Study on the impact flow field and the motion characteristics of vehicle with boost floatation aids falling on the water in a wave environment
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摘要: 为探究波浪环境下带助浮装置航行体下落冲击过程中的流场以及运动演化特性,基于CFD (computational fluid dynamics) 数值模拟技术,在方法上耦合了VOF (volume of fluid) 多相流模型、k-ω SST湍流模型、Schnerr-Sauer空化模型以及Stokes五阶非线性波理论,建立了一套针对入水冲击问题的数值计算方法,并采用速度边界法进行造波。经验证,试验与数值结果在下落位移上对比差异较小,该数值方法可靠有效,且造波结果与Stokes五阶非线性波理论吻合较好。然后,基于构建的数值方法,在不同波浪环境下对带助浮装置航行体下落入水冲击过程进行了数值模拟,计算带助浮装置航行体冲击过程的位移、速度、加速度以及助浮装置受力情况,分析冲击过程中航行体的运动学参数、动力学参数以及入水空泡流场演化过程,总结了波浪环境下带助浮装置航行体的入水冲击特性。结果表明,波浪环境对下落冲击过程的影响主要体现在运动衰减段,水平方向的冲击相较于垂直方向的冲击受到波浪环境的影响要大得多,不同海况对航行体的水平冲击造成的影响主要是通过影响入水空泡的形成与溃灭过程实现的。Abstract: Based on the computational fluid dynamics (CFD) numerical methods, a set of reliable and effective numerical methods for investigating the flow field and evolution characteristics of motion during the process of falling vehicle with boost floatation aids impacting the water in wave environment was established coupled with volume of fluid (VOF) multiphase flow model, k-ω SST turbulence model, Schnerr-Sauer cavitation model and Stokes fifth-order nonlinear wave theory. The numerical simulation of the process of falling into water under a horizontal cylinder showed that the difference between the experimental results and the numerical results in falling displacement was small, which verifies the validity of the numerical method of water falling impact. The wave generation results obtained by the velocity boundary numerical wave generation method were in good agreement with Stokes fifth-order nonlinear wave theory. Based on the established numerical method, numerical simulation was carried out on the water falling impact process of the vehicle with boost floatation aids under different wave sea states. The kinematic and dynamic parameters of the vehicle and evolution of water-entry cavity flow field during the impact process were analyzed, and the water falling impact characteristics of the vehicle with boost floatation aids under wave environment were summarized. The results show that the impact of wave environment on the falling impact process is mainly reflected in the motion attenuation section. The horizontal impact is much more affected by the wave environment than the vertical impact and the influence of different sea conditions on the horizontal