水陆两栖飞机典型横截面入水撞击实验研究

选取当今世界几种水陆两栖飞机典型横截面为研究对象,通过进行不同投放高度和不同质量的楔形体入水冲击实验,动态测量楔形体入水冲击过程中的压力并记录自由液面变化情况,研究了不同实验件的自由液面变化、冲击压力随时间变化及局部压力分布规律等。通过对比分析,发现带舭弯的弧形横截面型式有利于降低水陆两栖飞机在复杂海况下的着水冲击载荷,可作为水陆两栖飞机设计的参考线型。     

二维弹性结构入水冲击过程中的流固耦合效应

描述了一个研究弹性结构入水冲击过程中水弹性效应的数值方法,在弹性结构入水冲击过程中,流体域作用在结构上的水动力载荷由边界元法获得,而结构的弹性动力响应则由有限元方法求解,通过线性给离散Ｂｅｒｎｏｕｌｌｉ方程将有限元方程和边界元方程耦合到一起,从而获得了求解流场和结构动力响应的相互耦合的运动方程。在数值考虑了自由表面的非线性边界条件,通过引入射流单元以及最大射流厚度,较好地处理了冲击引起的射流问题。     

旋转圆球入水空泡特性与流场结构的大涡模拟研究

圆球旋转入水过程对于基于先导物投放的新型入水降载方式具有重要研究价值. 采用大涡模拟方法结合均质多相流模型和VOF界面捕捉算法, 对低弗劳德数条件下疏水圆球高速旋转入水的自由运动过程进行了数值模拟, 研究了转速对入水空泡演化、流场结构和水动力特性的影响. 采用动网格与滑移网格技术实现圆球的自由运动, 并基于试验结果对比验证了数值模拟的可靠性与正确性. 旋转运动的升力效应导致圆球入水弹道发生偏转并从水面携带横向楔形射流侵入空泡内部. 采用入水砰击速度与转速进行归一化分析, 结果表明入水转速的增加显著改变了圆球的动力特性: 水平方向的速度和位移以及升力峰值都随入水转速的增加而变大, 但升力峰值受到入水速度的限制; 而垂直方向的速度和加速度以及空泡断裂深度几乎不受转速增加的影响, 并且空泡深闭合发生前圆球转速变化不大. 入水转速的增加也使液面飞溅环和空泡断裂的非对称性增强, 在较低转速时发生空泡表面闭合, 而在较高转速时则发生空泡深闭合. 对于空泡深闭合模式, 入水转速的增加带来更强的横向楔形射流, 并且抑制了空泡断裂产生的高压以及相应涡结构的生成, 致使圆球在入水砰击阶段承受更低的侧向压力.      

基于ALE算法的V形楔形体入水的水动力特性分析

结构入水问题是一种复杂的流固耦合过程,涉及到固体力学、流体力学、冲击动力学和计算力学等相关力学分支的交叉与融合.论文基于非线性显式动力分析方法,采用任意拉格朗日-欧拉算法(水域采用欧拉描述,固体结构采用拉格朗日描述),并用罚函数方法控制结构与流体之间的耦合作用,对二维V形楔形体垂直入水的初期过程进行了数值仿真.通过数值仿真,分析了楔形体底部压力分布情况,讨论了网格密度、接触刚度以及阻尼系数对数值计算结果的影响,并将数值结果与Wagner理论解进行了对比分析,验证了ALE方法的可靠性.     

自由液压流体流动问题的数值分析研究

对国内外具有自由液面的流体流动问题研究的数值求解方法进行了较为全面的评述。文中首先讨论了自由液面流体流动问题的数值求解方法，包括差分法、有限元法和边界元法等，然后说明了自由液面的跟踪和网格更新等问题。     

SPH固壁边界处理方法的改进

提出一种适用于光滑质点水动力学（SPH）方法的改进的边界处理方法。在这种方法中,边界粒子的压力可通过其周围的流体粒子的压力插值得到,从而改进了耦合边界法在边界上压力不准的问题。运用这种改进的边界处理方法模拟了二维方形水箱中的非线性晃荡问题以及二维楔形体自由入水问题。模拟结果与实验结果吻合较好,证明了此改进的边界处理方法是有效的。     

用于ALE有限元模拟的网格更新方法

任意拉格朗日欧拉法(ALE)可以通过定义参考网格的运动,实现自由液面跟踪,完成液体晃动的数值计算. 综合用于更新网格节点的3种基本计算方法,将多方向更新网格速度的技术应用于任意拉格朗日欧拉网格节点的速度计算. 给出了水平圆柱形贮箱和椭圆形贮箱内液体晃动算例,实现了多方向更新网格运动与晃动流场计算的耦合,使ALE方法能胜任复杂几何边界下的自由液面流动的数值模拟.      

