管材缩径起皱失稳数值模拟

轻质薄壁件越来越多地应用于航空、航天和汽车等高新技术工程领域,但起皱失稳一直是薄壁件成形过程中主要的缺陷之一。因此,轻质薄壁件成形过程中的塑性失稳起皱预测研究就显得尤为重要。本文针对塑性成形中薄壁管件结构由于存在多种初始缺陷(如厚度不均,尺寸偏差等)从而导致失稳模拟预测与实际情况不符的现象,以AA6061固体颗粒介质管材缩径工艺为算例,结合ABAQUS中最适合复杂接触工况的DYNAMIC算法以及Draker-Pager颗粒介质成形数值模拟模型,对无初始缺陷圆柱壳体、引入屈曲模态初始缺陷圆柱壳体、引入厚度不均初始缺陷圆柱壳体以及同时引入屈曲模态和厚度不均缺陷的圆柱壳体进行了缩径成形模拟,将四种条件下的管件失稳波形模拟结果与试验结果进行了比较。结果表明:同时引入屈曲模态和厚度不均缺陷的动态显示有限元法可以准确预测出管材缩径失稳后的形貌,并将模拟仿真与实验的应力-位移曲线进行对比,验证了此模拟方法的可靠性。

煤体失稳的数值模拟研究

煤岩突出现象通过计算屈曲模态和分析鼓突失稳来加以研究。这个模型考虑了岩石的几何非线性和弹塑性两个方面。基于该模型的有限元模拟结果与本文报告的实际观察相一致。

透射边界高频失稳机理及其消除方法——SH波动

用有限元法求解近场波动问题,须选取人工边界条件以实现对无限域稳定、高效的数值模拟.该文探讨了SH波导有限元数值模拟中透射边界引发的高频失稳问题.从离散模型出发,分析了内节点与人工边界节点运动方程频散曲线之间的匹配关系,揭示了高频失稳的一种机理,即二者相互耦合所得计算方案支持自发从人工边界向计算区域内行进的高频波动.提出通过调整内节点运动方程以改变这一匹配关系,从而消除失稳的措施.理论分析与数值结果表明该措施能有效地消除高频振荡失稳.

WENO型有限体积格式高超声速数值激波稳定性分析

高阶格式在提高计算精度、刻画流动细节等方面具有巨大优势,然而在模拟多维强激波时,高阶格式通常会产生激波失稳问题,制约了其在超声速/高超声速流动模拟中的应用。本文针对加权基本无振荡格式(WENO)型有限体积格式,建立了矩阵稳定性分析方法,为定量分析高阶格式捕捉强激波的稳定性提供了有效工具。利用矩阵稳定性分析方法,对三阶WENO型有限体积格式的激波失稳问题进行定量研究。结果表明,与一阶格式相比,三阶格式面临更严峻的激波失稳问题。在空间三阶精度下,解算器的性质与激波强度是影响数值格式激波稳定性的重要因素。此外,对数值激波失稳的空间位置进行了研究,发现激波失稳源于数值激波结构内部,因此激波结构内部的中间状态是影响格式捕捉强激波稳定性的关键因素。本文为分析三阶WENO型有限体积格式的激波失稳问题提供了有效方法,并对三阶格式数值激波失稳问题的内在机理进行研究,为更深入地理解高阶格式激波失稳问题的内在机理,发展强稳定的高阶格式奠定了基础。

应力加载路径对板壳塑性起皱失稳极限的影响研究

随着航天、航空、国防、汽车等制造技术领域对金属产品轻质化、强韧化的要求不断提高,塑性失稳成为制约其整体成形的难题之一,高效准确预测和预防薄壁构件成形过程中起皱失稳缺陷具有十分重要的意义。以楔形件拉伸试验作为研究对象,通过建立楔形件数值失稳起皱模型,结合分岔原理,提出一种板壳起皱极限图(Wrinkling limit diagram,WLD)数值求解及绘制方法。针对在金属板壳试件成形过程中应力加载路径对起皱失稳的显著影响,通过分析楔形件不同区域的应力加载路径,探究板壳变形时应力加载路径对临界起皱极限的影响。结果表明:对于存在彼此独立、不同受力状态起皱区域的试件,应对不同区域分别建立WLD。不同区域单元应力加载路径对WLD的影响可归纳为临界皱屈主应力比对主应变比的影响:当临界皱屈单元的主应力比越大时,临界皱屈主应变比就越大,即WLD斜率越大,工艺抗皱性越弱;同时,通过比较楔形件不同起皱失稳区域的承载变化过程可知,起皱单元的临界主压应力增加速率会影响板料的临界起皱失稳时刻。

Drum instability of thinning spinning ultra thin-walled tubes with large diameter-to-thickness ratio

In order to explore drum instability problems of thinning spinning ultra thin-walled tubes with large diameter-to-hickness ratio, experiments of thinning spinning ultra thin-walled tubes with different clearances between the mandrel and the tube were carried out. The phenomena of drum instability were analyzed. Drum instability mechanism was studied. The important influence of the mandrel-locked ring on stable thinning spinning was found. Besides, two important parameters, namely drum ratio and drum stiffness, were proposed to characterize the drum instability of spinning ultra thin-walled tubes with large diameter-to-thickness ratio. What's more, numerical simulations were applied to explore the influences of different clearance ratios and diameter-to-thickness ratios on the drum instability. As a result, it is found that the mandrel-locked ring is the key to the stability and precision of spinning; drum ratio can reflect the degree of the deformation of the tubes; drum stiffness is a comprehensive index to measure the influences of the tube's own parameters on the spinning instability; both the clearance ratio and diameter-thickness ratio have significant influences on the drum ratio and drum stiffness.

Wet Compression Performance of a Transonic Compressor Rotor at its Near Stall Point

In order to study the effects of wet compression on a transonic compressor,a full 3-D steady numerical simulation was carried out under varying conditions.Different injected water flow rates and droplet diameters were considered.The effect of wet compression on the shock,separated flow,pressure ratio,and efficiency was investigated.Additionally,the effect of wet compression on the tip clearance when the compressor runs in the near-stall and stall situations was emphasized.Analysis of the results shows that the range of stable operation is extended,and that the pressure ratio and inlet air flow rate are also increased at the near-stall point.In addition,it seems that there is an optimum size of the droplet diameter.