Development of Mechanics in Support of Rocket Technology in Ukraine

The paper analyzes the advances of mechanics made in Ukraine in resolving various problems of space and rocket technology such as dynamics and strength of rockets and rocket engines, rockets of different purpose, electric rocket engines, and nonstationary processes in various systems of rockets accompanied by phase transitions of working media. Achievements in research on the effect of vibrations and gravitational fields on the behavior of space-rocket systems are also addressed. Results obtained in investigating the reliability and structural strength durability conditions for nuclear installations, solid- and liquid-propellant engines, and heat pipes are presented     

Evolution of damage-induced nonlinearity in proximity of discontinuities in concrete

Understanding the mechanisms and modalities of damage progression close to discontinuities in solids, such as joints, is of great importance for applications in different fields. The interaction between damage and elasticity causes a nonlinear elastic response of the sample to a stress excitation (e.g. in the ultrasonic frequency range). Extracting physical or mechanical information on the sample properties from recorded ultrasonic signals requires a realistic model and an efficient detection method, as it will be discussed in this paper. We study here the successive phases that concrete samples with discontinuities enter by progressively increase the applied external load. Considerations on the mechanisms of damage progression are derived from experimental data using a Preisach–Mayergoyz space approach, developed in order to capture all the observed behaviors.     

以``大科学''观进行力学教学

 阐述大学的力学教学应该将培养学生的思维方法、学习方法、学习兴趣和科学观念作为责 任，把力学当作一门承上启下的课程来进行教学实践，让学生们通过力学的学习尽快进入大 学生这一角色. 力学是大学第一门物理课程，力学教师有义务为后面的课程开辟道路，做好 铺垫，这是力学教学的任务.     

Criterion of Hyperbolicity in Hyperelasticity in the Case of the Stored Energy in Separable Form

We consider the equations of hyperelasticity for isotropic solids in the Eulerian coordinates in a special case where the specific stored energy is a sum of two functions. The first one, the hydrodynamic part of the energy, depends only on the solid density and the entropy, and the second one, the shear energy, depends on the invariants of the Finger tensor in such a way that it is unaffected by the volume change. A new sufficient criterion of hyperbolicity for such a system is formulated: if the sound velocity is real and a symmetric 3×3 matrix determined in terms of the shear energy is positive definite on a one-parameter family of surfaces of the unit-determinant deformation gradient, the equations are hyperbolic.     

混凝土中爆炸模拟的数值方法比较

应用AUTODYN商业软件对装药在混凝土靶介质中的爆炸过程进行数值模拟,对比了混凝土中爆炸模拟的不同方法。将各数值方法模拟得到的混凝土毁伤特征尺寸与相关实验数据进行比较,研究了各数值方法的准确性及描述混凝土毁伤特征的可靠性参数。结果发现：Euler算法计算的压力最大,SPH和Lagrange耦合算法计算的压力最小;Euler算法和SPH算法计算的压力衰减过程会发生比较显著的扰动。以不同的平均网格尺寸对数值模拟结果的影响分析了各种数值方法的稳定性。比较靶体上同一测点的压力时间历程表明网格变化对测点处压力影响不大。通过应用各种数值方法模拟再现混凝土中的爆炸过程,揭示了各种方法的优缺点,明确了现有商业软件模拟混凝土中爆炸的适用性。     

Formation of shock waves in flow of charge carriers in semiconductors

A hydrodynamic model of the physics of semiconductors is studied numerically. It is shown that the solution of the problem of an (n⁺-n-n⁺) ballistic diode has a shock wave. This problem is solved using an iterative method. An economical conservative semi-implicit difference scheme is developed for search of a numerical solution. Siberian State Geodetic Academy, Novosibirsk 630108. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 3–10, September–October, 1999.     

General solutions of weakened equations in terms of stresses in the theory of elasticity

The general solutions of some weakened systems of equations expressed in terms of stresses in the isotropic theory of elasticity are analyzed. These systems are not equivalent to the classical one and involve the equilibrium equations and only three of the six equations of compatibility (either diagonal or off-diagonal ones). In the framework of elasticity theory, an equivalence of the formulations of quasistatic boundary value problems based on such systems and expressed in terms of stresses is discussed.     

