number of conclusions on the relation between the drag coefficient and the speed and direction ofthe wind,the sea depth and so on,were obtained.

EFFECTS OF WAVE-CURRENT INTERACTIONS ON BOTTOM STRESS AND CURRENTS

This paper presents a high-resolution (2′X2′) numerical model of coastalcoupled wave-current interaction with explicit consideration of the effects of wave-currentinteraction on bottom stress. For two selected storms with measured data in the Yellow River coastalarea of the Bohai Sea, it is shown that the bottom stress calculated by using a coupledwave-current model is increased, as one would expect, compared with the bottom stress computed withan uncoupled current model. Moreover, the current velocity field is also changed, but thecorresponding current directions show less influence in the two simulations. The extents of changesin bottom stress and current velocity vary with storm intensities. The results further imply thatthe coupled wave-current model should be used as the basis for simulating the current velocity andsea level in the near shore region.

Wave-current bottom shear stresses and sediment re-suspension in the mouth bar of the Modaomen Estuary during the dry season

On the basis of the measurement data pertaining to waves, current, and sediment in February 2012 in the mouth bar of the Modaomen Estuary, the Soulsby formulae with an iterative method are applied to calculating bottom shear stresses （BSS） and their effect on a sediment resuspension. Swell induced BSS have been found to be the most important part of the BSS. In this study, the correlation coefficient between a wavecurrent shear stress and SSC is 0.86, and that between current shear stresses and SSC is only 0.40. The peaks of the SSC are consistent with the height and the BSS of the swell. The swell is the main mechanism for the sediment re-suspension, and the tidal current effect on sediment re-suspension is small. The peaks of the SSC are centered on the high tidal level, and the flood tide enhances the wave shear stresses and the SSC near the bottom. The critical shear stress for sediment re-suspension at the observation station is between 0.20 and 0.30 N/m2. Tidal currents are too weak to stir up the bottom sediment into the flow, but a WCI （wave-current interaction） is strong enough to re-suspend the coarse sediment.

Effects of bottom shear stresses on the wave-induced dynamic response in a porous seabed:PORO-WSSI (shear) model

When ocean waves propagate over the sea floor,dynamic wave pressures and bottom shear stresses exert on the surface of seabed.The bottom shear stresses provide a horizontal loading in the wave-seabed interaction system,while dynamic wave pressures provide a vertical loading in the system.However,the bottom shear stresses have been ignored in most previous studies in the past.In this study,the effects of the bottom shear stresses on the dynamic response in a seabed of finite thickness under wave loading will be examined,based on Biot＇s dynamic poro-elastic theory.In the model,an ＂u-p＂ approximation will be adopted instead of quasi-static model that have been used in most previous studies.Numerical results indicate that the bottom shear stresses has certain influences on the wave-induced seabed dynamic response.Furthermore,wave and soil characteristics have considerable influences on the relative difference of seabed response between the previous model（without shear stresses） and the present model（with shear stresses）.As shown in the parametric study,the relative differences between two models could up to 10% of p0,depending on the amplitude of bottom shear stresses.

Simulation of Random Waves and Associated Laminar Bottom Shear Stresses

This work presents a new approach for simulating the random waves in viscous fluids and the associated bottom shear stresses. By generating the incident random waves in a numerical wave flume and solving the unsteady two-dimensional Navier-Stokes equations and the fully nonlinear free surface boundaiy conditions for the fluid flows in the flume, the viscous flows and laminar bottom shear stresses induced by random waves axe determined. The deterministic spectral amplitude method implemented by use of the fast Fourier transform algorithm was adopted to generate the incident random waves. The accuracy of the numerical scheme is confirmed by comparing the predicted wave spectrum with the target spectrum and by comparing the nanlerical transfer function between the shear stress and the surface elevation with the theoretical transfer function. The maximum bottom shear stress caused by random waves, computed by this wave model, is compared with that obtained by Myrhaug＇ s model （1995）. The transfer function method is also employed to determine the maximum shear stress, and is proved accurate.

