The Shunting of Current and the Resultant Variation of Voltage in Channels of Plasmatrons with Self-Adjusting Length of Electric Arc

The characteristics of pulsation processes in dc plasmatrons with the cathode located in the channel center and the anode in the form of a cylindrical or a conical channel wall are investigated in plasmas of argon and nitrogen at atmospheric pressure. The fast Fourier transform of signals and their subsequent computer processing are used to obtain the dependences of the frequency of fluctuations of the arcing voltage on the working parameters of the plasmatron, namely, the electric arc current, the flow rate of plasma-forming gas, and the channel diameter. For plasmatrons with the self-adjusting length of the electric arc, analysis is performed of the mechanism of reclosing of the anode region of the arc, i.e., of the electric arc shunting associated with the stretching of the current filament by a flow of gas and with the electrodynamic interaction of different filament regions. A formula is derived which defines the dependence of the characteristic frequency of fluctuations of the arcing voltage on the external parameters of the problem, namely, the arc current, the flow rate of the working gas, and the characteristic channel diameter. It is demonstrated that the pattern of the dependence of the frequency of voltage fluctuations on the gas flow rate may vary with the values of the parameter of magnetohydrodynamic interaction. The formula generalizes the experimental results of numerous researchers obtained in a wide range of variation of the external parameters.     

液压缸非线性弹簧力约束下轧机吸振器参数的优化

考虑四辊轧机液压缸非线性弹簧力约束的因素,引入吸振器控制装置,建立带有轧机吸振器的轧机辊系振动动力学模型;通过对轧机吸振器基本参数的优化,得出吸振器最优的阻尼系数和刚度系数;仿真分析不同质量、弹簧力、摩擦力对轧机辊系振动幅频特性曲线的影响规律,得到轧机吸振器的最优质量可以有效提高系统稳定性,轧机吸振器的最优弹簧力可以缩小系统的不稳定区域,轧机吸振器的最优摩擦力可以有效降低幅频特性曲线的高度,为有效抑制轧机辊系垂直振动提供理论支持。     

桩端岩溶顶板的破坏特征试验与理论计算模型研究

基于分离相似设计方法,开展了顶板厚度和溶洞直径变化下桩端顶板的破坏特征模型试验,并构建了相应的安全厚度理论计算模型。1)溶洞顶板厚度的大小影响了桩基嵌岩端荷载的传递路径,厚度越大传递范围越广,形成的剪切带体积越大。顶板厚度t≤1.0d时(d为桩径)顶板临空面处易发生冲切破坏,此时溶洞顶板的自身稳定性起控制作用,顶板厚度越小,溶洞临空面处脱落体积越小;顶板厚度1.0d< t ≤2.0d时,表现为锥形冲切失稳驱动上部剪切错动的破坏;顶板厚度t >2.0d时,表现为上部剪切错动驱动临空面的锥形冲切失稳,且溶洞直径小于剪切错动体的横向宽度时,剪切破坏最终发生在桩-岩界面的竖向投影范围以内。顶板厚度较小,对应的Q-S曲线为典型的陡降型曲线,而厚度较大时Q-S曲线为典型的缓变型。2)顶板具有一定厚度情况下(t≥2.0d),洞径较小(l≤3.0d)时,桩端剪切变形较为显著,上部剪切错动达到一定程度后,顶板临空面才发生冲切破坏,此时Q-S曲线呈现缓变型趋势;洞径较大时(l >3.0d),顶板临空面处冲切现象较显著,且洞径越大锥形冲切块的体积越大,此时Q-S曲线呈陡降型变化特征。3)以锥形冲切破坏计算模型进行工程设计风险较大,而冲-剪破坏理论模型与顶板岩体强度、完整性、桩径、嵌岩深度、施工方法及工艺等相关,故现场条件下即可计算出顶板的最小安全厚度值。     

The method of ray expansions for solving boundary-value dynamic problems for spatially curved rods of arbitrary cross-section

