气垫平台破冰阻力的模型试验研究

通过开展低温拖曳冰水池物理模型试验,测试气垫平台在遭遇平整冰时的破冰过程和破冰阻力.在模型试验中,以一座现役破冰气垫平台为原型,建立了合理的模型试验相似律.依据相似律分别对原型平台的结构框架、气道结构、围裙结构和垫升系统等部分进行了模拟,从而得到一套与原型平台结构型式和垫升机制相似的模型平台.模型平台在试验拖车的拖曳下通过低温冰水池中的模型冰排,分别以垫升高度和航行速度为试验参数,对不同试验工况下气垫平台的破冰过程进行测试.通过对模型试验现象和结果的分析,深入解析了气垫平台的破冰过程,揭示了气垫平台的破冰机理.通过试验发现,非全垫升状态更有利于模型平台的破冰作业.气垫平台破冰的关键机理是在冰排底部形成稳定的气腔,从而促使冰排在结构的下压作用和气腔的上顶压力下发生弯曲破坏.在试验中测试了气垫平台破冰风压随结构姿态的变化,在时频域内对风压的变化情况进行了分析,并讨论了风压随航行速度的变化规律.以此为基础,对气垫平台破冰阻力随垫升高度和航行速度的变化规律进行分析,从而为该类气垫平台的结构设计和操船方法提供必要的基础性数据和参考依据.      

浅层水中沉底的两个装药爆炸的数值模拟研究

根据试验模型和试验结果,进行了浅层水中沉底的两个装药爆炸的数值模拟;通过与水中单个装药爆炸,以及无限水中两个装药同时爆炸的数值模拟结果的对比分析,研究了水底水面对沉底的两个装药同时爆炸产生的冲击波传播与相互作用的影响。结果表明:水底对冲击波压力峰值有削弱作用,水面使冲击作用冲量明显减小,冲击波相互作用压力叠加或多次冲击作用可提高爆炸威力。     

旋涡与水面相互作用研究

采用Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程的有限差分数值解来研究水下生成的旋涡在浮升过程中与水面的相互作用，旋涡的初始模型为Ｏｓｅｅｎ涡。数值模拟给出了在旋涡与水面相互作用过程中，水面形状和涡量场的演化，还讨论了Ｆｒｏｕｄｅ数、Ｒｅｙｎｏｌｄｓ数和Ｗｅｂｅｒ数对水面变形的影响。     

纳米碳管与刚性壁的正碰撞

用分子动力学方法模拟纳米碳管与刚性壁的正碰撞过程,并与弹性动力学方法的结果进行对比．在分子动力学模拟中,采用Tersoff-Brenner势描述碳管的原子间相互作用,用6～12形式的Lennard-Jones,势描述碳管与刚性壁间相互作用．结果表明,两种方法所得到的应力波传播速度吻合较好．与弹性动力学分析结果不同的是,在发生屈曲以前,纳米碳管与刚性壁的接触时间不仅与纳米碳管的长度近似成线性关系,还与管径及碰撞初速度有关．碰撞过程中,纳米碳管端部应力并非定值,但其平均值与弹性动力学计算结果相差不大．     

波流与结构物相互作用的数值模拟

采用高阶边界元法建立波流与任意形状结构物相互作用的时域数学模型。在小流速假设下,将速度势进行摄动展开,边界条件分解为双物体假设下的零阶波陡稳定问题和一阶波陡下的不稳定波浪问题。物面速度势及自由水面速度势的法向导数等未知量通过求解高阶边界元积分方程得到,而积分方程的求解则通过一个数值程序来实现。在每个时间步上采用四阶Runge-Kutta法更新下一时刻自由水面波面和速度势,自由水面上采用人工阻尼层消除散射波。通过波流与直立圆柱相互作用的数值计算研究了一阶激振力和二阶慢漂力随波数的变化关系以及圆柱周围波幅的分布曲线,并与已有频域结果和时域结果对比验证了所建模型的准确性,进而应用本文模型研究了波流与实际工程结构物相互作用的问题。     

低马赫数下斜爆轰波的结构

利用Euler方程和两步化学反应模型,对低马赫数入流时的驻定斜爆轰波进行了数值模拟,并重点研究了斜爆轰波的驻定过程和结构。数值结果显示,当入流马赫数较低时,即使其本身是附体的,在诱导区后侧的高压区的作用下,斜爆轰波也会从其起始位置向来流方向运动。在这种情况下,斜爆轰波会驻定在靠近斜面前缘的位置,诱导区的长度仅有1 mm左右。通过设置初始条件,让斜爆轰波在斜面前缘附近被触发,则其将一直维持在靠近斜面前缘的位置。     

