Numerical simulation of quench propagation at early phase by high-order methods

In this paper a high-order discontinuous Galerkin (DG) spectral element method (SEM) is introduced to deal with thermo-hydraulic quench simulation in superconducting magnets, specifically in the case of cable-in-conduit conductors (CICC). We will focus on 1D quench propagation at early phase which is important to understand the mechanism of quench propagation and protect the superconducting magnets. A second-order polynomial interpolation to the helium heat capacity is introduced in this study, and boundary conditions are imposed along the characteristics to overcome the typical difficulties encountered in numerical studies. Explicit fourth order Runge–Kutta time integration is used to match the high accuracy in space. Numerical results demonstrate the advantages in accuracy of this method when used to simulate early quench development.     

Numerical simulation of intergranular and transgranular crack propagation in ferroelectric polycrystals

We present a phase-field model to simulate intergranular and transgranular crack propagation in ferroelectric polycrystals. The proposed model couples three phase-fields describing (1) the polycrystalline structure, (2) the location of the cracks, and (3) the ferroelectric domain microstructure. Different polycrystalline microstructures are obtained from computer simulations of grain growth. Then, a phase-field model for fracture in ferroelectric single-crystals is extended to polycrystals by incorporating the differential fracture toughness of the bulk and the grain boundaries, and the different crystal orientations of the grains. Our simulation results show intergranular crack propagation in fine-grain microstructures, while transgranular crack propagation is observed in coarse grains. Crack deflection is shown as the main toughening mechanism in the fine-grain structure. Due to the ferroelectric domain switching mechanism, noticeable fracture toughness enhancement is also obtained for transgranular crack propagation. These observations agree with experiment.     

Numerical simulation of ultrasonic wave propagation in anisotropic and attenuative solid materials

The axisymmetric elastodynamic finite element code developed is capable of predicting quantitatively accurate displacement fields for elastic wave propagation in isotropic and transversely isotropic materials. The numerical algorithm incorporates viscous damping by adding a time-dependent tensor to Hooke's law. Amplitude comparisons are made between the geometric attenuation in the far field and the corresponding finite element predictions to investigate the quality and validity of the code. Through-transmission experimental measurements made with a 1 MHz L-wave transducer attached to an aluminum sample support the code predictions. The algorithm successfully models geometric beam spreading dispersion and energy absorption due to viscous damping. This numerical model is a viable tool for the study of elastic wave propagation in nondestructive testing applications.     

Numerical simulation of the guided Lamb wave propagation in particle reinforced composites

This paper deals with the investigation of the Lamb wave propagation in particle reinforced composites excited by piezoelectric patch actuators. A three-dimensional finite element method (FEM) modeling approach is set up to perform parameter studies in order to better understand how the Lamb wave propagation in particle reinforced composite plates is affected by change of central frequency of excitation signal, volume fraction of particles, size of particles and stiffness to density ratio of particles. Furthermore, the influence of different arrangements is investigated. Finally, the results of simplified models using material data obtained from numerical homogenization are compared to the results of models with heterogeneous build-up. The results show that the Lamb wave propagation properties are mainly affected by the volume fraction and ratio of stiffness to density of particles, whereas the particle size does not affect the Lamb wave propagation in the considered range. As the contribution of the stiffer material increases, the group velocity and the wave length also increase while the energy transmission reduces. Simplified models based on homogenization technique enabled a tremendous drop in computational costs and show reasonable agreement in terms of group velocity and wave length.     

水下稳态涡流场声传播的数值模拟和实验研究

声波穿过涡流场时声传播特性会发生改变,可由此判断涡流场的存在及其特征参数。文章首先推导了二维稳态涡流场的射线微分方程组,实现了声波通过涡流场的声线轨迹模拟。然后开展水池超声传感器实验,采用现代计算流体力学(Computational Fluid Dynamics,CFD)仿真方法获取涡流场参数。最后将通过涡流场声信号的时延结果,与实验测量值进行对比分析。结果表明,对于涡核半径为30 mm,最大切向速度为1.2 m·s^-1的涡流场,数值模拟误差小于10%,验证了基于射线声学的水下稳态涡流场声传播数值模拟的有效性。     

Numerical simulation of an adaptive optics system with laser propagation in the atmosphere