The method of ray expansions is developed for solving boundary-value problems connected with the propagation of planes of strong and weak discontinuity in spatially curved linearly elastic rods of arbitrary cross section. The equations of the three-dimensional theory of elasticity are utilized, which are written on the wave surface using the theory of discontinuities and then are integrated over the cross-sectional area. It is assumed that the plane of discontinuity remains perpendicular to the centroidal axis of the rod all the time during its propagation; the discontinuities in the normal stresses in the sections with the normals perpendicular to the centroidal axis can be ignored as compared to the discontinuities in stresses in the sections with the normals parallel to the centroidal axis. The cross-sections of the rod remain plane during the process of the rod deformation. These assumptions lead to the generation of two wave surfaces propagating in the spatially curved rod with the velocities of longitudinal-flexural and transverse-torsional waves of elastic rods. As this takes place, on the longitudinal-flexural wave, the bulk deformations experience a discontinuity not only at the sacrifice of shortening-elongation of the medium’s element locating along the centroidal axis, but also at the expense of thickening–thinning of this element in the directions of the principle axes of the rod cross-section. For the transverse-torsional wave, there exist discontinuities in the components of the velocities directed along the principle axes of the cross-section, in the angular velocity of the cross-section rotation as a rigid whole with respect to the centroidal axis, as well as in transverse deformations occurring due to the inhomogeneity of the transverse displacements. During the solution of boundary-value problems, the values to be found are represented in terms of the power series, the coefficients of which are the discontinuities in arbitrary order partial time-derivatives of the desired functions, while the time of arrival of the wave front is the independent variable; in so doing the order of the partial time-derivative coincides with the power exponent of the independent variable. The ray series coefficients are determined from the recurrent equations of the ray method within the accuracy of arbitrary constants, while the arbitrary functions themselves are found from the boundary conditions. Examples illustrating the efficiency of the ray method for solving the problems of dynamic contact interaction, resulting in the propagation of transient waves of strong discontinuity in spatially curved rods, are presented.     

Deformation of Rodlike Elements of Composite Materials Subjected to Uniform Compression

On the basis of generalization of the results of investigation of the operation of rodlike elements of composite materials, we make a conclusion that the influence of various factors on the deformability of these elements is studied incompletely. The comparison of the stress-strain states for different models of concrete and reinforced-concrete elements makes it possible to describe the effect of longitudinal reinforcement (coefficient of reinforcement, strength and deformation characteristics) on the critical level of strains in the cross section. According to the results of our investigations, we give recommendations concerning the evaluation of the critical value of the coefficient of reinforcement above which one may observe the effect of reinforcement on the deformability of the analyzed sections. It is shown that the compression strength of the brittle base of the composite with inhomogeneous structure decreases as a result of the fracture processes. It is also demonstrated that the existing procedure used for the evaluation of the indicated decrease in strength requires significant improvement. The problem of evaluation of the ultimate value of the coefficient of reinforcement is also analyzed.     

Inverse Problem of Evaluation of the Coefficient of Friction of Layers According to the Data of Measurements of the Surface Displacements

We consider a one-dimensional model of friction contact of two layers of different nature. The lower surface of the first layer is elastically fixed and the second layer is pressed to the upper surface of the first layer and moves along this surface with variable velocity. As a result of friction, heat is produced on the contact surface according to the Amonton's law. For known boundary and initial conditions, we pose the problem of evaluation of the friction coefficient and the intensity of friction heat flux according to given values of the vertical displacements of the upper surface of the second layer. The posed problem is reduced to the inverse contact problem of thermoelasticity described by the Volterra integral equation of the first kind. The solution of the problem obtained by the method of averaging of functional corrections enables us to study the time behavior of the indicated quantities for the entire period of interaction of the bodies and establish the dependence of the friction coefficient on the basic parameters of the process (sliding velocity, contact pressure, and temperature of the contact surface). The solution of the direct contact problem of thermoelasticity is used to perform the numerical verification of the proposed method for the solution of the inverse problem.     

MTMD系统抗震控制研究

MTMD利用一组频率分布在结构被控模态频率周围的单TMD控制结构的振动,它可以克服单TMD控制效果不稳定,适用激励频带过窄的缺点。研究了多自由度结构附加MTMD后的频率响应特性,计算了不同频谱特性的地震激励下MTMD系统的时程响应。通过计算和分析表明,MTMD系统可以增大单TMD抗震控制的适用范围,增强控制效果的鲁棒性;MTMD系统的设计需考虑结构所处的建筑场地条件,并应根据结构的具体情况优化选择MTMD系统的参数。     

Influence of heat-resistant coatings on the state of thermal stress of model of gas turbine engine blades in nonsteady heat exchange

The finite-element method was applied to calculate the thermal state and the state of stress and strain of a wedge-shaped model with a heat-resistant coating (heat shield) subjected to heat cycling by a gas stream. We analyzed the influence of the method of discretization of the section of the model and of the method of specifying the characteristics of the materials of the base and of the coating on the accuracy of the solution of the problem of thermoelasticity. We obtained data on the influence of the technological factors and of the thickness of the heat shield on the kinetics of the state of thermal stress of the material of the models. It was established that a ceramic layer changes the asymmetry of the cycle to the side of tensile stresses and shifts the maximum of the stresses into the range of lower temperatures.Translated from Problemy Prochnosti, No. 6, pp. 24–30, June, 1994.     