船-冰作用中海冰典型破坏模式的离散元数值模拟

在破冰船破冰过程中,冰排主要表现为挤压与弯曲两种破坏模式.本文基于黏结离散单元法对船-冰作用中的这两种典型破坏模式进行数值模拟.海冰离散元数值试样采用随机排布方式生成,采用单轴压缩试验与三点弯曲试验相结合的方式标定模型中的细观参数.将船-冰碰撞中的挤压和弯曲作用方式简化为直立或倾斜 平板与海冰的作用模式,构建挤压与弯曲破坏的海冰离散元数值试样,分析了挤压破坏模式中不同加载方位以及冰厚和加载速率对破坏模式的影响,以及弯曲破坏模式中不同冰排夹角以及冰厚和加载速率对破坏模式的影响.计算结果表明,离散元海冰数值模型可以很好地对冰排挤压与弯曲破坏现象进行模拟,可揭示冰排在不同破坏模式下的破坏机理.     

激波诱导环形SF6气柱演化的机理

基于可压缩多组分Navier-Stokes控制方程,结合5阶加权本质无振荡格式以及网格自适应加密技术和level-set方法,数值模拟了平面激波(Ma=1.23)与环形SF₆气柱(内外半径分别为8和17.5 mm)界面的相互作用过程。相比于之前的实验结果,数值模拟结果揭示了入射激波在界面内4次透射过程中的复杂波系结构,观察到透射激波在内部界面传播时形成自由前导折射结构并向自由前导冯诺依曼折射结构转换的波系演变过程;另外,界面内的复杂激波结构诱导内部下游界面上的涡量发生了3次反向;在界面演化后期,内部界面形成的“射流”结构与下游界面相互作用,诱导界面形成一对主涡、一对次级涡以及一个反向“射流”结构。定量分析了环形界面长度、宽度、位移、环量以及混合率的变化情况,结果表明,内部气柱的存在减弱了前期小涡结构合并形成大涡结构过程中对界面高度与长度的影响,同时提高了重质气体与环境气体的混合率。     

高强预应力钢绞线试验研究

为了研究高强钢绞线在多束并联使用情况下，其总体破断强度利用系数的取值范围，以及锚固预紧力与总体破断强度之间的关系。通过对试验结果的分析归纳，给出了最佳试验加载控制曲线，便于在实验室利用计算机对加载系统进行自动控制，得到理想的试验结果。本文对目前使用较为广泛的高强度低松弛钢绞线的力学性能进行了试验研究，对实验过程中安装因素进行了初步分析探讨，提出了相关的最佳实验加载方程和一些可供设计工作参考的安全强度利用系数。为国标中的相应强度标准提供了试验理论依据。     

预应力钢梁体外索的张力增量分析

将预应力钢梁看作梁与索的组合结构,充分考虑了二者之间的相互作用．利用变分原理推导出在外荷载作用下,体外索张力增量的计算公式,并通过预应力钢梁模型试验对计算结果进行了验证．     

水位变化对正倒锥体冰载荷影响的离散元分析

在海冰与锥体海洋结构的相互作用过程中,潮汐水位变化时海冰作用于锥体结构的位置改变对冰载荷具有显著影响.本文采用具有粘结破碎功能的离散元方法计算海冰与锥体作用的破坏过程.同时考虑海冰上下表面温度差异对海冰强度的影响,将离散元计算冰载荷及海冰破坏模式与渤海现场实测数据进行对比验证.离散元结果表明,海冰与正锥和倒锥碰撞时均发生弯曲破坏,且冰载荷均随水线处锥径的增大而增大.在水线处锥径相同的情况下,正锥冰载荷大于倒锥冰载荷,而正锥作用下海冰的断裂长度则较小.基于离散元计算结果和渤海现场观测资料分析了海冰与正锥、倒锥作用时冰载荷和断裂长度差异的主要原因.海冰与正倒锥交界线处作用时,一般发生弯曲破坏.当冰层中心高度与正倒锥交界线的高度相同时,海冰才会发生局部挤压破碎,但冰荷载并没有明显升高.由此可见,倒锥体结构可有效降低冰载荷从而具有较好的抗冰性能.以上研究表明离散元方法可确定海冰与锥体结构作用时的海冰破碎规律和冰载荷特性,为海洋工程结构的抗冰设计提供参考依据.      