A comprehensive model of laser propagation in the atmosphere with a complete adaptive optics (AO) system for phase compensation is presented, and a corresponding computer program is compiled. A direct wave-front gradient control method is used to reconstruct the wave-front phase. With the long-exposure Strehl ratio as the evaluation parameter, a numerical simulation of an AO system in a stationary state with the atmospheric propagation of a laser beam was conducted. It was found that for certain conditions the phase screen that describes turbulence in the atmosphere might not be isotropic. Numerical experiments show that the computational results in imaging of lenses by means of the fast Fourier transform (FFT) method agree well with those computed by means of an integration method. However, the computer time required for the FFT method is 1 order of magnitude less than that of the integration method. Phase tailoring of the calculated phase is presented as a means to solve the problem that variance of the calculated residual phase does not correspond to the correction effectiveness of an AO system. It is found for the first time to our knowledge that for a constant delay time of an AO system, when the lateral wind speed exceeds a threshold, the compensation effectiveness of an AO system is better than that of complete phase conjugation. This finding indicates that the better compensation capability of an AO system does not mean better correction effectiveness.     

Effect of polymer additives on propagation of turbulence front

A study is made of the propagation of a turbulent region formed by vibrations of plates with holes in solutions of polyoxyethylene and guar resin. Polymer additives are found to appreciably affect the motion of a turbulence front.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 2, pp. 220–224, August, 1982.     

Numerical simulation of fatigue crack propagation in compressive residual stress fields of notched round bars

In this paper, the influence of the residual compressive stresses induced by roller burnishing on fatigue crack propagation in the fillet of notched round bar is investigated. A 3D finite element simulation model of rolling has allowed to introduce a residual stress profile as an initial condition. After the rolling process, fatigue loading has been applied to three‐point bending specimens in which an initial crack has been introduced. A numerical predictive method of crack propagation in roller burnished specimens has also been implemented. It is based on a step‐by‐step process of stress intensity factor calculations by elastic finite element analyses. These stress intensity factor results are combined with the Paris law to estimate the fatigue crack growth rate. In the case of roller burnished specimens, a numerical modification concerning experimental crack closure has to be considered. This method is applied to three specimens: without roller burnishing, and with two levels of roller burnishing (type A and type B). In all these cases, the computational finite element predictions of fatigue crack growth rate agree well with the experimental measurements. The developed model can be easily extended to crankshafts in real operating conditions.     

Mathematical simulation of the front processes of combustion of condensed systems in view of the velocity of propagation of heat

It is suggested to use the hyperbolic heat equation for mathematical simulation of the front processes of combustion of energy-intensive condensed systems (CS). The correlation between the velocity of propagation of thermal wave with the velocity of front motion, the thermal effect of the reaction of thermal decomposition, and the value of heat flux delivered to the surface is analytically determined. Numerical solutions of hyperbolic heat equation are obtained for unsteady-state mode of ignition of energy-intensive materials. The agreement between calculated dependences and experimental data on the combustion of energy-intensive compounds is considered.     

Estimation of the fracture front propagation time in structural components

We discuss special features of the fracture front propagation in structural components (variable-thickness disks and plates, internally pressurized thick-walled vessels). The upper and lower bounds of the relative time of the front propagation have been estimated. Some recommendations regarding the application of the results obtained are provided. __________ Translated from Problemy Prochnosti, No. 6, pp. 13–24, November–December, 2007.     

不同强度分布激光水平传输稳态热晕效应的数值模拟

采用数值模拟的方法,以高斯光束、平顶光束、有中心遮拦的平顶光束为例,研究了激光在均匀大气传输过程中产生的稳态热晕效应。详细分析了发射功率、传输距离、光束直径、横向风速四种参数对热晕效应的影响,得到了以上三种光束的斯特列尔比和峰值偏移量随广义热畸变参数N的变化关系。数值仿真结果表明,在其他参数一致的情况下,发射功率越大、传输距离越长,热晕效应越强;而光束直径和横向风速的增加会减小热晕效应;不同激光强度分布对热晕效应的影响不同。在同样广义热畸变参数N的条件下,高斯光束的热晕效应最严重,平顶光束次之,空心光束的热晕效应最小。

     

Numerical simulation of the crystallization front inversion in oxide single crystals grown from melt using the Czochralski method

The process of pulling a Czochralski grown oxide single crystal out from melt has been numerically simulated for the first time from the moment of seeding until attaining a stationary growth regime. It is established that the phase boundary inversion at the lateral (shouldering) growth stage is a rather complicated process accompanied by intense oscillations of the crystallization rate. Diagrams illustrating variations of the crystal shape during this process are presented.     