高应力大巷底鼓机理及其稳定性控制方法

针对余吾煤业1号回风大巷在高应力扰动下出现的巷道底鼓严重问题,综合运用理论分析、数值计算和现场试验相结合的方法,系统研究了高应力大巷底鼓机理及其稳定性控制方法。理论计算结果表明,高应力巷道底板破坏深度达1.35 m,底板滑移面受到的滑动力达3.11 MPa,超过了底板砂质泥岩极限抗拉强度,导致巷道底板发生底鼓。从高应力巷道底鼓控制角度出发,提出了高应力巷道顶底板加强锚索与巷道两帮锚杆联合支护的巷道底鼓控制技术,数值模拟结果表明巷道底鼓量可降低67%,有效控制底鼓的同时,也可为巷道整体稳定提供支撑。通过现场试验,研究提出的巷道底鼓控制方法可使回风大巷顶板下沉值降低37%,底鼓值降低72%,两帮变形量降低53%,有效保证了回风大巷的稳定。研究结果对于指导深部矿井高应力环境下巷道底鼓控制具有重要实践意义。

Estimation of Bed Shear Stresses in the Pearl River Estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler （PC-ADP） and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary. The bed shear stresses were estimated by four different methods： log profile （LP）, eddy correlation （EC）, turbulent kinetic energy （TKE）, and inertial dissipation （ID）. The results show that （a） all four methods for estimating bed stresses have advantages and disadvantages, and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors; （b） the LP method was found to be the most suitable to estimate the bed stresses in non-stratified, quasi-steady, and homogeneous flows; and （c） in the estuary where the semi-diurnal tidal current is dominant, bed shear stresses exhibit a strong quarter-diurnal variation.

Investigation on structural component behaviours of double bottom arrangement under grounding accidents

Marine accidents have caused immense casualties on various parties in shipping and shipbuilding industries, including financial and structural losses. This situation makes ship accident becomes a critical subject in naval architecture and marine structures, as it needs continuous assessment and investigation to broaden insight and data of collision and grounding phenomena. The paper aims to investigate structural conditions of a ship arranged by double hull system under accidental scenario, namely ship grounding. Fundamental concept of structure-rock interaction in poweredhard grounding is adopted to design impact configuration for calculation using finite element(FE)simulation. Involved entities are defined as the structure represented by tanker vessel, and oceanic rock is deployed as the indenter in analysis. Calculation results indicate that the crashworthiness capability of structural part strengthened by longitudinal girder is higher than other selected locations on the structures against rock penetration. Localized flooding of storage oil may occur during raking damage is formed on structural part between two girders.

基于光纤感测的急倾斜煤层采动围岩变形特征研究

将分布式光纤传感技术应用于急倾斜煤层采场围岩变形及应力分布特征物理相似模型试验,通过在模型中沿煤层倾向布置传感光纤与沿煤层底板铺设压力传感器,实时监测模型内部变形场与应力场,并结合数值模拟分析了急倾斜煤层顶板应变分布及演化特征。结果表明:工作面上部区域应力值小于下部和中部区域,中部区域应力值最大;随工作面推进,采空区底板应力逐渐释放,应力值不断降低,采空区两侧边界煤柱下方煤层底板应力逐渐升高,且中、下部采空区底板应力恢复程度显著高于上部底板;沿煤层倾向布置的光纤应变曲线呈三段分布,具有前后2处应变波峰区及采空区对应的中部波谷区,前波峰远高于后波峰;工作面中上部顶板变形程度大于下部顶板;数值模拟与光纤检测结果一致,均表现出中上部顶板变形显著大于下部。研究弥补了物理相似模型试验监测手段短板。

钢轨轨底裂纹扩展影响因素分析

【目的】为分析钢轨材质、残余应力、温度应力在钢轨使用中的内外因对轨底裂纹扩展的影响开展研究。【方法】基于断裂力学理论结合轨底伤损实测,建立钢轨轨底裂纹扩展模型,对轨底伤损扩展失效时的临界裂纹尺寸、裂纹平均扩展速率、剩余寿命等指标进行量化分析。【结果】结果表明:钢轨材料断裂韧性与轨底初始伤损扩展的剩余寿命存在正相关关系,U95Cr H材质钢轨的轨底裂纹扩展剩余寿命小于U78CrV H材质小于U75V H材质;以某线路实际使用的钢轨材质U78CrV H为例,轨底残余应力由0增加至300 MPa,轨底裂纹扩展的临界尺寸增长约2.5%,剩余寿命下降约6%;残余应力为200 MPa时,温度应力由0增加至200 MPa,轨底裂纹扩展的临界尺寸变化不大,在1%内。【结论】研究结果可为轨底裂纹扩展研究和养护维修提供参考。

认知评估视角下领导底线心智对员工创新行为的双刃剑效应研究

通过来自创业和非创业团队的685个样本,从压力认知评估视角分析领导底线心智如何以及何时会对员工创新行为产生影响。研究结果表明:领导底线心智首先会让员工感知到基于底线目标的绩效压力;领导底线心智在一般情况下会对员工创新行为产生“双刃剑”影响效应,即领导底线心智在一般情况下不仅会通过员工对绩效压力的挑战性评估促进员工创新行为,同时也会通过员工对绩效压力的阻碍性评估抑制员工创新行为;团队类型在影响机制中起到了显著的调节作用,即创业团队中的领导底线心智主要通过绩效压力的挑战性评估对员工创新行为产生积极影响,而非创业团队中的领导底线心智主要通过压力的阻碍性评估对员工创新行为产生消极影响。

