【作者】 孙华；

【导师】 王德禹；

【作者基本信息】 上海交通大学 ， 船舶与海洋工程， 2018， 博士

【摘要】 近几十年,针对不同结构形状的入水冲击问题研究越来越受到重视。作为一种复杂的瞬态过程,入水冲击可以被认为是有固体、液体、气体参与的三相耦合现象,它广泛存在于自然环境和工程应用中。入水冲击会在短时间内于结构表面产生强烈的砰击载荷,引起结构的失效变形甚至破坏;同时,结构的弹塑性响应也会对流场运动产生影响。影响冲击载荷的因素较多,例如结构形状、冲击速度、弹性效应、气垫效应等等,因此需要通过解析、数值和试验方法预测冲击压力和对应结构响应,并系统深入地分析这些因素对它们的影响。针对上述提到的问题,本文具体地开展了以下研究工作:(1)本研究基于Verhagen的气垫模型,提出了一种计算弹性平板入水冲击问题的简化方法,主要适用于入水冲击初期水动力载荷峰值形成阶段。将简化方法得到的流固耦合计算结果与任意拉格朗日-欧拉(Arbitrary Lagrange-Euler,简称ALE)方法得到的数值仿真结果以及Chuang的试验结果进行了比较,验证了简化方法的适用性和准确性。简化模型体现了气垫厚度变化速率对冲击载荷的影响,气垫中的气体速度分布也会在后期影响冲击压力。通过对不同厚度、弹性模量和非结构质量平板的入水冲击问题研究表明:一般来说,当材料变形在弹性范围内时,结构的弯曲刚度越小时,冲击压力越小,冲击时间越长,平板挠度越大;结构的非结构质量越大,冲击压力和平板挠度越大,冲击时间越短。另外,三维效应的研究表明,本方法主要适用于长宽比大于1.5的矩形平板结构。(2)本研究将气垫简化模型中弹性平板的运动响应方程扩展为弹塑性平板的运动响应方程,进一步扩大了该模型的应用范围。通过与ALE方法的数值仿真结果对比,验证了简化模型的准确性。当入水工况相同时,发生了塑性变形的弹塑性平板冲击压力峰值小于弹性平板,平板中心最大挠度大于弹性平板。本研究讨论了材料属性(如弹性模量、硬化模量、屈服强度)和结构属性(如板厚和非结构质量)对于冲击响应的影响,并发现:弹性模量增大过程中,结构总体变形减小,但塑性变形增加,冲击压力峰值会先上升,再减小,后在小范围内波动,最终略有回升并趋于稳定。板厚、屈服应力、硬化模量均与压力峰值成正比例关系,与最大垂向挠度及残余变形成反比例关系,但变化规律各不相同;非结构质量与压力峰值、最大垂向挠度和残余变形值均成正比例关系。(3)为了研究典型加筋板结构在加筋面受到砰击载荷下的动态响应,本研究设计了一系列加筋板架的入水冲击试验。试验中,板架从0.5m,1.0m,1.5m和2.0m四个高度自由下落,流体砰击发生在板架加筋一侧。试验测量了入水过程中板架受到的冲击压力和典型位置的应变。根据测量结果可知:板架中心的压力峰值和板架各处的最大等效应力均与入水高度成线性正相关关系;冲击过程包含有强气垫效应,冲击持续时间为80-100ms,压力波峰平滑;部分桁材结构由于两端整体受压,腹板在横向气垫压力作用下极易发生侧倾失稳,引起较大的变形。(4)基于板架的入水冲击试验设计,本研究采用ALE方法建立了对应的数值仿真模型,并将显式有限元计算结果与试验测量数据进行了对比,验证了该方法对于求解此类问题的适用性。仿真模型模拟了整个入水冲击过程,并总结了其中的两种冲击类型:桁架面板和外侧板架直接受水流冲击;内侧板架主要由压缩气体产生冲击压力。通过与两倍宽度与深度水域模型的计算结果比较发现,水域宽度对冲击响应的影响略高于水域深度,但影响程度有限,因此在试验和计算中基本可以忽略。(5)通过数值仿真方法,研究了板架加筋面发生入水冲击时加强筋尺寸对于冲击响应的影响,结果表明:桁材结构主要对板架中心冲击压力的大小、开始时刻和持续时间产生影响,同时也会影响自身面板的局部载荷大小;桁材越高,自身等效应力越大,整体结构应力也会随冲击载荷的改变而变化;一般扶强材对于压力几乎没有影响,腹板高度增加,自身应力减小;所有加强筋的面板宽度几乎不影响载荷,但与其自身的等效应力呈反比例关系。另外,由于冲击过程存在强气垫效应,本文通过数值仿真,研究了气体属性对于冲击响应的影响,结果表明:与真空状态的模型相比,受空气垫效应影响的板架中心冲击压力减小至三分之一,冲击开始时刻更早,持续时间更长;一般来说,气体密度和声速越大,压力的增长速率与峰值越大,但当它们小到一定程度时,压力峰值反而会减小。本文的主要创新点归纳如下:(1)基于Verhagen的气垫模型,提出了一种预报弹性和弹塑性平板入水冲击过程中的冲击压力和结构变形的简化方法,它将结构、水、空气三者的耦合关系简化为结构与空气、空气与水的相互作用,从新的角度对弹塑性平板入水冲击的初期过程进行描述。(2)在弹塑性平底结构入水冲击简化模型计算中提出了最小气垫厚度的概念与对应的经验公式,弥补了原Verhagen模型在计算后期不易收敛的缺陷。通过定量分析平底结构入水冲击过程的气垫参数变化,明确提出将气垫厚度变化速率作为冲击载荷变化的关键参数,并揭示了其它气垫参数对冲击响应的影响规律。(3)通过试验和数值模拟方法,首次研究了板架加筋面的入水冲击问题,揭示了该结构发生砰击时的完整过程,得到了模型在不同高度下自由入水时受到的冲击压力和变形响应值,揭示了入水高度、加强筋尺寸和气体属性对于冲击响应的影响规律。更多还原

