基于LS-DYNA的高速破片水中运动特性流固耦合数值模拟

基于大型有限元分析软件ANSYS/LS-DYNA,建立三维长方体高速破片在水介质中运动的有限元动力分析模型,采用ALE方法对破片在水下运动过程进行流固耦合数值模拟,获得了破片的速度衰减曲线。研究了速度衰减规律、破片墩粗变形规律以及冲击波传播过程。得到高速破片的侵彻能力随速度的变化规律:当初速度大于910~1 115m/s时破片头部将产生显著变形,并大大影响其侵彻阻力;当破片速度较小时,水中侵彻距离随破片初速的增大而增大,当破片速度达到某临界值以后,侵彻距离将随初始速度的增大而逐渐减小。     

Nonequilibrium dissociation of diatomic molecules in flows with convective and diffusive particle transport

The nonequilibrium effects in the kinetics of the thermal dissociation which occurs in a streaming gas of diatomic molecules are investigated. Expressions are obtained for the macroscopic reaction rate and the vibrational energy distribution of the molecules, taking into account the influence of the gas motion. Cases of flows with convective and diffusive particle transport are considered. The dissociating molecules are simulated by cutoff harmonic oscillators. The vibrational kinetics is described in the framework of the so-called diffusion approximation.Translated fron Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 147–153, November–December, 1985.     

Microwave detection of molecules formed by shock wave induced reactions

The paper describes experimental investigations in a shock tube concerning the formation of new molecules using He as a driver gas and a mixture of Kr, CH and NH as driven gas, where Kr serves as diluant. By microwave absorption technique it was possible to detect HCN, CHO and CHNH as reaction products. Obviously, these molecules were formed in the plasma behind the shock wave. In addition, the “swan bands” of C have been observed with optical methods. Received 16 November 1997 / Accepted 7 May 1998     

Viscoplasticity of metals: Comments on statistical approaches to dislocation reactions

Dislocations, i.e., line defects in the crystal structure, are at the origin of viscoplastic deformation in metals. In the literature, the statistical approaches to the dislocation dynamics in terms of dislocation densities are often used. There, the key ingredient for modeling the ubiquitous strain hardening is the interplay between the mobile and forest (i.e., immobile) dislocations, which is captured in terms of reaction-type contributions in the evolution of the dislocation densities. In this paper, we demonstrate that a certain class of such models is in conflict with thermodynamic principles. The origin of this conflict is identified. Specifically, the absence of the reversal processes for any of the reactions is problematic and sharply contrasts to usual chemical reactions. Possible solutions for restoring the thermodynamic admissibility are discussed.     

Nonequilibrium thermal dissociation of diatomic molecules with VV,VT, and VT′ exchanges

Nonequilibrium dissociation in a mixture of anharmonic oscillators — molecules of a diatomic gas and its atoms — is investigated with allowance for the temperature gradient in the boundary layer. The effect of VT, VV, and VT′ exchanges on the dissociation rate is considered, and the nonequilibrium correction to the macroscopic reaction rate, which depends, in particular, on the temperature gradient and degree of dissociation, is determined. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.1, pp. 171–179, January–February, 1994.     

Thermomechanics of solids: Nonequilibrium and irreversibility

Reconciliation of thermal and mechanical interdependence is made possible by casting away the concept of heat. Defined are the dissipation and available energy density as two mutually exclusive quantities; thermal and mechanical changes become two aspects of the same process that coexist and are in continual operation. This is accomplished by interlacing the rotation, deformation and change in element size into one single operation; irreversibility is embedded inherently into the theory. No a priori assumption needs to be made on the constitutive relations which are, in fact, derived for each individual element and time increment. Only the initial slope of the reference material and load history need to be specified. Instead, the surface and volume energy density are assumed to be exchangeable without letting the change of volume with surface area to vanish in the limit, a simplification of classical physics and continuum mechanics that results in the decoupling of thermal and mechanical effects. Complete nonlinearity and finiteness of deformation are retained such that boundary problems can be solved directly by specifying the tractions and/or displacements. Nonequilibrium/irreversible solutions are shown to possess definite limits and to be bounded by the equilibrium/irreversible states whose solutions are proved to be unique. The existence of the isoenergy density function provides an elegant means of resolving the multidimensionality of the problem; the translation of unidimensional data to multidimensional states.     

