舰船舰面空气流场的CFD数值模拟探讨

CFD技术已经广泛应用于舰船水流场的分析计算研究领域,而在舰船空气流场研究中的应用趋势正在逐步增强,目前国内在该领域的研究工作还处于起步阶段.从现代舰船气流场的研究发展现状、舰船空气流场CFD数值计算方法的选用以及实例数值计算,探讨了数值模拟舰船气流场的可行性,并通过将计算实例的数值计算结果与其对应状态的风洞试验结果进行对比分析,验证了计算结果的合理性,结果显示,利用CFD工具在定常条件下对舰船空气流场的模拟计算方法是可行的.     

翼/板结合部涡旋流动结构与壁面脉动压力的大涡模拟研究

壁面脉动压力是湍流非定常特性的重要表征,而且是重要的流体动力声源,所以对于脉动压力的研究已经成为流声耦合研究领域的重要课题。文章采用大涡模拟方法结合动态Smagorinsky亚格子涡模型,数值计算了两型翼/板结合部在不同流速下的湍流脉动压力,并展示了结合部马蹄涡流动结构。通过将计算结果与试验结果进行对比分析可知,数值计算方法能准确地预报湍流脉动压力,从而为理解翼/板结合部非定常流动的物理机理奠定了基础。     

船体结构表面裂缝在应力腐蚀耦合作用下的扩展机理数值研究

用数值方法研究了船体表面微裂缝在海水腐蚀以及外荷载耦合作用下扩展的机理.实验表明,在给定材料和腐蚀环境的条件下,外荷载越小,微裂缝扩展并最终导致材料断裂破坏过程越长;但当荷载小于某个值(门槛值或称疲劳极限)时,这种延迟的腐蚀断裂破坏现象可以避免.为研究方便,文中假设了在腐蚀环境中工作的材料为各向同性材料.根据数值计算结果,共发现了三种可能的裂缝扩展形式,并证实了导致腐蚀疲劳断裂临界条件的存在性.最后通过计算,针对不同材料以及不同环境条件,勾勒出导致腐蚀疲劳断裂相应的临界条件曲线.     

船舶双层底结构与台形礁石碰撞能量及搁浅阻力分析

提出一套基于双层底油轮搁浅于台型礁石场景下的结构损伤变形非线性机理模型和解析计算方法,并通过数值仿真计算验证该机理模型和解析方法的准确性。在整合双层底板材变形解析计算模型和加强筋变形解析计算模型的基础上,提出的结构变形机理模型能够同时考虑船底板材和加强筋的变形模态。以双层底油轮的一个舱段作为研究对象,使用数值仿真软件LS_DYNA在较大的撞深和礁石倾角变化范围内进行仿真计算,并进行比较。研究结果表明该解析计算模型的总变形能和平均水平搁浅阻力与数值结果吻合得较好,从而验证了机理模型和解析计算方法的可靠性。研究成果可以方便地应用到双层底船舶搁浅场景的结构性能快速预报,以及船舶耐撞性能结构设计中。     

某码头工程滑坡的三维有限元分析

对发生在深圳的软粘土土坡的滑坡进行了三维有限元分析，发现利用三维有限元分析方法可以较好地模拟土坡的破坏，有助于了解土坡的破坏机理，其结果不仅对工程实践具有指导作用，而且对研究土坡的破坏机理，探索计算土坡稳定的适用计算方法都有现实意义。     

舰船结构在爆炸载荷作用下的破坏研究综述

在研究国内外相关文献的基础上,针对舰船结构在爆炸冲击载荷作用下的破坏所涉及到的研究方法,对板壳结构和水面舰船的破坏理论分析、试验研究方法、数值分析方法给予总结。根据国内外研究的进展情况,对水面舰船的破坏研究现状和研究的空白领域给予简要介绍。指出了尚待进一步研究的问题。     