极地船舶冰区操纵性能的六自由度离散元分析

极地船舶操纵破冰性能是破冰船设计建造过程中需重点考虑的问题。为分析极地船舶操纵性能,本文发展了冰区船舶六自由度操纵破冰运动模型,采用扩展多面体离散元模拟海冰,舵桨操纵模型提供破冰力。开展平整冰区定常直航模拟计算,并与Lindqvist船体冰阻力经验公式展开对比验证;开展雪龙号敞水35°舵角操纵性仿真,并与实船试航结果进行对比。在此基础上,开展冰厚及舵角影响下的船体结构冰载荷及破冰轨迹计算,模拟得到操纵破冰航行中船体垂荡、横摇及纵摇运动时程;最后分析操纵破冰船体线载荷分布与直航破冰下的差异。     

The effect of basal topography on ice sheet dynamics

Classical shallow-ice theory assumes that bed topography under ice sheets has slopes comparable to the surface slope of the ice sheet. A modification of the classical steady-state theory which allows for significant bed topography on shorter length scales has recently been developed by Morland (Proc. R. Soc L. Ser. A., 456, 1711-1739 ), but his theory requires explicit integration of the ice-flow equations over the topography length scale, which may be below the grid size of typical numerical ice sheet models. Here we present a method for parameterising the effect of basal topography of wavelengths much greater than ice thickness but much smaller than the horizontal extent of the ice sheet on the bulk flow of the ice sheet. In particular, we are able to show through the use of a multiple-scales expansion technique that the effect of such topography is described by a simple correction factor applied to the classical expression for ice flux. This correction factor dispenses with the need to integrate explicitly over the topography length scale and could allow the effect of such topography to be included in numerical models with limited grid size. Examples are given for the practical implementation of this `correction factor method' in calculations of the steady-state shape of ice sheets. Received August 20, 2002 / Accepted February 3, 2003/ Published online May 9, 2003 / L. W. Morland     

Dynamic ice loads on conical structures

Two series of model tests were performed to observe the dynamic ice loads on conical structures. The variable testing parameters include the water line diameter of the model cone and ice parameters. During small water line diameter tests, two-time breaking is found to be the typical failure of ice on steep conical structure, and also be controlled by other factors, such as ice speed and the cone angle. During big water line diameter tests, the ice sheet failed nonsimultaneously around the cone. Several independent zones of bending were found in the nonsimultaneous failure process of ice. With the increase of the ratio of D/h and the number of independent zones, the total ice force was found being gradually reduced.     

Two-dimensional discrete element simulations ofice–structure interaction

The paper presents the application of a discrete element technique for the study of the plane strain problem of the interaction between a moving ice sheet and a flexible stationary structure. The discrete element technique accounts for the generation of failure within an initially intact ice sheet. The failure of the ice corresponds to situations where the ice can exhibit combinations of brittle fragmentation and viscoplastic flow. The modelling also accounts for size dependency in the strength of the ice after fragmentation. The inter-fragment interactions are modelled by non-linear constraints which includes Coulomb frictional behaviour. The computational scheme is used to evaluate the time history of the average contact stresses and the distribution of local contact stresses at the ice–structure interface in the fragmentation zone.     

基于近场动力学数值方法的冰-吊舱推进器接触判断研究

吊舱推进器在极地船舶中的应用, 可避免冰区航行中转向、调头困难等操纵问题, 是极地船舶广泛采用的推进形式. 在冰-吊舱推进器切削过程中, 吊舱推进器受到了极端冰载荷的作用, 对吊舱推进器结构强度和极地船舶的安全性带来严重的危害. 为了研究不同操纵状态的吊舱推进器与冰切削时冰载荷的变化规律, 首先, 详细介绍了近场动力学方法研究物体断裂问题的理论基础, 分析该方法模拟冰材料的可行性. 基于近场动力学方法和面元法耦合推导了适用于冰破碎问题模拟的材料破坏准则和冰载荷计算方法. 其次, 提出了不同操纵状态的吊舱推进器与冰的接触判断方法, 建立了冰-吊舱推进器切削状态的数值计算模型, 实现了冰-吊舱推进器切削动态变化过程的数值仿真. 最后, 分析了吊舱推进器在直航、斜航以及操舵状态与冰切削时冰块破碎、螺旋桨和桨叶冰载荷以及吊舱单元整体扭矩的变化情况. 计算结果表明: 本文提出的不同操纵状态的吊舱推进器与冰切削时的接触判断方法能够真实地模拟冰-吊舱推进器的切削过程, 并能获得该过程中冰块的破坏现象和冰载荷变化特性, 可为冰区海洋结构冰载荷数值预报技术的发展、冰区吊舱推进器结构的优化设计和运营提供指导.      