动压推力轴承的边界元分析

本文利用Green第二公式,将Reynolds方程转化为沿边界的积分方程,并将非线性项作为自由项的一部分处理,采用常单元离散边界Γ,用迭代技术求出油膜压力分布,与有限差分法和有限元法比较,边界元法的结果更接近解析解.     

应用基于GPU的SPH方法模拟二维楔形体入水砰击问题

光滑粒子流体动力学(SPH)法是一种无网格的拉格朗日效值方法,广泛应用于计算流体领域模拟复杂自由表面流问题.SPH方法的主要缺点就是计算量过大,而基于GPU的并行计算方法可使SPH计算得到有效加速.本文应用基于GPU的SPH并行计算方法研究了二维楔形体的入水砰击问题.数值计算结果与文献中对应的解析解比较一致,验证了基于GPU的SPH方法的精度和可靠性.仿真结果同时显示基于GPU的并行计算方法可使SPH计算速度得到显著提高.     

两套节点格林元嵌入式离散裂缝模型数值模拟方法

对于原始嵌入式离散裂缝模型(EDFM),在计算包含裂缝单元的基质网格内的压力分布时采用了线性分布假设,这导致了油藏开发早期对非稳态窜流量的计算精度不足.因此,本文提出了一种两套节点格林元法的EDFM数值模拟方法.两套节点格林元法的核心思想是将压力节点与流量节点区分开,一套压力节点设置在单元顶点,另一套流量节点设置在网格边的中点,满足局部物质守恒、具有二阶精度的同时,可适用于任意网格类型.本文将两套节点格林元法与EDFM耦合,采用了非稳态渗流控制方程的边界积分形式推导了基质网格与裂缝网格之间传质量的新格式,代替了线性分布假设以提高模拟精度;此外,修正后的EDFM能适应任意形态的基质网格剖分,拓展了原始EDFM仅适用于矩形基质网格、难以考虑复杂油藏边界的局限性.研究表明:通过对比商业模拟软件tNavigator?LGR模块与原始EDFM,验证了本文模型具有较高的早期计算精度;以复杂油藏边界-缝网-SRV分区模型为例,通过对比SFEM-COMSOL商业模拟软件,验证了本文模型处理复杂问题的适应性.本文研究可用于裂缝性油藏开发动态的精确模拟.     

Numerical simulation of water entry of twin wedges

The hydrodynamic problem of twin wedges entering water vertically at constant speed is analysed based on the velocity potential theory. The gravity effect on the flow is ignored based on the assumption that the ratio of the entry speed to the acceleration due to gravity is much larger than the time scale of interest. The problem is solved using the complex velocity potential together with the boundary element method through three stages. When the body touches water, the similarity solution is obtained for each wedge in isolation. This is used as the initial solution at the second stage for the time stepping technique for each wedge in a stretched system defined through the ratio of the Cartesian system to the distance the wedge travelled into water. When the disturbed zone of each wedge begins to affect the flow generated by the other wedge, the stretched system is abandoned and the original system is used. At the third stage the full interactions between the two wedges are included. Various results are provided for the wave elevation, pressure distribution and force at different deadrise angles. They are compared with those obtained from a single wedge and the interaction effect is investigated.     

MPS方法数值模拟楔形体入水问题

入水问题是船舶海洋工程中典型的流动问题。当船舶在恶劣海况中航行或海洋平台遭遇恶劣天气时,结构物和水体之间往往会出现剧烈的砰击作用。砰击发生时,伴随着结构物湿表面的变化、自由液面的翻卷和破碎等强非线性现象。本文采用本课题组自主开发的基于移动粒子半隐式法MPS(Moving Particle Semi-Implicit Method)的求解器MLParticle-SJTU对二维楔形体入水问题进行了数值模拟。选取斜升角为30°的楔形体模拟其入水过程,研究了不同粒子布置方式对于计算结果如垂向水动力和自由面变化的影响,并与MLM砰击模型(Modified Logvinovich Model)的结果进行了比较,吻合较好。在此基础上,进一步研究了不同斜升角对计算结果(垂向水动力和自由面变化)的影响,其中流动分离前的垂向水动力与MLM结果相近,表明了MPS方法能有效地模拟入水问题。     