循环弹塑性本构模型在港口沉箱码头抗震理论中的应用

在港口抗震中采用循环弹塑性本构模型对港工建筑物的地震破坏机理进行数值分析与振动台实验。选取在港口建设中最常见的沉箱码头作为研究对象,采用循环弹塑性本构模型,FD-FE耦合方法模拟地震荷载作用下地基土液化导致的沉箱码头岸壁的整体失稳。然后通过水下大型振动台的模型实验进行验证。数值分析与模型试验破坏趋势基本吻合。结论表明：循环弹塑性本构模型在港口工程抗震理论研究中有一定的参考应用价值,为循环弹塑性本构模型在港口建设实际工程中的推广起到一定的铺垫作用。     

超大型集装箱船全浪向波浪增阻预报技术研究

随着IMO最小推进功率规范的实施和ISO-15016实船测试航速修正方法的修订改版,波浪增阻研究引起广泛关注,成为当前船舶耐波性领域研究的热点和难点。虽然国际上针对迎浪情况的船舶波浪增阻已开展过较为广泛的研究,但对于斜浪和随浪等全浪向中的波浪增阻预报尚未取得突破性进展。该文首先阐述全浪向中的波浪增阻数值计算与模型试验方法,然后针对2万箱超大型集装箱船开展全浪向中的波浪增阻数值计算与模型试验研究。通过与商业软件和荷兰MARIN水池试验结果的比较,证明数值预报方法有效且计算结果优于商业软件、模型试验方法可行。研究成果可用于船舶在全浪向中的波浪增阻预报,进而可用于实船测试航速修正和最小推进功率规范校核等,具有较好的工程应用价值。     

基于离心试验和数值模拟的加筋高陡边坡综合分析方法

以重庆港某码头工程陆域加筋边坡为原型,采用离心模型试验和数值模拟相结合的综合分析方法,研究加筋高陡边坡的加筋机理及稳定性。离心模型试验结果表明：加筋边坡与无加筋边坡的破坏模式不同,无加筋边坡坡肩垮塌,而加筋边坡在1/6~1/3边坡高度处出现应力集中;边坡填料尤其是边坡中、下部填料的强度性质对边坡的安全系数起决定性的作用,如果仅将边坡上部变为强度较低的填料,加筋边坡安全系数变化甚小。数值分析方法先进行离心模型试验验证,再用于分析计算原型加筋边坡的安全系数和应力位移场分布,保证了数值分析计算结果的正确性,研究成果为设计提供了技术支持。本研究方法可作为研究加筋边坡工程或其他岩土工程参考。     

动爆载荷作用下舱室结构毁伤效果研究

运用数值仿真计算方法,研究动爆载荷作用下舱室结构的毁伤效果.动爆载荷对舱室结构毁伤效果的不对称性主要体现在装药运动正、负向舱壁的破坏模式及破坏程度不同.基于仿真计算工况,总结出舱室结构的5种不对称破坏模式.依据量纲分析理论,提出一组适用于动爆载荷作用下舱室结构不对称破坏模式的无量纲表达式,并将无量纲表达式应用于动爆载荷...     

考虑小裂纹和保载时间效应的疲劳可靠性分析

大深度载人潜水器的载人舱材质是高强度金属,不可避免地会受到疲劳损害现象的挑战。实际的载人舱结构中存在着由外部因素和内部因素引起的不确定性,其疲劳寿命将受到这些不确定性的影响。在前期的研究中,课题组基于统一的疲劳裂纹扩展率模型进行了疲劳可靠性分析方法的探讨,可作为进一步进行载人舱疲劳寿命预报的基础。但是,该模型无法反映小裂纹和保载时间的影响,而这正是载人舱结构和所受载荷的重要特征,应进行深入研究。所以,课题组提出了一个小裂纹扩展率模型和两个反映保载时间效应的裂纹扩展率模型,以更好地解释载人舱用钛合金金属的蠕变疲劳特性。文中基于这三个模型,结合不同的理论方法,进行了疲劳可靠性分析,考察了可靠度分析方法之间的不同、输入参数以及不同的裂纹扩展理论模型对结构的疲劳可靠性的影响。     