Dynamic thermo-mechanical coupled simulation of statistically inhomogeneous materials by statistical second-order two-scale method

In this paper,a statistical second-order twoscale(SSOTS) method is developed to simulate the dynamic thcrmo-mechanical performances of the statistically inhomogeneous materials.For this kind of composite material,the random distribution characteristics of particles,including the shape,size,orientation,spatial location,and volume fractions,are all considered.Firstly,the repre.sentation for the microscopic configuration of the statistically inhomogeneous materials is described.Secondly,the SSOTS formulation for the dynamic thermo-mechanical coupled problem is proposed in a constructive way,including the cell problems,effective thermal and mechanical parameters,homogenized problems,and the SSOTS formulas of the temperatures,displacements,heat flux densities and stresses.And then the algorithm procedure corresponding to the SSOTS method is brought forward.The numerical results obtained by using the SSOTS algorithm are compared with those by classical methods.In addition,the thermo-mechanical coupling effect is studied by comparing the results of coupled case with those of uncoupled case.It demonstrates that the coupling effect on the temperatures,heat flux densities,displacements,and stresses is very distinct.The results show that the SSOTS method is valid to predict the dynamic thermo-mechanical coupled performances of statistically inhomogeneous materials.     

Quasiequilibrium model of the kinetics of coupled modes of CO2 molecules

The correct allowance for the influence of anharmonicity in the vibrational spectrum of CO₂ on the level distribution of molecules under nonequilibrium conditions, when the vibrational temperature departs significantly from the gas temperature, has become especially urgent in connection with obtaining generation on a number of long-wavelength transitions of CO₂ molecules [1, 2]. The shifts in the levels of coupled modes (symmetric and deformation) are due mainly to Fermi resonance and can reach a considerable value, comparable with the gas temperature even for low levels. In [3] the main features of the quasisteady level distribution of coupled modes were clarified within the framework of the Treanor model of vibrational kinetics. The influence of the ascending flux of quanta, excited by VV exchange under nonequilibrium conditions, on the vibrational distribution was considered in [4–6]. In the present paper we propose a quasiequilibrium model of CO₂ kinetics, obtained without presuming quasisteadiness of the ascending flux of quanta, and making it possible, in contrast to [3–6] to describe the dynamics of the variation of the distribution of molecules among multiplets as a result of processes of VV exchange and VT relaxation between multiplets, with allowance for possible processes of pumping by outside sources. With a Boltzmann population distribution within the multiplets, having the translational temperature of the gas, the problem of studying relaxation in coupled modes is reduced to the equations for an effective anharmonic oscillator with levels corresponding to the multiplets of CO₂ molecules. In this case the levels of the effective oscillator are degenerate with a multiplicity equal to the number of levels in the corresponding multiplet, and they have an anharmonicity constant dependent on the gas temperature. The population distribution of the effective oscillator can be studied by methods developed for the investigation cf a one-mode anharmonic oscillator. The proposed quasiequilibrium model was used for a numerical calculations of the temporal evolution of the distribution function of CO₂ molecules over the levels of coupled modes under the conditions of an extremely maintained discharge.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 16–22, May–June, 1986.     

On the secondary resonance of a MEMS resonator: A conceptual study based on shooting and perturbation methods

Nonlinear dynamics of a clamped–clamped capacitive micro-beam resonator subjected to subharmonic excitation of order one-half is studied. The micro-beam resonator is sandwiched with two piezoelectric layers throughout the length, and as a result of piezoelectric actuation a tensile/compressive axial load is induced along the length which is used as a frequency tuning tool. The resonator is subjected to a combination of a bias DC and harmonic AC electrostatic actuations. In order to determine the frequency response subharmonic resonance condition, both perturbation and shooting methods are applied. The stability of the periodic solutions and the bifurcations types are also studied. It is shown that the application of perturbation method imposes some limitations on the order of magnitudes of the terms in the differential equation of the motion; as a result out of the domain where the ordering assumption of the perturbation solution does not hold, some periodic solutions as well as some vital bifurcation points are missed. It is shown that on the frequency domain, the resonator exhibits both softening and hardening behaviors whereas this is not predicted by the perturbation scheme. The effect of DC and AC actuation voltages on the qualitative response of the system is determined. It is shown that based on the polarity of the piezoelectric actuation, the frequency response curves can be shifted both in forward and backward directions which can be used in the design of novel RF MEMS filters/sensors.     

Theoretical statistical solution and numerical simulation of heterogeneous brittle materials

The analytical stress-strain relation with heterogeneous parameters is derived for theheterogeneous brittle materials under a uniaxial extensional load, in which the distributions of theelastic modulus and the failure strength are assumed to be statistically independent. This theoreticalsolution gives an approximate estimate of the equivalent stress-strain relations for 3-D heterogeneousmaterials. In one-dimensional cases it may provide comparatively accurate results. The theoreticalsolution can help us to explain how the heterogeneity influences the mechanical behaviors, Further, anumerical approach is developed to model the non-linear behavior of three-dimensional heterogeneousbrittle materials. The lattice approach and statistical techniques are applied to simulate the initialheterogeneity of heterogeneous materials. The load increment in each loading stage is adaptivelydetermined so that the better approximation of the failure process can be realized. When the maximumtensile principal strain exceeds the failure strain, the elements are considered to be broken, which canbe carried out by replacing its Young‘s modulus with a very small value. A 3-D heterogeneous brittlematerial specimen is simulated during a full failure process. The numerical results are in good agreementwith the analytical solutions and experimental data.     