Heat transfer in the neighborhood of a point of inflection in the leading edge of a plate in hypersonic flight

A study is made of the three-dimensional flow of a viscous gas around a flat plate with an inflection in the generator of the leading edge in the case of strong interaction between the exterior hypersonic flow and the boundary layer. Numerical solutions to the problem are obtained. It is shown that near points of inflection of the profile of the leading edge of a flat wing strong self-induced secondary flows can be formed together with associated local peaks of the heat fluxes and the friction.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 40–45, May–June, 1980.     

Advances in the Application of High-order Techniques in Simulation of Multi-disciplinary Phenomena

This paper describes the development of a comprehensive high-fidelity algorithmic framework to simulate the three-dimensional fields associated with multi-disciplinary physics. A wide range of phenomena is considered, from aero-acoustics and turbulence to electromagnetics, non-linear fluid-structure interactions, and magnetogasdynamics. The scheme depends primarily on "spectral-like," up to sixth-order accurate compact-differencing and up to tenth-order filtering techniques. The tightly coupled procedure suppresses numerical instabilities commonly encountered with high-order methods on non-uniform meshes, near computational boundaries or in the simulation of nonlinear dynamics. Particular emphasis is placed on developing the proper metric evaluation procedures for three-dimensional moving and curvilinear meshes so that the advantages of higher-order schemes are retained in practical calculations. A domain-decomposition strategy based on finite-sized overlap regions and interface boundary treatments enables the development of highly scalable solvers. The utility of the method to simulate problems governed by widely disparate governing equations is demonstrated with several examples encompassing vortex dynamics, wave scattering, electro-fluid plasma interactions, and panel flutter.     

Effect of Elevated Velocity of Particles in Groundwater Flow and Its Role in Colloid-facilitated Transport of Radionuclides in Underground Medium

Colloid-facilitated transport of radionuclides by groundwater can increase the level of ecological hazard from radioactive contaminant migration in geological medium. The reason for this is that the migration velocity of a radioactive colloid can be higher than that of radionuclides carried by the groundwater as a solute. On the basis of their field and laboratory studies, a few researchers have concluded that the velocity of the colloid can even exceed the interstitial velocity of the groundwater by a few times. A theoretical analysis of this effect is carried out in this article. The analysis is based on the assumption that the only mechanism responsible for the effect is caused by a redistribution of the colloid in a cross section of the groundwater flow in a representative volume to such domains of the cross section where the local velocity of the flow is higher than the average velocity over the whole cross section. This redistribution can be caused by drift forces arising as a result of the Magnus effect. The influence of these drift forces on a particle’s movement is considered in two extreme cases; that of relatively large and relatively small colloid particles. Particles are considered relatively small if the thermal motion of water molecules exerts a decisive influence on the particles’ movement. Otherwise the particles are considered as being relatively large. It is shown that in the case of relatively large particles this redistribution can be caused by the instability of their movement. The redistribution in the case of relatively small particles can be caused by an influence of the drift forces on characteristics of Brownian motion. It follows from the results of the theoretical analysis that an influence of the drift forces in both cases does not lead to an increase in the particles’ migration velocity at near-horizontal direction of the groundwater flow. Data from experimental studies of the elevated velocity of colloids in porous medium are analyzed. It is shown that some findings of colloidal migration velocity exceeding the interstitial velocity of the groundwater are a result of misinterpretation of experimental results.     

Prediction of salt conditions in irrigated soils in the presence of drainage

The equations of convective diffusion are solved for two flat layers, using the method of finite integral transformations. Exact solutions are obtained to the problem in the presence of a descending or ascending filtration flow.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 113–117, November–December, 1972.     