金属疲劳裂纹扩展速率的贝叶斯正则化BP神经网络预测

人工神经网络是进行预报裂纹扩展率的一个重要方法。文章针对不同金属的疲劳裂纹扩展速率分别建立贝叶斯正则化BP(Back Propagation)神经网络,将各材料在不同应力比R下的疲劳裂纹扩展速率试验数据分为两部分,一部分用来进行训练网络,另一部分用来测试训练好的网络,检验其泛化能力。将从文献中获取的4种不同金属材料的疲劳试验数据作为算例,来检验网络的性能。计算结果表明贝叶斯正则化BP神经网络不仅对训练样本有很好的拟合能力,而且对于未训练过的测试样本也有较好的预测能力,即有较强的泛化能力。同时,指出了建立网络时减少门槛值附近的试验样本点,可以提高网络的预测能力。研究结果表明,该方法可以方便地获得不同应力比R下的疲劳裂纹扩展速率,从而达到减少试验次数,充分利用已有数据的目的。并且可以进一步应用于其他金属的疲劳裂纹扩展速率的预报。     

Crack growth direction effects on corrosion-fatigue behaviour of offshore wind turbine steel weldments

In this study corrosion-fatigue tests have been conducted on fracture mechanics specimens extracted from an S355 G10+M structural steel welded plate. The tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. The corrosion-fatigue test results from this study have been compared with the data available on the base metal as well as air tests on the same material. Moreover, the obtained results have been compared with the corrosion-fatigue data available in the literature on a wide range of steels and also the fatigue trends for welded joints in free-corrosion condition recommended in the BS7910 Standard. The effect of the specimen orientation, with respect to the weld region, is also examined in this study and it has been found that higher corrosion-fatigue crack growth rates are generally observed in the tests with 0° orientation. The results have also shown that the corrosive environment has significant effects on the fatigue crack growth acceleration at the beginning of the tests; however, as the crack propagates, the environmental damage effect on crack growth behaviour becomes less pronounced. The results presented in this study are discussed in terms of improvement in the structural integrity assessment of offshore wind turbine monopiles.     

A unified fatigue life prediction method for marine structures

Marine structures such as ships and offshore platforms are mostly designed with damage tolerance and this design philosophy requires accurate prediction of fatigue crack propagation process. Now more and more people have realized that only a fatigue life prediction method based on fatigue crack propagation (FCP) theory has the potential to satisfy the accuracy requirement and to explain various fatigue phenomena observed. In the past several years, the authors’ group has made some efforts in developing a unified fatigue life prediction (UFLP) method for marine structures. The key issue for this development is to establish a “correct” crack growth rate relation. In this paper the improvement of the crack growth rate model is dealt with first. A new crack growth rate model based on the concept of partial crack closure is presented. The capability of the model is demonstrated. Secondly, studies on the engineering approaches to determine the parameters in the new crack growth rate model are carried out and validated by comparing with the experimental results on a wide range of alloys. Thirdly, the preliminary studies on some significant problems such as load sequence effect are presented. Finally, further studies for the application of the UFLP method to the fatigue strength assessment of marine structures are pointed out.     

A probabilistic damage tolerance concept for welded joints. Part 1: data base and stochastic modelling

The present paper presents the necessary crack growth statistics and suggests stochastic models for a reliability analysis of the fatigue fracture of welded steel plate joints. The reliability levels are derived from extensive testing with fillet-welded joints for which the entire crack growth history has been measured, not only the final fatigue life. The statistics for the time to reach given crack depths are determined. Fracture-mechanics-derived crack growth curves are fitted to the measured experimental curves and the best fit defines the growth parameters involved for each test specimen. The derived statistics and distribution function for these parameters are used as variables in a Monte Carlo simulation (MCS). In addition a Markov model is developed as an alternative stochastic model. It is a Markov chain for which the discrete damage states are related to chosen crack depths in the material. This model works directly with the experimental time statistics. It is a “stochastic bulk approach” not involving any random variables or fracture mechanics modeling. Both models are fitted to the data base and scaled to in-service conditions. Both methods are compared and discussed. The aim is to provide data for the variables used in a MCS and to develop a Markov chain for fast reliability calculation, especially when predicting the most likely influence of numerous future inspections.     