Assessment of PIV-based analysis of water entry problems through synthetic numerical datasets

The phenomenon of hull-slamming, that is, the sudden impact of a solid body on the water surface, is critical in the design of naval structures. Thus, the development and validation of schemes to predict the slamming load and elucidate energy exchange during water entry are of fundamental importance in a wide range of engineering applications. Recent studies have demonstrated the possibility of using direct flow measurements from particle image velocimetry (PIV) to investigate the kinetics of water entry. Specifically, these efforts have contributed a first characterization of the hydrodynamic loading on impacting wedges and of the energy imparted to the water pile-up and the spray jets. Here, we seek to provide a thorough assessment of such a PIV-based approach through synthetic datasets, in which PIV parameters, such as the camera acquisition rate and the size of the interrogation area, are systematically varied, without experimental confounds. We implement a direct computational framework to study the two-dimensional flow physics generated during the water entry of a rigid wedge. Water and air are treated as immiscible phases and their relative motion is utilized to track the free surface dynamics. Our results show that the PIV-based methodology allows for an accurate reconstruction of the pressure field from the measured velocity field, except for early stages of the impact and for a small region close to the free surface. We also demonstrate that the reconstruction is only marginally affected by the spatial resolution, while a sufficiently high acquisition frequency is required to correctly predict the pressure field in the pile-up region. The proposed computational framework can also find application in the analysis of less studied aspects of water entry problems, such as cycling loading, flow transitions and separation, and formation of spray jets.     

Oblique water entry of a wedge into waves with gravity effect

The hydrodynamic problem of a two dimensional wedge entering waves with gravity effect is analysed based on the incompressible velocity potential theory. The problem is solved through the boundary element method in the time domain. The stretched coordinate system in the spatial domain, which is based on the ratio of the Cartesian system in the physic space to the vertical distance the wedge has travelled into the water, is adopted based on the consideration that the decay of the effect of the impact away from the body is proportional to this ratio. The solution is sought for the total potential which includes both the incident and disturbed potentials, and decays towards the incident potential away from the body. A separate treatment at initial stage is used, in which the solution for the disturbed potential is sought to avoid the very large incident potential amplified by dividing the small travelled vertical distance of the wedge. The auxiliary function method is used to calculate the pressure on the body surface. Detailed results through the free surface elevation and the pressure distribution are provided to show the effect of the gravity and the wave, and their physical implications are discussed.     

Translationally nonequilibrium free jet boundary layer in the wake behind a wedge

Macroscopic equations obtained as a thin-layer version of the 13-moment Grad equations derived from kinetic considerations are used for describing the translationally nonequilibrium monatomic gas flow in a hypersonic free jet boundary layer formed in the wake behind a wedge. This model makes it possible to investigate flows with strong violations of equilibrium with respect to the translational degrees of freedom. A method of constructing the solution of this kinetically justified problem based on the solution of an analogous problem in the Navier-Stokes interpretation is proposed. It is established that for the kinetic variant of the problem considered the gas flow velocity distribution along the separating streamline in a plane orthogonal to the wedge generator coincides with the distribution obtained in solving the Navier-Stokes variant. It is found that taking into account the nonequilibrium nature of the flow with respect to the translational degrees of freedom of the gas particles has no effect on the base pressure and the wake angle.     

Hydroelastic analysis of water impact of flexible asymmetric wedge with an oblique speed