【Abstract】 The water impact on various structures has garnered a great deal of research attention over recent decades.As a complex transient process,such impact can be considered as a three-phase coupling model encompassing solid,liquid,and gas.This phenomenon is common in the natural environment and across countless engineering applications.Water slamming is a phenomenon characterized by high pressure acting over a body surface for a very brief duration.The impact pressure could cause great structural deformation and even local yields;the elastic and plastic response also influence the flow of fluids.There are many factors that can influence the impact loads,such as structural dimension,impact velocity,hydroelastic effect,air cushion effect,etc.So it is very important to investigate the prediction of the hydrodynamic response and the corresponding influencing factors thoroughly and in-depth by analytic,numerical and experimental methods.As above mentioned,this thesis carried out the following works:(1)A new simplified analytical method of water impact on the elastic plate is proposed based on Verhagen’s model.The analysis is focused on the initial stage during which the highest hydrodynamic loads are generated.The fluid-structure coupling results by simplified method are compared to numerical results from Arbitrary Lagrangian–Eulerian method in LS-DYNA code and existing experimental measurements to validate the feasibility and accuracy of the simplified method.It is indicated that the change rate of air cushion thickness will influence the impact loads,and the distribution of the speed of air in the cushion also affects the loads in the later stage.The water entry problems of plates with different thickness,elastic modulus and added mass are studied in the paper.It shows that as the structural bending rigidity is lower,the impact pressure decreases and the duration and plate deflection increases.As the added mass increases,the pressure peak and plate deflection are larger,and the duration is shorter.In addition,the proposed model is mainly suitable for the rectangle plate whose length-width ratio is larger than 1.5.(2)The kinematic equation of elastic plate is replaced with that of the elasticplastic plate in the proposed model,and the application of simplified method is expanded.The results by simplified method are compared with numerical results by ALE method to validate the accuracy of this approach.In the same working condition,the pressure peak of elastic-plastic plate is smaller than that of elastic plate,and the deflection is greater.Influences of material properties(e.g.,elastic modulus,hardening modulus and yield stress)and structural properties(e.g.,plate thickness and added mass)on the impact pressure and structural deformation are discussed in detail.It is indicated that the total deformation of plate decreases while the plastic deformation increases as the elastic modulus is larger.The pressure peak will rise first and then decreases,then fluctuates on a small scale,and finally turns to stability.The plate thickness,yield stress and hardening modulus have,respectively,positive relationship with the pressure peak,and negative relationship with the maximum vertical deflection and residual deformation.The pressure and deformation are larger as the added mass increases.(3)The water entry tests of a typical stiffened panel are carried out at different drop heights.The structure free falls from the drop heights of 0.5m,1.0m,1.5m and 2.0m,and impact happens on the stiffened side.The impact pressures and strains on the panel during water entry are measured.It shows that the pressure peak at the center and effective stresses on the stiffened panel increases linearly as the drop height increases.The great air cushion effect exists in the impact process,so the pressure curve is smooth,and the impact duration is between 80 and 100 ms.Due to the compression on both ends of girder,the elastic tripping may easily occur under the lateral pressure induced by air cushion,and cause huge deformation.(4)A numerical model is built and calculated by ALE method based on the experimental design.The numerical results are compared with the experimental measurements to validate the feasibility of this method.The impact process is simulated by numerical approach and two types of impact are summarized: the direct water impact on the outer panel and the stiffener flanges;the impact transmitted by air on the inner panel.It is also indicated that the influence of the width of water domain is a little greater than that of the depth on the hydrodynamic response by calculation of models with double-width and double-depth water domain.But their influences are still limited and can be ignored.(5)Through numerical approaches,relationships between hydrodynamic response and dimensions of stiffeners are analyzed.It is indicated that the girder mainly affect the pressure peak,start time and duration at the center of the panel,as well as its local loads.When the girder is higher,its effective stress is greater,and the overall stress will change due to the change of the loads.Lower stiffeners have no effects on the impact pressure,and will decreases its own stress when its web height increases.The widths of all the stiffener flanges have positive relationship with its stress.In addition,due to the strong cushion effect in the impact process,the effects of gas properties on impact pressures are discussed.Compared with the model in the vacuum condition,impact pressure at the center of the panel decreases to one third when affected by the cushion effects,and the impact begins earlier and lasts longer.In general,the growth rate and peak value of the pressure increases as the density and sound velocity of the gas increases.But when they are too small to a certain degree,the pressure peak may decrease in turn.The innovations of this thesis are listed as follows:(1)A new simplified method to predict the impact pressure and structural deformation of elastic and elastic-plastic plate is proposed based on Verhagen’s model.The coupling relationship among structure,air and water is simplified as interactions between structure and air,as well as air and water by this method.Then the water entry of an elastic or elastic-plastic plate in initial stage can be described in a new angle.(2)The concept of minimum air cushion thickness and corresponding formula are defined in the calculation of the simplified model to fill the gap of no convergence during the later stage of impact in Verhagen’s model.Through the quantitative analysis of parameters of air cushion during water entry process of flat-bottom structure,the change rate of air cushion thickness is determined as the key factor to influence the impact load.Also,influences of other parameters on the hydrodynamic response is studied.(3)The water impact on the stiffened side of stiffened panel is studied by experimental and numerical approach firstly.The whole impact process is revealed.The impact load and structural response at different drop heights are adopted.Relationships between the drop height,stiffener dimension,gas property and dynamic response are analyzed and summarized.更多还原