Approximate method to determine the model parameters in a new crack growth rate model

Marine structures are subjected to complex loading histories and one of the most significant failure modes is fatigue. Accurate prediction of the fatigue life of marine structures is very important for both safe and economic design and operation. Now many researchers and engineers have realized that fatigue crack propagation theory can provide more rational basis to predict the fatigue life of metal structures. At the same time, more and more fatigue crack growth models are proposed along with a good understanding of metal fatigue mechanisms. However, it is difficult to determine a large number of model parameters, which restricts their use in practical engineering problems. Therefore, it is significant to study the approximate methods for estimating the model parameters in good fatigue crack growth models.In our previous work, an extended McEvily model for fatigue crack growth analysis of metal structures was proposed. This model shows promising capability to explain various fatigue phenomena. In order for the convenient use in estimating fatigue life of marine structures, the concepts and approximations of the model parameters are comprehensively studied in this paper. Based on that, more reasonable assumptions and empirical formulas to determine the parameters are recommended. The approximate method is validated by experimental results of several types of materials, which could be successfully used in simple and effective engineering analysis for marine structures.     

Fitness-for-purpose assessment of cracked offshore wind turbine monopile

In this paper, the procedure for flaw acceptability assessment is examined through a case study of a semi-elliptical surface crack in an offshore monopile as it grows till it forms a through thickness crack. Using the procedure prescribed in an industrial standard (BS 7910), the fracture ratio, K_r is shown to increase monotonically with increasing crack depth. The load ratio, L_r, is initially insensitive to the crack depth. However, there is a rapid increase in L_r when the crack depth to thickness ratio exceeds 80%. L_r values obtained from detailed 3D FE limit analysis using elastic-perfectly-plastic material behaviour do not exhibit the asymptotic behaviour predicted by BS 7910 as the flaw transitions from deep crack to through-thickness crack. Furthermore, K_r predicted by BS 7910 is shown to be an over-estimation for the typical dimensions of offshore monopiles. The findings suggest that a structure with a deep flaw may be identified as unacceptable based on BS 7910 when it may still possess a non-trivial amount of structural residual life. This is a concern for monopiles where crack growth as a large flaw forms a significant part of the total life.     

改进的统一疲劳裂纹扩展速率预报模型对合金在常幅载荷下普遍适用性的研究

精确预报金属结构的疲劳对确保结构安全及指导结构设计与维修具有重要的意义.作者们基于McEvily模型提出了一个改进的统一疲劳裂纹扩展速率模型,其将疲劳裂纹扩展的三个扩展区域统一起来,并能解释更多的疲劳试验现象.文中对该模型进行了详细阐述,同时对模型参数的工程预报方法进行了讨论.为了进一步检验本模型的可靠性,还对不同载荷比下各种材料疲劳裂纹扩展率的预报结果与实验结果进行了对比,对比结果证明了该模型的准确性及其在常幅载荷下对不同材料的普遍适用性.     

试样厚度对船用钢疲劳裂纹扩展速率影响的试验

为研究试样厚度对船用钢疲劳裂纹扩展速率的影响,设计并实施两组不同厚度的紧凑拉伸试样进行疲劳裂纹扩展速率试验,同时建立了三维疲劳裂纹扩展有限元模型,分别基于线弹性理论和弹塑性理论对应力强度因子进行了计算,并分析了试样厚度对裂纹扩展速率的影响。试验与计算结果的综合分析表明：相同应力水平下,薄试样裂纹尖端的塑性区明显大于厚试样,且裂纹尖端应力强度因子值大于理论经验计算结果可达23.25%,因此,在材料裂纹扩展速率试验前,特别是试样厚度尺寸较小时,应充分考虑试样的厚度效应,参考基于弹塑性理论计算得到的应力强度因子结果,同时有必要针对当前试样及材料进行专门的裂纹扩展速率试验,以得到准确裂纹扩展参数结果。