Current paper deals with hydroelastic impact of asymmetric and symmetric wedge sections with oblique speed into calm water. It is aimed to provide a better insight regarding fluid–structure interaction of the wedge sections of a high-speed craft into water in more realistic condition, in the presence of heel angle and oblique speeds. The defined problem is numerically investigated by coupled Finite Volume Method and Finite Element Method under two-way approach consideration. Accuracy of the proposed model is assessed in different steps. The results of current method are compared against previous experimental, numerical and theoretical methods and good agreement is displayed in these comparisons. Subsequently, the method is used in order to examine the fluid and structure behavior during the elastic impact of the wedge into water. Accordingly, four different physical situations are simulated. In the first part, symmetric impact with no oblique speed is simulated. The results of this part show fluctuations in vertical force and pressure of the midpoint during the impact time. Also, the relation of deadrise with deflection and pressure is observed in this part. In the second part, heel angle is also taken into consideration. It is concluded that the pressure and deflections at the right side of the wedge reduce, but these parameters increase at the left side. Moreover, it is observed that, the pressure at the midpoint of the left side of the wedge with deadrise angle of 10°, becomes negative, when the wall of the flexible wedge reaches its largest deflection. It is also observed that, the pressure at left side of the wedge with deadrise angle of 20°, reaches zero. Such behavior does not occur for the wedges of 30° and 45° deadrise angles. In the third part of simulations, oblique water entry of a flexible wedge of 20° deadrise angle is simulated, and no heel angle is considered. Harmonic behavior is observed for the vertical force, horizontal force, pressure of the midpoint and its deflection. First peaks of all of these variables are larger than the second peak. The obtained results lead us to conclude that an increase in oblique speed yields larger deflection and pressure at the right side. Meanwhile, no significant effect is observed for the left side of the wedge. Also, larger oblique speed is found to yield larger forces and angular moment. Final part of simulations involves the oblique water entry of a flexible wedge of 5° heel angle. Comparison of the results in the final part with that of third part, show that heel angle affects the pressure and deflection at both sides of the wedge. It is also observed that pressure and deflections of the left side increase, while those of right side increase. It is also seen that, similar as in the case of no heel angle, an increase in oblique speed leads to an increase of pressure and deflection at the starboard. It also leads to an increase in frequency of the vibration at right side.     

Elastic wedge impact onto a liquid surface: Wagner's solution and approximate models

The problem of elastic wedge impact onto the free surface of an ideal incompressible liquid of infinite depth is considered. The liquid flow is two-dimensional, symmetric and potential. The side walls of the wedge are modelled as Euler beams, which are either simply supported or connected to the main structure by linear springs. The liquid flow, the deflection of wedge walls and the size of wetted region are determined simultaneously within the Wagner theory of water impact. We are concerned with the impact conditions of strong coupling between the hydrodynamic loads and the structural response. The coupling is well pronounced for elastic wedges with small deadrise angles. This is the case when the fully nonlinear models fail and approximate models based on the Wagner approach are used. In contrast to the existing approximate models, we do not use any further simplifications within the Wagner theory. Calculations of the velocity potential are reduced to analytical evaluation of the added-mass matrix. Hydrodynamic pressures are not evaluated in the present analysis. In order to estimate the maximum bending stresses, both stages when the wedge surface is partially and totally wetted are considered.Three approximate models of water impact, which are frequently used in practical computations, are examined and their predictions are tested against the present numerical solution obtained by the normal mode method within the Wagner theory. It is shown that the decoupled model of elastic wedge impact, which does not account for the beam inertia, provides a useful formula for estimating the maximum bending stress in thick wedge platings.     

90°锥头弹丸不同速度下垂直入水冲击引起的空泡特性

用高速摄像拍摄了90°锥头弹丸低速入水的空泡形态演变过程,全面讨论了不同入水冲击速度下空泡的闭合方式及其演变过程,分析了空泡闭合时间、闭合点水深和弹头空泡长度随入水速度的变化规律以及不同水深位置空泡直径的变化规律;研究了水幕闭合和近液面空泡收缩上升所形成的射流现象及其相互耦合作用过程,探讨了空泡深闭合后其壁面波动规律。结果表明:随着入水速度的增加,空泡分别发生准静态闭合、浅闭合、深闭合和表面闭合,每种闭合方式对应的一个速度区间;弹头产生空泡的临界入水速度为0.657 m/s;不同水深位置的空泡直径呈现非线性变化;随着水深的增加空泡扩张初速增大,空泡最大直径减小,扩张段缩短,收缩段延长;同一时刻水深越大空泡扩张收缩的加速度也越高;水幕闭合后会产生向上和向下两股射流,向下射流速度较大时会对弹丸运动产生影响;近液面空泡收缩上升时会产生强度正比于空泡体积大小和闭合点水深的射流,并与上两股射流相互耦合形成一股更强的向上射流;空泡深闭合后长度缩短和产生的向下射流使弹丸受力发生改变,弹丸速度因受力产生的变化带动了流体质点速度的波动,进而导致空泡壁面发生波动,壁面波动遵循空泡截面独立扩张原理。