Study on the Mechanism of Detonation to Quasi-detonation Transition

In this paper, the mechanism of detonation to quasi-detonation transition was discussed, a new physical model to simulate quasi-detonation was proposed, and one-dimensional theoretical and numerical simulation was conducted. This study firstly demonstrates that the quasi-detonation is of thermal choking. If the conditions of thermal choking are created by some disturbances, the supersonic flow is then unable to accept additional thermal energy, and the CJ detonation becomes the unstable quasi-detonation precipitately. The kinetic energy loss caused by this transition process is firstly considered in this new physical model. The numerical results are in good agreement with previous experimental observations qualitatively, which demonstrates that the quasi-detonation model is physically correct and the study are fundamentally important for detonation and supersonic combustion research.

氨热法生长氮化镓晶体的数值模拟

氮化镓晶体是继单晶硅之后的一种新型的半导体材料. 本文利用有限体积法模拟了氨热法生长氮化镓晶体中流场的瞬态特性, 研究了隔板开孔10％时流场结构、温度场、浓度场. 发现对于隔板开孔率(10％)的情形, 中心开孔及边缘间隙的平均速度表现为振荡的特性, 中心开孔速度大多是正的, 边缘开孔大多是负的. 大的温度梯度发生在在高压釜壁面与液体的交界处与隔板周围. 物质由多孔介质区向生长区输运

气泡/液滴Marangoni迁移数值模拟研究

本文利用Level Set方法,数值模拟了微重力情况下气泡/液滴的Marangoni迁移现象,分析了Marangoni数对迁移速度的影响.数值模拟结果表明,随着Marangoni数的增大,非线性热对流效应的影响会逐渐增大,导致沿相界面温度分布趋于均匀,从而降低迁移运动的驱动力,使气泡/液滴的迁移速度随Marangoni数的增加而逐渐减小.

结构钢中绝热剪切带形成与扩展的光学观测与数值模拟

绝热剪切带(Adiabatic Shear Band,ASB)是许多金属材料在冲击载荷作用下发生破坏的主要原因之一,它是近年来冲击动力学和损伤力学研究的前沿和热点.相关的理论研究主要针对一维剪切条件,分析应力、应变、剪切速度、材料热物理和力学性能、初始缺陷大小之间的关系,得到一个由多个物理量组合而成的量来判别材料出现剪切带的难易.ASB的实验主要利用Hopkinson压杆、扭杆、压剪炮等加载技术,研究钛合金、钨合金、高强结构钢等材料的剪切带特征,包括局部温度和变形分布、剪切带出现的阈值等.但是,对剪切带演化过程的在位观察及其动态实时演化的研究还较少见,妨碍了人们对由于剪切局部化而导致的材料破坏机理的深入认识.针对45钢,在Hopkinson压杆上,开展了不同冲击加载条件下剪切带演化过程的在位观察及可视化研究.利用自行设计的高分辨力的光学观测系统和基于数字相关理论的图像处理软件,捕捉了单一试样在冲击加载条件下ASB逐渐形成和扩展的过程.同时,利用LS-DYNA商业程序对试样的冲击压缩过程进行了数值模拟,所得主要结果与实验观测基本一致.

基于SPH方法的溃坝流动数值模拟

应用光滑粒子流体动力学方法对溃坝问题进行了数值模拟,在现有SPH方法的理论基础上对密度近似方程进行了重新初始化处理,分析了密度重新初始化对溃坝流动问题的影响,并对SPH数值模拟所得到的结果与试验结果以及移动粒子半隐式方法得到的计算结果进行了比较和分析.结果表明,对密度近似方程进行重新初始化保持了流场内的质量守恒,同时整个计算域内的压力分布更加规则,SPH方法数值模拟得到的结果与实验结果和MPS方法得到的结果非常吻合,验证了改进方案及所编程序的可靠性和计算结果的准确性