Modeling ocean wave propagation under sea ice covers

Operational ocean wave models need to work globally, yet current ocean wave models can only treat ice covered regions crudely. The purpose of this paper is to provide a brief overview of ice effects on wave propagation and different research methodology used in studying these effects. Based on its proximity to land or sea, sea ice can be classified as: landfast ice zone, shear zone, and the marginal ice zone. All ice covers attenuate wave energy. Only long swells can penetrate deep into an ice cover. Being closest to open water, wave propagation in the marginal ice zone is the most complex to model. The physical appearance of sea ice in the marginal ice zone varies. Grease ice, pancake ice,brash ice, floe aggregates, and continuous ice sheet may be found in this zone at different times and locations. These types of ice are formed under different thermal-mechanical forcing. There are three classic models that describe wave propagation through an idealized ice cover: mass loading,thin elastic plate, and viscous layer models. From physical arguments we may conjecture that mass loading model is suitable for disjoint aggregates of ice floes much smaller than the wavelength, thin elastic plate model is suitable for a continuous ice sheet, and the viscous layer model is suitable for grease ice. For different sea ice types we may need different wave ice interaction models. A recently proposed viscoelastic model is able to synthesize all three classic models into one. Under suitable limiting conditions it converges to the three previous models. The complete theoretical framework for evaluating wave propagation through various ice covers need to be implemented in the operational ocean wave models. In this review, we introduce the sea ice types, previous wave ice interaction models, wave attenuation mechanisms,the methods to calculate wave reflection and transmission between different ice covers, and the effect of ice floe breaking on shaping the sea ice morphology. Laboratory experiments,field measurements and numerical simulations supporting the fundamental research in wave-ice interaction models are discussed. We conclude with some outlook of future research needs in this field.     

Numerical investigation of ice breaking by a high-pressure bubble based on a coupled BEM-PD model

In this paper, by combining the boundary element method (BEM) and peridynamics (PD), a bubble-ice interaction model is established, which can investigate the dynamic interactions between a high-pressure bubble and an ice plate with particular focus on the mechanical behaviors of ice breaking. The bubble dynamics are solved by BEM based on the potential flow theory. Ice cracks initiation and propagation are simulated by the bond-based peridynamics which is validated by a three-point bending test. The fluid–structure interaction (FSI) is achieved by matching the normal velocity and hydrodynamic loads at the fluid–structure interface. To validate the proposed FSI model, an experiment is carried out in which an oscillating bubble is generated under an ice plate by underwater discharge system. The whole interaction process is captured by a Phantom V711 high-speed camera. Qualitative agreements are achieved between the numerical and experimental results. The underlying mechanism of cracks initiation, propagation, branching, and coalescence of the ice plate is found to highly depend on three parameters, i.e., bubble–ice distance, ice thickness and bubble size. The present study is expected to provide further assists in the understanding of ice breaking problems.     

冰激结构频率锁定振动的发生机理及简单分析方法

冰激结构频率锁定振动是冰区海洋工程结构的危险工况.对频率锁定振动过程的传统机理解释没有体现这一过程的全部物理特征,导致现有的分析方法无法准确分析这一问题.本文基于对现场测量结果的分析,提出了一种海冰韧性损伤-破碎过程与结构振动耦合导致频率锁定振动的机理.该机理认为,海冰在直立结构频率锁定振动过程中发生韧性损伤-破碎行为,海冰的韧性损伤-破碎与结构运动相位耦合,导致了频率锁定振动. 海冰对结构作用产生的载荷为锯齿形,作用过程可以分为加载和卸载两个阶段,其中加载阶段时间长度约为卸载阶段3倍以上.在加载阶段,结构从平衡位置先与海冰运动方向相反振动到反向最大振幅位置后回摆,然后与海冰同向运动到正向最大振幅位置,这段过程中海冰与结构接触部位内部产生裂纹并扩展但未发生主要的破碎,在此阶段海冰发生韧性损伤;在卸载阶段,结构从最大振幅位置向平衡位置回摆,结构与海冰运动方向相反,这一过程中应变速率的突然增大导致裂纹加速扩展并失稳开裂,此时带有韧性损伤的海冰发生破碎.基于这一新的机理解释,本文提出了一种冰激结构频率锁定振动幅值的简单分析方法.该方法认为海冰破碎长度是频率锁定振动的关键参数. 理想状态下,海冰破碎长度约为结构水线处振动幅值的2.2倍.当冰速接近海冰破碎长度与结构自振周期的比值时,结构会发生频率锁定振动.该方法对评估海洋工程结构频率锁定振动的发生概率及疲劳损伤具有指导意义.