弹性结构入水砰击载荷特性三维数值模拟研究

依据双渐进法建立研究弹性结构入水砰击过程中弹性效应的数值方法。在弹性结构入水瞬态砰击过程中,自由液面速度势满足自由液面的非线性条件,并通过考虑结构运动非线性的伯努利方程将流体域控制方程与结构弹性动力响应方程耦合求解,计算结果与试验结果进行了比较分析。结果分析表明,流固耦合效应对弹性结构入水砰击载荷及结构弹性动力响应影响不可忽略。圆柱体倾斜入水过程中水平、垂向合力在头部入水和尾部入水时刻出现两个峰值;入水角度对船舶入水砰击安全性至关重要。     

轴线间距对圆柱体低速并联入水空泡演化影响试验研究

为分析圆柱体并联入水过程中入水空泡间相互干扰对于空泡演化特性影响规律,采用基于高速摄像的方法,开展两圆柱体同步并联入水试验研究。通过圆柱体单独入水和双圆柱体并联入水试验对比,分析了圆柱体同步并联入水空泡演化特性与单独入水空泡演化特性的差异,揭示了同步并联入水双空泡演化机理,并进一步给出了入水空泡随轴线间距的变化特性。试验结果表明:并联入水过程中,双空泡形态在空间上呈现出了较好的镜面对称特征;对于深闭合入水空泡,随着两圆柱体轴线间距的增大,空泡在两圆柱体轴线内侧区域的发展程度逐渐提升,且水深较浅处的发展程度提升幅度大于较深处;对于表面闭合入水空泡,随着两圆柱体轴线间距增大,内测区域空泡逐渐趋于饱满,云化现象也逐渐减弱。     

SWATH船在砰击载荷作用下动态响应研究

针对SWATH在波浪中航行时,其湿甲板底部易遭受砰击从而引起船体振动的现象,对SWATH的砰击动态响应和强度进行了研究。利用SWATH在规则波中船波相对运动推导了随空间和时间变化的砰击载荷,采用有限元数值仿真,计算了SWATH湿甲板受砰击载荷作用的动态响应,分析了其位移和应力分布规律;并与静强度法砰击响应进行了比较,对湿甲板强度设计方法进行了探讨,为船舶结构设计提供参考     

双球并联入水空化及运动特性实验研究

物体入水问题近年来受到广泛关注,但关于运动体并联入水的研究较少。相对于单运动体入水,多运动体并联入水在军事中也有很强的应用背景,例如水上多枚鱼雷空投、超空泡射弹齐射入水等,这种多层次饱和打击能够大幅提高对水下运动目标的毁伤效果。为了探索双运动体并联入水的流体动力特点,开展了不同横向间距下的双球同步入水实验,分析了入水后双空泡演化过程与运动特性。结果表明,并联入水后由于双球内侧流场和压力发生变化,水-汽-壁三相接触线倾斜在球体表面,且倾斜角度随双球间距增大而减小;双球的空泡形态和三相接触线具有良好的彼此对称性。当双球以较小的横向间距(≤2.5D)并联入水时,空泡尾部内侧壁面会发生融合,形成"臼齿形"空泡,且其溃灭首先发生在壁面融合处,溃灭尾迹交织呈云团形;当间距增加至3.5D时,空泡溃灭尾迹向内侧偏、交汇呈马蹄形;当间距进一步增加到5.5D和7.5D时,双球空泡尾迹相互干扰很弱,与单球空泡尾迹形态比较接近。当双球横向间距小于3.5D时空泡表面闭合诱导产生的水柱顶部发生分离并两侧抛洒,随着横向间距的不断增加,空泡发展受相邻运动体的影响逐渐减弱,但均呈现出良好的对称性,其长度、直径与溅起水花等现象都趋近于单球入水情况。     