Modeling of diffusion in the presence of damage in polymer matrix composites

It is now well known that Fick’s Law is frequently inadequate for describing moisture diffusion in polymers and polymer composites. Non-Fickian or anomalous diffusion is likely to occur when a polymer composite laminate is subjected to external stresses that could give rise to internal damage in the form of matrix cracks. As a result, it is necessary to take into account the combined effects of temperature, stress, and damage in the construction of such a model. In this article, a modeling methodology based on irreversible thermodynamics applied within the framework of composite macro-mechanics is presented, that would allow characterization of non-Fickian diffusion coefficients from moisture-weight-gain data for laminated composites. A symmetric damage tensor based on continuum damage mechanics is incorporated in this model by invoking the principle of invariance with respect to coordinate transformations. For tractability, the diffusion-governing equations are simplified for the special case of a laminate, with uniformly distributed matrix cracks, that is subjected to a uniaxial tensile stress. The final form for effective diffusivity obtained from this derivation indicates that the effective diffusivity for this case is a quadratic function of crack density. A finite element procedure that extends this methodology to more complex shapes and boundary conditions is also presented. Comparisons with test data for a 5-harness satin textile composite are provided for model verifications.     

Dissipative-convergent intermittency in dynamics of tidal waves in estuaries

The effect of dissipative-convergent intermittency in dynamics of tidal waves in estuaries is described. The effect manifests itself in spatial modulation of the tide amplitude. The effects of convergent channel and turbulent friction are competitive in dynamics of tidal waves in bays, gulfs, and estuaries. This manifests itself in the alternating excess of the action of one effect over another as the tidal wave penetrates deep the estuary. The effect of dissipative-convergent intermittency is manifested most completely in “strange” bays in which the mean water depth is commensurable with the Stokes layer thickness. It is shown that the spatial distribution of the tidal wave amplitude has a minimum in the convergent channel. If the minimum point lies beyond the estuary, then the effect of convergent channel predominates and the wave height increases to the vertex. If the minimum point lies in the neighborhood of the estuary vertex then the effect of turbulent friction predominates along the entire bay length and the wave height decreases to the bay vertex. Finally, if the minimum point finds itself in the middle part of the estuary, then the case of the “strange” bays arises, namely, in such bays the wave height decreases to crossing the minimum point and begins increases again after its passage. The local increase in the tidal wave amplitude can be manifested further in the mouth of the inflowing river.     

Transport of radionuclides in isolated fractures in crystalline rocks

This paper presents a generalization of a computational method for the prediction of solute dispersion in fractured porous media. This method is specially useful for the prediction of subsurface flows in crystalline rocks. The model now includes a linear kinetics mechanism to represent the effects of sorption of radionuclides in the rock matrix. The method is improved in its accuracy and provides results useful for the assessment of radionuclide migration in high-level, radioactive waste repositories. Results including verification (analytical) and physical test simulations are given. These results provide a partial validation of the numerical model.     

Sustainment of Oscillations in Localized Turbulent Structures in Pipes

The solution of the Navier–Stokes equations which reproduces some qualitative features of localized turbulent structures developed in circular pipes at transitional Reynolds numbers is numerically investigated. In the phase space this solution corresponds to the limiting state of the solution which evolves along the separatrix dividing the regions of attraction of the solutions corresponding to the laminar and turbulent flow regimes. Relative simplicity of the spatial and temporal behavior of the limiting solution on the separatrix makes it possible to investigate it in detail. In particular, the nonlinear mechanism of the onset of streamwise vortices responsible for sustainment of near-wall streaks whose instability ensures the presence of fluctuations is revealed.     

Advances in modeling of water in the unsaturated zone

This paper reviews recent advances in analytical and numerical solution of problems of water flow through rigid soils in the unsaturated zone. The Richards model remains the most widely accepted and fertile framework for water flow analyses. More general formulations are reserved for the analysis of problems involving macroporosity, thermal effects, and air pressure effects. Many exact and approximate solutions have been derived for particular boundary value problems of homogeneous soils using methods such as quasi-linear analysis, Green-Ampt analysis, perturbation, and the kinematic wave approximation. Numerical simulators have become bigger and more accurate due to improvements in the areas of nonlinear solution procedures, mass conservation, computational efficiency, and computer hardware. Problems of natural heterogeneity have been addressed primarily through application of various stochastic methods to the Richards model. The stochastic formulations generally refute the concept of simple equivalent homogeneous properties, but do themselves offer a certain limited potential for a predictive capability.