厚松散层薄基岩综放面切顶留巷底鼓变形特征及控制对策

基于厚松散层、薄基岩、综放开采条件下沿空留巷底板变形严重的难题,以赵固一矿为背景,理论研究深部切顶留巷底板载荷演化规律,提出卸压、支护的综合防控技术,并进行现场试验。研究表明:留巷底鼓的主因是留巷期间实煤帮与挡矸系统处产生垂直应力集中且底板岩层出现三角形塑性滑移区;提出增加切顶高度、提升顶帮支护结构的稳定性、强化采空区帮柔性挡矸的综合措施;留巷治理前后底板变形分别为724、299 mm,底鼓总量降低58.7%,现场实施效果明显。

煤层群采动下围岩应力演化规律及协同控制技术研究

针对煤层群开采过程中巷道支护困难问题,以贵州土城矿212回风石门为工程背景。综合采用现场调研、数值模拟、相似模拟及现场试验等手段,揭示了212回风石门应力演化规律,并提出了“卸−转−固”协同控制技术。研究结果表明:212回风石门遭受破坏的主要原因是煤层群采动过程中存在的地质力学问题导致了围岩失稳。巷道底板及两帮在采动过程中产生不同程度的应力集中。当遭受垂直应力挤压时,巷道底部承受的挤压力较大,而顶部围岩承受的拉伸力较大,由于力学不平衡导致围岩的破坏。基于此提出了“卸−转−固”协同控制技术。通过爆破卸压的方式,利用爆破产生的冲击波引起围岩的震动和应力波动,使表层围岩中原本集中的应力分散到更深的围岩区域,降低表层围岩的应力集中程度。同时,利用爆轰和封孔工艺进一步加固卸压孔周围的围岩,形成两个承载结构。即由巷道支护体形成的内承载体和由深部围岩形成的外承载体。两者相互作用有效承受巷道浅部及深部围岩的应力,并转移到支护结构,起到保护和稳定围岩的作用。利用该技术在212回风石门现场试验,结果显示:使用该技术区域应力长期趋于稳定甚至缓慢降低,巷道顶底板及两帮移近速率分别降低了74.49%及47.67%,底鼓量降低了77.2%。而未使用该技术区域应力出现不同程度的上升,表面位移收敛严重。由此可得,围岩控制效果显著。该技术已成功推广到贵州其他不同地质环境的煤矿,均取得了显著效果。

交错起爆下爆炸应力波的碰撞机制与破岩效果

对双孔交错起爆方式下孔间爆炸应力波的碰撞机制和破岩效果开展了研究,基于应力波正、斜碰撞理论研究了孔间爆炸应力波的相互作用机制,证明了双孔交错起爆方式下孔间应力波碰撞引起的应力增强效应;借助ANSYS/LS-DYNA有限元程序中岩石的RHT模型和炸药的JWL状态方程,模拟了交错、孔底和孔口起爆方式下孔间应力波的大小和破岩效果;最后,结合现场试验对比分析了不同起爆方式下爆炸应力波的相互作用及含砾石岩体的破碎块度分布特征。结果表明:双孔交错起爆下两应力波首先在孔间正碰撞,碰撞后的压力与应力波稳定传播时的压力比为2.4;当入射角在0°~44°区间时,应力波斜碰撞,压力比由4.1降至2.3;当入射角处于44°~90°区间时,应力波发生马赫反射,压力比由3.5降至1.0。交错、孔底起爆方式下,爆破块度尺寸小于250 mm的比例分别为25.5%和20.9%,爆破块度尺寸大于750 mm的比例分别为9.2%和17.5%。双孔交错起爆引起的应力波碰撞增强效应可有效改善含砾石岩体的钻孔爆破破碎效果。

沥青混凝土面板与土工膜防渗连接方式研究——以江苏句容抽水蓄能电站为例

江苏句容抽水蓄能电站上水库采用库周沥青混凝土+库底土工膜相结合的防渗措施,这两种防渗措施在抽蓄电站中尚属首次结合运用,其中土工膜和沥青混凝土之间连接接头的设计及施工难度较高,尚无成果经验借鉴。为此,采用三维有限元计算和模型试验相结合的方法,研究了适应高坝以及回填库盆大变形条件的库周沥青混凝土面板+库底土工膜的3种接头连接型式。研究结果表明:在沥青混凝土面板底部设置与土工膜的混凝土连接板的方案较优;同时,针对土工膜与沥青混凝土面板直接锚固的方案,若能验证锚固用环境砂浆与沥青混凝土的长期运行可靠性,仍不失为一种创新性方案。