圆柱体入水空泡壁面运动特性与空泡演化关系实验研究

为了获得入水空泡壁面的运动特性与空泡演化之间的关系,进而对入水空泡演化过程开展定量划分研究。采用高速摄像的方法对低弗劳德数条件下的圆柱体入水开展实验研究。通过对比具有水平速度的圆柱体以不同倾角入水空泡,分析了入水空泡壁面运动特性与空泡演化过程的关系,并基于空泡壁面运动特性划分空泡演化各阶段,分析了空泡演化在各阶段的流动机理。实验研究结果表明:空泡壁面的运动特性与空泡演化过程紧密关联,根据空泡壁面位移和运动速度变化规律可定量划分空泡演化阶段。空泡的闭合形式对于具有水平速度的圆柱体入水所形成的空泡的演化过程有显著影响,若空泡发生表面闭合,则空泡演化包含了空泡敞开、空泡体积膨胀、空泡拉长以及溃灭后四个阶段,无量纲分界时间分别为T=2.8、T=5.4和T=6.9;若空泡发生深闭合,则其演化过程仅包含空泡敞开、空泡闭合以及空泡溃灭三个阶段,并以时间T=2.8和T=8.4为分界。根据空泡壁面的运动特性划分空泡演化各阶段的时间特性非常明确且没有相互重叠。     

基于重叠网格法的飞机水上降落水动力砰击载荷研究

为分析飞机在水上降落过程中的砰击载荷及运动规律,基于数值模拟研究NACA TN2929飞机水上降落过程,考查飞机的初始速度、初始仰角对降落过程的影响。求解雷诺平均纳维-斯托克斯方程(RANS)模拟飞机与水的相互作用,采用VOF方法捕捉自由面,采用重叠网格技术及DFBI模型模拟飞机与流体的耦合运动。通过计算楔形体和圆锥体自由入水过程验证模型的可靠性,通过网格和时间步长的收敛性分析确定合适的网格和时间步大小,与实验结果对比确定计算结果的正确性。模拟飞机以不同初始速度与不同仰角的着水过程,分析飞机底部压力的变化规律,研究飞机初始速度、初始仰角对砰击载荷及运动姿态的影响规律。数值计算结果表明飞机在入水0.2 s左右砰击载荷会达到峰值,载荷峰值与飞机着水的速度的平方线性相关;飞机机身底部压力峰值出现在水面与机身的正向交界处;飞机初始仰角增加,砰击载荷峰值会有所减小。     

基于砰击载荷的船体梁可靠性分析

基于随机过程理论,建立了船体梁在砰击、波浪及静水载荷联合作用下功能函数的随机过程模型,考虑波浪载荷和砰击载荷幅值的相关性,采用上穿率及并联系统可靠性分析方法求解此随机过程的可靠性.最后通过算例分析了舰船结构的阻尼率、砰击率及相关系数对可靠性的影响,并计算分析了舰船结构各种失效模式的可靠性.分析表明结构的阻尼率决定了砰击载荷对结构可靠性的影响程度.阻尼率较小时,砰击载荷对结构的可靠性影响很大,且舰船结构的受压屈曲破坏是结构失效的主要模式.     

回转体匀速垂直入水早期空泡数值模拟研究

基于VOF(Volume of Fluid)方法和有限体积法求解气、水两相流动的RANS方程,并结合动网格技术,对物体垂直入水空泡流动进行了数值计算研究。通过对球体垂直入水早期空泡形态进行数值计算,并将计算结果与May A 理想空泡模型拟合结果进行对比分析,二者具有较好的一致性,验证了数值计算方法的有效性。在此基础之上,进一步研究了150°锥角回转体垂直入水空泡生成过程,空泡壁面运动特性和空泡表面闭合特性,给出了垂直匀速入水空泡形态随时间变化规律,空泡壁面随入水过程时间变化运动规律,以及空泡表面闭合时间与入水速度之间的关系。     

楔形体匀速和自由落体水弹性砰击的半解析法求解EI北大核心CSCD

针对二维弹性楔形体入水过程的流固耦合问题,提出一种基于耦合Wagner理论和模态叠加法的半解析解方法。结构湿表面的速度势基于Wagner理论求解并考虑了结构弹性影响。砰击压力根据伯努利方程求解,为提高求解精度考虑了伯努利方程中速度平方项。通过平均弹性速度修正模型推导出附加质量和阻尼矩阵,将其代入固体动力学方程从而建立统一的流固耦合方程,耦合方程通过基于隐式的Newmark-β算法实现求解。通过计算楔形体垂直恒速和自由落体入水两种运动状态,并与基于半解析、数值和试验的文献结果进行了对比,验证了所提理论的可靠性。     