液化土层中桶形基础承载力弱化的数值模拟

对动冰载作用下饱和砂土层液化过程中桶形基础承载力的弱化规律进行了数值模拟。建立液化土层的简化计算模型,采用FLAC3D软件分析了等效动冰载作用下土层不同位置的液化度及其主要影响因素;进而将具一定液化度土层的抗力作用等效为沿桶壁的不同刚度的非线性弹簧作用,在给定的位移破坏标准下确定不同液化度土层中桶形基础的承载力,分析液化土层中桶形基础承载力的弱化特征。在文中荷载条件和计算模型下,当土层顶面液化度为0.60、底面液化度为0.06时,土层中桶形基础的承载力降低12%。

动载作用下桶形基础周围土体变形的数值模拟

针对动冰载作用下桶形基础周围土体的液化变形进行了数值模拟.建立饱和砂层中桶形基础的简化计算模型,研究动载作用下砂层竖向和水平向的液化变形.首先与桶形基础动载离心机试验比较,验证了计算模型的可靠性.分析了砂层竖向和水平向液化变形特征,以及砂层变形随动载特性和土性参数变化的规律.研究结果表明:砂层变形随着动载频率减小,振幅加大,砂层弹性模量降低而增大.在该文计算模型和荷载条件下,砂层竖向最大变形发生在距荷载作用边界0.25倍桶高位置处,水平最大变形发生在荷载作用边界处.当动载激振时间超过5h后,土顶面竖向变形约为土底面竖向变形的3倍,距荷载作用边界2.0倍桶高位置处的水平变形仅为荷载作用边界处水平变形的3.3%

水幕减爆防护技术数值仿真

高能炸药爆轰爆炸在极短的时间内产生强烈的冲击波,对周边的结构造成严重的破坏,并导致附近人员的伤亡.水幕减爆作为一种新颖的防护技术,通过在装药或者需要保护结构的外面布置水袋,利用水的加热、压缩和蒸发转移爆炸产生的能量,改变爆炸冲击波的传播方式,减弱爆炸冲击波的影响.建立安全可靠、稳定有效的水幕减爆装置需要大量的参数分析,从而进行优化设计.数值实验仿真提供了研究水幕减爆成本低廉并且没有任何危险的可行方式.传统的网格方法模拟水幕减爆问题存在很多困难.该文应用无网格粒子方法SPH对不同状况下的水幕减爆问题进行了一系列研究,得到一些初步的研究结果,对水幕减爆防护装置的设计和布置具有一定的指导意义.

水冷塔空气涡流导引装置的数值模拟

使用FLUENT软件, 对水冷火电厂的水冷塔换热进行了数值模拟, 解决了物模实验难以模拟热力因素的困难. 同时还对采用"空气涡流导引装置技术"的水冷塔和传统水冷塔, 在不同环境风影响下的换热效率进行了对比. 结果表明, 采用"空气涡流导引装置技术"的水冷塔换热效果优于传统水冷塔, 抵抗大风的能力更强

A finite-element algorithm for modeling variably saturated flows

A general numerical algorithm in the context of finite element scheme is developed to solve Richards’ equation, in which a mass-conservative, modified head based scheme (MHB) is proposed to approximate the governing equation, and mass-lumping techniques are used to keep the numerical simulation stable. The MHB scheme is compared with the modified Picard iteration scheme (MPI) in a ponding infiltration example. Although the MHB scheme is a little inferior to the MPI scheme in respect of mass balance, it is superior in convergence character and simplicity. Fully implicit, explicit and geometric average conductivity methods are performed and compared, the first one is superior in simulation accuracy and can use large time-step size, but the others are superior in iteration efficiency. The algorithm works well over a wide variety of problems, such as infiltration fronts, steady-state and transient water tables, and transient seepage faces, as demonstrated by its performance against published experimental data. The algorithm is presented in sufficient detail to facilitate its implementation.