impact of the vehicle is mainly achieved by influencing the formation and collapse of the water-entry cavity. The calculated displacement, velocity, acceleration and boost floatation aids force during the impact process of vehicle with boost floatation aids can be provided as a reference for the structural design and safety test guidance of the vehicle recovery under wave environment.
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Key words:
- vehicle with boost floatation aids /
- water-entry impact /
- impact load /
- wave load
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金属玻璃作为原子无序堆垛结构的代表性材料, 有独特的原子短程有序、长程无序的微观结构, 兼有金属和玻璃特性, 是具有广泛应用前景的新型结构和功能材料[1]。金属玻璃力学性能的研究有助于理解其变形和损伤破坏机理、提高其结构性能。
对一般环境下金属玻璃的弹塑性变形、损伤及破坏等力学性能已有了大量研究, C.A.Schuh等[2]和M.M.Trexler等[3]分别对相关研究进行了综合评述。在已有研究中, 金属玻璃的屈服强度特性是重点关注的内容之一。很多准静态实验研究结果表明, 金属玻璃的屈服强度与应力状态有关。J.Lu等[4]采用围压法研究了受约束状态下Zr41.25Ti13.75Cu12.5Ni10Be22.5金属玻璃的屈服强度特性(最大压力约2GPa), 结果显示屈服强度压力硬化效应非常明显。近年来, 高压、高应变率等极端条件下金属玻璃的冲击波响应特性逐渐引起重视, 目前已有Zr基金属玻璃的冲击绝热线[5-6]、层裂现象[7-8]和弹塑性变形行为[9-12]的一些实验及理论模拟研究。F.P.Yuan等[9]运用压剪炮技术对Zr41.25Ti13.75Cu12.5Ni10Be22.5金属玻璃屈服强度的压力相关性进行了研究(最大压力8.8GPa), 实验结果与准静态不同:压剪加载下压力或法向应力对屈服强度影响很小; 而S.J.Turneaure等[10]和俞宇颖等[11]的27GPa压力范围内平靶冲击实验结果表明, 锆基金属玻璃的冲击加载波阵面存在剪应力衰减。总之, 金属玻璃的屈服强度特性研究限于较低压力范围, 而且相关结论并不一致, 须进一步研究。此外, 表征金属玻璃强度特性的另外一个物理量——剪切模量也仅有2GPa压力范围内的超声测量结果[13], 还未见冲击波加载下的高压剪切模量数据。
本文中, 对一种锆基金属玻璃进行平靶冲击, 通过测量样品/透明窗口界面冲击加载-卸载粒子速度剖面, 获得37~66GPa压力范围的屈服强度和剪切模量数据; 结合实验测得的强度数据, 对锆基金属玻璃冲击波阵面剪应力松弛现象[10-11]进行分析。
1. 实验
为简化冲击加载-卸载过程样品中的波系作用, 利于粒子速度剖面的处理分析, 实验采用如图 1所示的反向碰撞方式, 即由待测样品(锆基金属玻璃)作为飞片直接撞击透明的单晶LiF窗口。LiF窗口碰撞面镀有1μm铝膜作为光学测试的反射面, 为保护长历时测量过程中铝膜不受破坏, 铝膜前粘接了8μm铜箔。飞片衬垫为低阻抗的聚碳酸酯, 实现对冲击后样品的卸载。DISAR(displacement interferometer system for any reflector)技术[14]用于测量锆基金属玻璃样品/LiF窗口界面粒子速度剖面, 飞片速度采用磁测速技术测量。
实验用金属玻璃为Zr51Ti5Ni10Cu25Al9(原子百分比), 平均密度为约6.740g/cm3, 超声测量的常态纵波和横波声速分别为4.820和2.193km/s[6]。根据测定的纵波和横波声速, 可以得到体波声速为4.101km/s, 剪切模量为32.4GPa, 泊松比为0.369。样品名义尺寸为∅28mm×3mm, 表面抛光处理, 平行度2~5μm。LiF窗口尺寸为∅28mm×12mm, 密度为2.638g/cm3, 冲击波速度D=5.148km/s+1.353u(u为粒子速度)[15]。
2. 结果与分析
在∅30mm二级轻气炮上进行了4发冲击加载-卸载实验, 冲击速度为2.889~4.480km/s, 锆基金属玻璃样品产生的压力为37~66GPa。实验的参数列于表 1中, 其中ρ0为锆基金属玻璃样品初始密度, Hs为样品厚度, W为冲击速度, σH为冲击压力, τH+τc为屈服强度, G为剪切模量。