砰击荷载下金字塔点阵夹层板动力响应分析与估算

该文以轻质金字塔点阵夹层平板结构为对象,对其在不同撞水速度下(1m/s~6m/s)的流-固耦合动力学行为进行数值和理论计算分析。首先建立了气-液-固三相数值模型,通过详细计算获得了砰击压力的分布特征与结构砰击变形特征,分析表明当砰击压力脉宽和结构自振周期(湿模态)之比大于4时流-固耦合效应不显著。与同等质量实体板相比,轻质金字塔点阵夹层平板平均流-固砰击压力与结构最大变形均显著降低,而且存在#x0201c;局部气垫#x0201d;现象(在实体平板撞水的过程中未发现该效应)。提出一种将砰击变形位移场分解为总体变形和局部变形,然后进行叠加的计算模型,该种工程近似计算方法获得的变形估算值与数值计算结果吻合较好。     

空心圆柱低速垂直入水试验研究

为获得空心圆柱垂直入水空泡演化规律和运动特性,利用高速摄像技术对其低速垂直入水过程进行了试验研究。通过对空心圆柱体入水空泡的演化过程的观察和运动参数的测量,讨论分析空心结构对空泡形态演变的影响和不同入水速度下空泡演化过程和模型运动阻力变化。结果表明：空心结构诱发通孔射流使深闭合下的空泡壁由点分离形式变为线分离,出现新的空泡壁收缩形式,运动体尾部出现环状空泡,稳定状态空泡出现气泡分阶段脱落和\     

卷破波斜向作用下方形桥墩砰击荷载研究

沿海桥梁常受到破碎波浪作用,其中卷破波砰击严重威胁结构安全。该文通过理论和试验相结合的方法研究了方形桥墩受到的卷破波砰击荷载。以结构破波荷载经验计算公式和MLM理论模型为基础,建立了方形截面桥墩卷破波砰击荷载理论计算模型。通过卷破波水槽试验探究砰击角度与砰击荷载的关系;对比不同砰击角度理论和试验下方形桥墩截面砰击压强时程,验证了理论模型的有效性,结合试验与理论模型确定了不同砰击角度下方形桥墩卷曲因子的取值范围。研究表明：卷破波砰击方向与方形桥墩迎水轴向夹角越小,砰击荷载越大;该文提出的理论模型能很好地描述方形截面砰击压强时程随砰击角度的变化情况;根据试验结果拟合得出了方形截面卷曲因子随砰击角度的变化公式。     

发动机泵体精密热模锻成形工艺研究

目的为了提高发动机泵体综合机械性能和降低制造成本,采用精密热模锻技术来实现泵体的精确成形。方法通过确定锻件分模面位置,建立了泵体精密热模锻几何实体模型;在此基础上,建立了泵体热模锻过程三维有限元模型和模拟参数,实现了精密热模锻过程有限元模拟模型。结果通过数值模拟,获得了成形过程中坯料的速度场、等效应变场和温度场及载荷-行程曲线,揭示了泵体热模锻过程中金属充填模具型腔的情况及其变形机理,获得了温度场应变分布以及载荷、打击能量随行程的变化规律,优化了预成形时拍方坯料长度等参数,为确定成形工艺参数提供了科学依据。结论经试验验证,新工艺成形的锻件非加工外形面尺寸精度达到了零件要求,数值模拟结果与实验结果一致。     

An experimental investigation of class A surface finish of composites made by the resin transfer molding process

Achievement of high class surface finish is important to the high volume automotive industry when using the resin transfer molding (RTM) process for exterior body panels. Chemical cure shrinkage of the polyester resins has a direct impact on the surface finish of RTM molded components. Therefore, resins with low profile additives (LPA) are used to reduce cure shrinkage and improve surface quality of the composite parts. However, little is known about the behaviour of low profile resins during RTM manufacturing and their ultimate effects on the surface quality of molded plaques. In this work, the effects of controlled material and processing parameters on the pressure variations, process cycle times and ultimately on the surface quality of RTM molded components were investigated. Taguchi experimental design techniques were employed to design test matrices and an optimization analysis was performed. Test panels were manufactured using a flat plate steel mold mounted on a press. Pressure sensors were inserted in the mold cavity to monitor pressure variations during different stages of cure and at various locations in the mold cavity. It was found that a critical amount of LPA (10%) was required to push the material against the mold cavity and to compensate for the resin cure shrinkage. A significant increase in pressure was observed during the later stages of resin cure due to the LPA expansion. The pressure increase had a significant effect on the surface roughness of the test samples with higher pressures resulting in better surface finish. A cure gradient was observed for low pressure injections which significantly reduced the maximum pressure levels.     