顺序输送混油过程的二维数值分析

采用二维对流扩散模型对成品油顺序输送中的混油问题进行了数值模拟研究,运用目前求解对流扩散方程比较有效的两步法,对层流态和湍流态顺序输送的混油段流动分别进行了数值计算,分析了层流态和湍流态下的顺序输送规律,表明湍流态输送比层流态输送的混油量要小,并得到了混油截面平均浓度随时间的变化特点,以及湍流扩散系数对混油浓度的影响规律

Hydroelastic dynamic characteristics of a slender axis-symmetric body

The slender axis-symmetric submarine body moving in the vertical plane is the object of our investigation. A coupling model is developed where displacements of a solid body as a Euler beam (consisting of rigid motions and elastic deformations) and fluid pressures are employed as basic independent variables, including the interaction between hydrodynamic forces and structure dynamic forces. Firstly the hydrodynamic forces, depending on and conversely influencing body motions, are taken into account as the governing equations. The expressions of fluid pressure are derived based on the potential theory. The characteristics of fluid pressure, including its components, distribution and effect on structure dynamics, are analyzed. Then the coupling model is solved numerically by means of a finite element method (FEM). This avoids the complicacy, combining CFD (fluid) and FEM (structure), of direct numerical simulation, and allows the body with a non-strict ideal shape so as to be more suitable for practical engineering. An illustrative example is given in which the hydroelastic dynamic characteristics, natural frequencies and modes of a submarine body are analyzed and compared with experimental results. Satisfactory agreement is observed and the model presented in this paper is shown to be valid.

基于CDEM的高桩码头承载力数值模拟

主要采用基于连续介质的离散元方法(CDEM)对高桩码头结构进行数值模拟。对CDEM方法在结构分析中应用验证可行的基础上,采用随机网格的方法对不规则抛石棱体进行模拟,根据实际的高桩码头建立数值计算模型,分析了其在不同加载条件下承载力的大小,得出码头结构破坏以及破坏程度的临界荷载。同时提取结构关键点的位移,将其与相对应的不同的加载等级进行详细分析,得出了破坏前后不同的直线型与抛物线型增长方式,为海洋工程结构的设计与分析提供了有价值的参考。

岩体中爆腔内压力脉动特征和爆炸能量分布的数值模拟

分析了对岩体中爆炸能量分布和爆腔内压力研究的现状, 为进行这方面的研究, 在块体离散元基础上独立开发了可变形多尺度计算模型和爆源模型, 改进后的数值模型适合研究爆源近区的爆炸效应. 通过该模型对岩体中爆腔内压力和爆炸能量分布进行了数值模拟, 分析了爆腔内压力脉动特征, 指出爆源近区岩体结构性质对爆腔内压力脉动规律影响不大, 爆腔内压力峰值与炸药量无关; 结合热力学和弹性力学知识分析了不同条件下爆炸能量分布规律, 研究了岩体破坏对爆炸能量分布的影响, 给出了岩石断裂破坏消耗能量、变形能、用于抛掷的能量的比例

PREDICTION OF VORTEX-INDUCED VIBRATION OF FLEXIBLE RISER USING AN IMPROVED WAKE-OSCILLATOR MODEL

Based on improving the wake-oscillator model, an analytical model for vortex-induced vibration (VIV) of flexible riser under non-uniform current is presented, in which the variation of added mass at lock-in and the nonlinear relationship between amplitude of response and reduced velocity are considered. By means of empirical formula combining iteration computation, the improved analytical model can be conveniently programmed into computer code with simpler and faster computation process than CFD so as to be suitable to application of practical engineering. This model is validated by comparing with experimental result and numerical simulation. Our results show that the improved model can predict VIV response and lock-in region more accurately. At last, illustrative examples are given in which the amplitude of response of flexible riser experiencing VIV under action of non-uniform current is calculated and effects of riser tension and flow distribution along span of riser are explored. It is demonstrated that with the variation of tension and flow distribution, lock-in region of mode behaves in different way, and thus the final response is a synthesis of response of locked modes.