高速侵彻弹体层合木靶脱壳

利用实验和数值模拟方法研究一种利用层合松木靶作为脱壳装置的机械式脱壳方法。首先讨论了一种正交各向异性材料模型用于高速侵彻木材的可行性及其参数变化规律,结合美军的高速侵彻实验数据对数值模拟方案进行了验证与确认。在此基础上,讨论了不同弹靶作用下含弹托弹体对松木靶的侵彻/贯穿规律。数值模拟与实验研究结果表明：在垂直入射条件下,通过合理的层合木靶设计可对次口径发射弹体有效脱壳,高速侵彻弹体可垂直入射靶板,弹体速度衰减可控;在初始攻角入射条件下,层合靶将使高速侵彻弹体攻角放大。随入射速度增加,弹体贯穿层合木靶消耗动能增加,体现了木材具有明显的应变率增强效应。

     

Direct numerical simulation of flexible molecules and data-driven molecular conformation

The present work aims at performing a molecular dynamics modeling of suspensions composed of flexible linear molecules. Molecules are represented by a series of connected beads, whose dynamics is governed by three potentials: the extension potential affecting the elongation of segments connecting consecutive beads, the one governing the molecule bending and finally the Lennard-Jones describing the interaction of non-consecutive beads. A population of non-interacting molecules is simulated in elongation and shear flows for different flow and molecule parameters. The flow-induced conformation is analyzed in the different considered situations. Finally a model for predicting the evolution of the population conformation will be obtained by using deep-learning.     

The effect of gas motion on the reaction kinetics of vibrationally excited molecules

The effect is considered of gas motion on the kinetics of reactions whose energy threshold is overcome as the result of vibrational excitation of the reactant molecules. The conditions are determined for which such an effect may be realized. An expression is obtained for the rate of thermal dissociation of diatomic molecules considered as harmonic oscillators representing a small impurity in a monoatomic inert gas; the expression depends explicitly and nonlinearly on the divergence of the flow velocity.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 100–105, November–December, 1984.     

基于详细化学反应动力学的激光点火模型

为研究含能材料在激光作用下的点火特性,从基本守恒方程组出发,基于详细化学反应动力学模型,建立了含能材料激光点火的气相模型.利用所建立的模型对RDX在激光辐照下的点火过程进行了数值模拟,得到了点火过程中的瞬时温度分布和组分分布,对点火特性进行了分析.计算得到的不同激光功率密度下的RDX点火延迟时间与文献结果一致.     

A review of the multiple scale and reductive perturbation methods for deriving uncoupled nonlinear evolution equations

The method of multiple scales and the reductive perturbation method are reviewed and compared for calculating uniformly valid solutions of ion-acoustic waves. It is shown that they differ only in the choice of characteristic scales used in nondimensionalizing the problem. The inconsistent initial scaling used in the reductive method is later corrected by introducing artificially scaled variables to describe the solution in a form which remains uniformly valid in the far field. As a result this solution obeys the same evolution equations in both methods. Uncoupled nonlinear evolution equations describing waves traveling in opposite directions are derived using the multiple scale method for a number of different physical problems including the case of a general Galilean invariant system in conservation form.     

Analog computer simulation of a duffing oscillator and comparison with statistical linearization

We present the results of an analog computer simulation of a Duffing oscillator, i.e. of a damped anharmonic oscillator with a cubic non-linearity driven by Gaussian white noise. The simulations were performed for wide ranges of parameter values. The experimentally obtained spectral densities are compared with those obtained analytically using the method of statistical linearization.     

Bifurcation analysis of a nonlinear pendulum using recurrence and statistical methods: applications to fault diagnostics

The problem of maintenance of expensive and heavy systems has increased the need for powerful tools to analyze their performance. The methods of recurrence plots (RPs) and statistical measurement have been used as data-driven tools for diagnostics with no possibility of classifying the nature of defect and poor ability to localize it. In order to enhance the efficiency of the forecast, innovative approaches consist of using physics-based features to train a data-based assessment methods. This requires proper analysis of the physical system using appropriate methods. For this purpose, this paper focusses on the bifurcation dynamics of nonlinear systems using the recurrence and statistical methods. Considering the nonlinear pendulum as a model, the qualitative behavior of the system is discussed through the bifurcation diagram of some recurrence quantification analysis (RQA) parameters, namely the recurrence rate, determinism, and laminarity. These parameters are used to measure the level of complexity and transition from regular to chaotic motion and vice versa. Statistical parameters such as crest factor, skewness, and kurtosis are used to identify various bifurcation and amplitudes in the system, and to measure the orientation and the level of asymmetry. Plots of recurrence diagrams and histograms are presented to support our observations. Examples of detection of dynamic changes using these two methods are provided. The interesting results obtained in this paper show that statistical methods complement results obtained from RPs. In addition, the paper demonstrates how the RPs can be employed in conjunction with the physics-based model.