表 1 平靶冲击实验参数及结果Table 1. Experimental conditions and results for four plate-impact experimentsNo. ρ0/(g·cm-3) Hs/mm W/(km·s-1) σH/GPa (τH+τc)/GPa G/GPa 1 6.744 3.142 2.889 37.28 1.73 47.59 2 6.743 3.120 3.604 49.69 1.88 63.25 3 6.736 3.016 3.640 50.33 1.99 62.96 4 6.655 3.007 4.480 66.42 2.39 79.47 由DISAR测得的4发实验锆基金属玻璃样品/LiF窗口界面粒子速度剖面如图 2所示。卸载过程中呈现明显的弹塑性特征, 表明在66GPa冲击压力范围内锆基金属玻璃没有发生冲击熔化。根据波传播特性, 可由粒子速度剖面(见图 2), 得到沿着卸载过程的拉格朗日纵波声速:
cL=Hst−Hs/Ds (1) 式中:Ds为样品的冲击波速度, t为来自样品后界面的卸载波到达样品/窗口界面时间(以碰靶为起始时刻)。在18~100GPa冲击压力范围, 该锆基金属玻璃的冲击波速度Ds=4.241km/s+1.015u[6]。
图 3给出了由上述加载-卸载粒子速度剖面得到的卸载过程拉格朗日纵波声速cL随粒子速度u的变化。其中, 粒子速度u由样品/窗口界面粒子速度uw结合增量型阻抗匹配法计算得到, 由此得到的粒子速度计及了卸载波在样品/窗口界面反射造成的影响[16]。与金属材料相类似, 锆基金属玻璃卸载过程也呈现准弹性行为特征, 即卸载过程弹、塑性波速为光滑过渡, 而没有发生突降[17]。尽管冲击压力不同, 但塑性声速与粒子速度关系基本一致。将塑性段声速线性外延可得相应的拉格朗日体波声速cB。
根据J.R.Asay等[18]提出的双屈服面强度测量方法, 对沿卸载过程的声速进行计算, 可得到:
τH+τc=−34ρ0∫ucuHc2L−c2Bc2L du (2) 式中:uH和uc分别为Hugoniot状态对应粒子速度和卸载进入塑性屈服时对应的粒子速度(见图 3), τH和τc分别为Hugoniot状态剪应力和临界剪应力, τH+τc为屈服强度。
冲击压缩下(Hugoniot态)的剪切模量:
G=34ρ20ρ(c2L−c2B) (3) 式中:ρ0为材料的初始密度, ρ为冲击压缩下(Hugoniot态)的密度, cL和cB分别为Hugoniot态对应的拉格朗日纵波和体波声速(见图 3)。
计算得到的屈服强度和剪切模量列于表 1中。屈服强度和剪切模量随冲击压力的变化如图 4所示。在涉及的冲击压力范围, Zr51Ti5Ni10Cu25Al9金属玻璃的屈服强度和剪切模量均随冲击压力的增加而增加, 出现了压力硬化效应。其中, 屈服强度在0~37GPa压力范围变化很小, 这与F.P.Yuan等[9]应用压剪炮技术测量的6.3~8.8GPa压力范围Zr41.25Ti13.75Cu12.5Ni10Be22.5金属玻璃屈服强度变化情况一致; 在37~66GPa范围, 屈服强度则明显增加。
与上述的压力硬化效应不同, 已有的实验结果表明金属玻璃的冲击加载波阵面存在剪应力衰减现象。S.J.Turneaure等[10]对17GPa冲击压力范围内的实测Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.54金属玻璃粒子速度剖面进行了数值模拟, 发现采用应变软化强度模型计算的剖面才能与实验结果符合。俞宇颖等[11]则通过轴向应力与静水压线的比较获得了10~27GPa冲击压力范围Zr51Ti5Ni10Cu25Al9金属玻璃的冲击加载波阵面剪应力, 表明该金属玻璃的冲击加载波阵面剪应力存在明显衰减, 而且衰减幅度随着冲击压力的增加而增加。
通常, 材料强冲击导致的损伤/破坏和高温是造成材料强度降低的两种主要因素。如果金属玻璃冲击加载波阵面剪应力衰减是由冲击加载导致的损伤/破坏所引起的, 那么由损伤/破坏材料的Hugoniot态卸载获得的屈服强度和剪切模量也应出现衰减, 但本文中强度测量结果显示一定程度的压力硬化效应, 基于此可以排除损伤/破坏因素; 如果金属玻璃冲击加载波阵面剪应力衰减是由温度软化所引起的, 同样由Hugoniot态卸载获得的屈服强度和剪切模量也应出现衰减, 而且应随冲击压力增加而更明显衰减, 这显然与本文中强度测量结果不相符, 温度因素也可以排除。因此, 导致金属玻璃冲击加载波阵面剪应力衰减的, 并非损伤/破坏或温度软化, 而应有其他控制因素。最近, B.Arman等[12]对平面冲击波加载下二元体系Cu46Zr54金属玻璃的塑性、层裂及原子结构演化进行了分子动力学模拟, 发现金属玻璃冲击波阵面上的剪应力衰减与加载过程材料内部具有较强剪切的原子团簇数量减少有关。但由于分子动力学模拟的粒子速度剖面与实测结果还存在一定差异, 因此上述剪应力衰减的微观机理还需进一步研究确认。
3. 结论
对Zr51Ti5Ni10Cu25Al9金属玻璃进行了反向碰撞实验, 测得了金属玻璃样品/LiF窗口界面粒子速度剖面, 由此获得了37~66GPa压力范围的屈服强度和剪切模量数据。结果表明, 在上述实验压力范围金属玻璃的屈服强度和剪切模量均随冲击压力的增加而增加, 具有一定程度的压力硬化效应; 进一步分析表明, 金属玻璃冲击加载波阵面剪应力的衰减, 并非由冲击损伤/破坏或温度软化等因素导致。
哈尔滨工业大学材料科学与工程系沈军教授提供样品了材料, 张毅、王为、叶素华、傅秋卫、汪小松、景海华、蓝强、方茂林、向曜明和靳开诚等在实验测试中给予了帮助, 在此表示感谢。 -
表 1 工况设置
Table 1. Working conditions setting
工况 波高/m 波周期/s 1 0.50 3.0 2 0.75 3.0 3 1.00 3.0 4 1.25 3.0 5 0.00 / -
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