飞机铝合金深锥型面零件多道次充液拉深技术

目的随着新一代飞机在隐身和战斗性能方面的提高,飞机钣金零件的复杂程度和制造精度要求也越来越高。对于深型腔复杂型面零件,充液拉深是一种有效的精密制造方法。方法针对难成形、复杂型面的某型飞机铝合金深锥零件,利用理论分析、有限元模拟和工艺试验相结合的方法,设计了多道次充液拉深技术方案,并建立有限元分析模型。基于等裕量函数法和零件锥面特征,分配并优化了不同道次的材料变形量。结果对多道次充液拉深成形过程中出现的起皱和破裂的失效形式进行了研究,分析了预成形高度,液室压力和压边力等关键工艺参数对零件成形质量的影响,获得了优化的预成形高度和液室压力加载轨迹。结论结果表明,提出的多道次充液成形技术能够实现复杂型面,大拉深比的铝合金零件的整体精确成形,采用优化的工艺参数能够成形出壁厚均匀,表面质量好,锥面精度高的零件。     

3D domain decomposition for violent wave‐ship interactions

A 3D Domain‐Decomposition (DD) strategy has been developed to deal with violent wave‐ship interactions involving water‐on‐deck and slamming occurrence. It couples a linear potential flow seakeeping solver with a Navier–Stokes method. The latter is applied in an inner domain where slamming, water‐on‐deck, and free surface fragmentation may occur, involving important flow nonlinearities. The field solver combines an approximated projection method with a level set technique for the free surface evolution. A hybrid strategy, combining the Eulerian level set concept to Lagrangian markers, is used to enforce more accurately the body boundary condition in case of high local curvatures. Main features of the weak and the strong coupling algorithms are described with special focus on the boundary conditions for the inner solver. Two ways of estimating the nonlinear loads by the Navier–Stokes method are investigated, on the basis of an extrapolation technique and an interpolation marching cubes algorithm, respectively. The DD is applied for the case of a freely floating patrol ship in head sea regular waves and compared against water‐on‐deck experiments in terms of flow evolution, body motions, and pressure on the hull. Improvement of the solver efficiency and accuracy is suggested. Copyright © 2013 John Wiley & Sons, Ltd.     

Temperature and pressure evolution in fast heat cycle injection molding

An accurate mold temperature control during injection molding processes allows obtaining objects with better surface finishing, more accurate surface replication, and reduced frozen-in orientation. The control of these properties is particularly important when the thickness of the molded part is very small as in the case of microinjection. In this work, thin multilayer heaters are adopted to obtain a very fast mold surface temperature evolution during the process of an isotactic polypropylene (iPP). The effect of mold temperature history on the pressure developed inside the cavity is analyzed and correlated to both gate solidification and mold deformation. Results confirmed that, with a fast control of the cavity surface temperature, a reduction of the injection pressure is achieved, without affecting the cycle time. The understanding of the phenomena that occur during the fast temperature evolution on the cavity surface can allow the microstructural calibration of the molded parts.     

Numerical Investigation of Dynamic Response and Failure Mechanisms for Composite Lattice Sandwich Structure under Different Slamming Loads

This paper mainly investigates the slamming dynamic response and progressive damage evolution of the composite lattice sandwich structure under different slamming velocities and deadrise angles. Based on the Coupled Eulerian–Lagrangian (CEL) method, an integrated numerical model of sandwich structures is developed to simulate the slamming process, in which the progressive damage evolution of composite material is considered with VUMAT subroutine. The reliability and accuracy of the corresponding numerical models are verified through the comparison between numerical and experimental results. Then, the typical slamming behavior of composite lattice sandwich structure is analyzed in detail, including hydrodynamic force, jet flow/water pressure distribution, progressive damage evolution and absorption energy. Subsequently, the influences of slamming velocity and deadrise angle on the hydrodynamic response and damage modes of the sandwich structures are investigated based on the developed numerical models. The results demonstrate that the slamming velocity and deadrise angle have significant influences on the hydroelastic behavior and damage modes of composite lattice sandwich structures. In the process of slamming, matrix damage and delamination damage are more prone to appear in the chine region, while the fiber damage more likely occurs under the higher slamming velocity and lower deadrise angle cases.