焊接位置对激光-电弧复合焊接底部驼峰的影响

目的通过改变焊接位置来抑制底部驼峰的形成进而降低生产成本。方法使用高速摄像机观察平向焊接和横向焊接2种不同焊接位置下熔池的匙孔和底部驼峰的形成过程,并对底部凸起受到的金属蒸气反冲压力、熔融金属重力以及表面张力进行受力分析。结果在S355J2W耐候钢的激光-电弧复合焊接平焊过程中,受金属蒸气反冲作用力的影响,熔融金属迅速从匙孔底部排出,流入匙孔底部凸起区域,底部凸起的增长速度极快,仅需7.5 ms即可形成大尺寸的驼峰。而在横向焊接中,匙孔前壁凸起数量减少,导致金属蒸气反冲压力减小,匙孔排出的熔融金属量减少,底部凸起的增速减缓,在13.6 ms时才达到最大高度,使驼峰在底部熔池凝固之前难以形成。结论采用横向焊接的方式可以有效避免驼峰的产生,降低生产成本。同时,该方式在焊缝背部成形效果方面也表现良好,为优化焊接工艺提供了重要参考。

基于有限元的复合水泥混凝土路面面层层底应力分析与结构设计

为满足重交通下复合水泥混凝土路面的使用要求,以贫混凝土作为基层材料,以沥青砂浆作为功能层设计水泥混凝土路面。采用软件ANSYS对不同路面结构进行力学响应分析。研究结果表明:基层材料由水泥稳定碎石变为贫混凝土时,水泥混凝土面层层底的荷载应力减小14.87%,温度应力减小2.70%,温度-荷载耦合应力减小10.02%;贫混凝土弹性模量由12 GPa增至16 GPa时,荷载应力、温度应力、温度-荷载耦合应力分别减小3.60%、1.09%和4.14%,应力下降幅度减小;荷载应力、温度应力的数值分析结果与理论计算结果的相对差小于10%,温度-荷载耦合应力的相对差较大。针对重交通、中等交通量等级设计复合水泥混凝土路面结构,应力校核结果表明,2种设计路面均满足使用要求。

基于深海矿车-稀软土相互作用的矿车触底过程分析

多金属结核赋存在深海5.0 km左右的平原区域,该区域的稀软沉积物含水率高、承载力低。深海采矿车在触底过程中,矿车下方稀软沉积物受到冲击后易发生扰动破坏,从而极易引发矿车沉陷,这将严重影响矿车的正常作业,甚至会导致停机。矿车是否沉陷与其触底后的应力分布形式密切相关,但尚无理论对其进行分析。通过开展采矿模型车不同触底速度下的触底试验,揭示了矿车触底后下方土体竖向应力随时间的变化规律及分布范围。在传统理论基础上,推导并验证了一种考虑履齿影响的竖向应力值的理论公式。研究发现,触底后下方土体的应力值会快速增长并趋稳定,且达到稳定应力值所需时间与触底速度成反比。随着触底速度增大,矿车下方应力泡形状会从平滑单峰向单峰两侧有平台的形状变化,车体正下方的应力传播得最远。相对于平板,履齿板应力影响深度显著增加。经误差分析,改进的理论公式比传统理论与试验结果吻合效果更好,提升了深海矿车触底后应力计算精度。文中研究围绕矿车触底后的海床应力分布进行了一系列讨论,可为深海矿车低扰动、防沉陷设计提供理论参考。

单箱多室宽幅高架桥梁端底板应力研究

结合某市政高架桥,对单箱多室宽幅高架桥梁端底板应力进行研究。采用桥梁博士和ANSYS有限元软件建立有限元模型,进行传统单梁模型计算分析,并针对实际工程模型在4种工况下的应力进行详细分析,根据分析结果提出相应的改进措施,如调整横梁宽度、改变腹板钢束弯起角度及增减底板钢束。

空心墩转体桥墩底应力分析

以某2×85 m预应力混凝土T构桥为例,建立了该桥的空间有限元模型,计算分析了桥梁转体阶段墩底的应力分布。分析表明:由于转体桥下部结构的构造特点使得空心墩墩底实心段顶部会产生较大的拉应力,设计中应采取有效的措施减小其产生。与球铰相对应的空心墩内宜设置中间隔板;在桥墩墩底实心段或在上承台设置预应力。结合工程实践,推荐在上承台设置预应力的方式,且应侧重横桥向预应力筋的配置。