Numerical simulation of light propagation through highly-ordered titania nanotube arrays: dimension optimization for improved photoabsorption

Propagation of electromagnetic waves in the ultraviolet-visible range (300 to 600 nm) through a unique highly-ordered titania nanotube array structure is studied using the computational technique of Finite Difference Time Domain (FDTD). Through numerical simulation the transmittance, reflectance and absorbance of the nanotube-arrays are obtained as a function of tube length and diameter. The nanotube-arrays are found to completely absorb light having wavelengths less than approximately 330 nm. For wavelengths above 380 nm absorption increases as a function of nanotube length, while above 435 nm absorption increases with decreasing pore size. Computational simulations closely match experimental measurements, indicating the suitability of the computational technique for guiding material optimization.     

Numerical models of shear fracture propagation

Two-dimensional numerical models using the displacement discontinuity method and the PATH algorithm show that shear fractures can propagate in three different patterns, depending on the lateral normal stress (LNS). Under 0 or very small LNS they propagate by open wing cracks, as shown in many previous results. Under increasing LNS, the extension consists first of a short wing crack followed by a succession of small closed and open segments forming a stair-step pattern. As the LNS increases, the length of the closed segments increases, thus changing the general orientation of the extension. Under sufficiently high LNS the transition is complete, and propagation takes place as a closed shear fracture. These results provide a remarkable confirmation of the theoretical predictions made by Cherepanov [J. Appl. Math. Mech. (English transl. of Prikl. Mate. Mekh.) 30 (1966) 96]. They also explain the difference between the fracture parameters in tension and in shear.     

Drying of porous building materials: hydraulic diffusivity and front propagation

One-dimensional drying of a porous building material is modelled as a nonlinear diffusion process. The most difficult case of strong surface drying when an internal drying front is created is treated in particular. Simple analytical formulae for the drying front and moisture profiles during second stage drying are obtained when the hydraulic diffusivity is known. The analysis demonstrates the origin of the constant drying front speed observed elsewhere experimentally. Application of the formulae is illustrated for an exponential diffusivity and applied to the drying of a fired clay brick.

---

Résumé Le séchage d'un matériau poreux est décrit par l'équation de diffusion non linéaire. Pour un coefficient de diffusion donné, des formules analytiques simples sont obtenues pour les profils hydriques et pour le front de séchage. Le cas, difficile à traiter, où la surface du matériau est éventuellement sèche, est considéré en détail. L'analyse montre l'origine de la vitesse constante du front de séchage, qui a été observée dans des études expérimentales indépendantes. L'application des formules au séchage d'une brique d'argile est illustrée pour un coefficient de diffusion qui dépend exponentiellement du contenu hydrique.

     

Numerical investigation of similarity laws for blast simulation: Open-field propagation and interaction with a biomechanical model

In a military context, blast loadings are encountered, with more or less severe consequences on structures in interaction with a shock wave, which travels in a field. With a need of protecting the fighter, the understanding of this physical dynamic phenomenon is of extreme importance to avoid body trauma induced by the primary blast. In this context, this study explores simulations of blast loading, and its interaction with a human torso biomechanical model previously developed, based on referenced experiments from the literature. The concept of similarities is also studied (peak pressure equivalence for long distance and large explosive mass and short distance and small explosive mass). Validations of numerical results are conducted using experimental data from the literature in terms of pressure peak as a function of scaled distance, and also in terms of pressure time history in the biomechanical model. Good agreement between the numerical results and the experimental peak pressure is achieved.     

构件跌落碰撞的数值模拟研究

针对构件跌落碰撞问题,将Hertz-damp模型与有限元方法结合,给出了结构碰撞位移突加约束条件,建立了碰撞结构体系的动力方程。该方法能考虑碰撞体质量分布对碰撞过程的影响,同时可描述结构碰撞过程中的刚度非线性和能量损失。建立了构件跌落碰撞模型,初步分析了跌落构件质量、质量分布以及跌落高度等参数对碰撞过程的影响。研究表明:结构碰撞模拟中碰撞体质量沿接触面法线轴向不对称时应考虑其质量分布,否则会高估碰撞反应;碰撞荷载峰值和单次碰撞持续时间与跌落构件质量和跌落高度均成指数关系,碰撞荷载峰值随跌落构件质量和跌落高度增大而增大,而单次碰撞持续时间则随跌落构件质量增大而增大,随跌落高度增大而减小。这些规律能为减轻结构碰撞破坏研究提供参考。