Reliability analysis of a microwave tower for fluctuating mean wind with directional effect

The reliability analysis of a 75 m tall steel lattice tower is presented for the annual variation of mean wind velocity taken as a function of both mean wind speed and direction. A parametric study is also conducted in order to investigate the effect of different important parameters on the component and system reliability of the tower. Variation of the mean wind velocity is determined from the continuous records of the hourly mean wind speed and the hourly mean values of the two components of the wind velocity. Annual probability of maximum response of a component of the tower exceeding a threshold level is determined as the mean number of crossing per day of the velocity vector out of the safe resistance boundary surface defined as a function of mean wind speed and direction. Using the above method of analysis, reliability against allowable stress failure of critical members of the tower and the consequent, system reliability are determined for a number of parametric variations. It is shown that the probability of failure of the tower increases with the increase in the standard deviation of mean wind speed, the zero crossing rate and the correlation coefficient between the velocities in the two directions. Further, the variation of the probability of failure is found to be sensitive to the variation of the above parameters in the lower range of the threshold levels of stresses.     

R290/R600a混合工质制冷系统泄漏点对系统性能的影响

混合工质在不同位置的泄漏所引起的系统中工质对组分比例发生变化会对制冷系统的性能产生很大影响。根据混合法则,从理论上研究R290/R600a工质对泄漏率在5%~40%范围内8种泄漏率条件下,系统工质对组分变化的规律。按照理论计算的要求建立制冷系统工质泄漏实验台,对实验台在4种泄漏率条件下的蒸发压力、冷凝压力、制冷量和COP进行研究。计算和实验结果表明,在蒸发器出口和冷凝器入口处的泄漏使低沸点组分R290在工质对中的比例增大,系统制冷量增加且COP下降;在蒸发器入口和冷凝器出口处的泄漏使高沸点组分R600a在工质对中的比例增大,系统制冷量下降且COP上升。     

Uncertainty quantification in the health consequences of the boiling liquid expanding vapour explosion phenomenon.

A methodology for estimating the risk owing to the phenomenon of boiling liquid expanding vapour explosion (BLEVE) in the presence of uncertainties both in the model and in the parameters of the models is presented. BLEVE takes place when a tank containing liquefied petroleum gas (LPG) is exposed to fire and fails catastrophically. Two models have been used in the estimation of the intensity of thermal radiation from the resulting fireball, namely the solid-flame model assuming an emission power independent of the combustion mass and the point-source model that estimates the emissive power as a function of the combustion mass. Three measures of the BLEVE consequences, the intensity of thermal radiation, the dose of thermal radiation and the probability of loss of life as a result of the exposure to the thermal radiation and as a function of the distance from the center of the tank have been considered. Uncertainties in the exact values of the parameters of the models have been quantified and the resulting uncertainties in the three consequence measures have been assessed. A sensitivity analysis on the relative contribution of the uncertainty in each of the input variables to the uncertainties of the consequence measures has been performed. One conclusion is that the uncertainties in the probability of loss of life are mainly due to the uncertainties in the model of the physical phenomenon rather than to the uncertainties of the dose-response model.     

Reflectance-based calibration of SeaWiFS. I. Calibration coefficients

We present a calibration approach for the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) based on the reflectance properties of the instrument's onboard diffuser. This technique uses SeaWiFS as a reflectometer, measuring the reflected solar irradiance from the Earth and from the onboard diffuser. Because the Sun is the common source of light for both measurements, the ratio of the SeaWiFS-measured radiances from the Earth and the diffuser provide the ratio for the reflectances of the two samples. The reflectance characterization of the onboard diffuser is the calibration reference for this approach. Knowledge of the value of the solar irradiance is not required for these measurements because it falls out of the ratio. Knowledge of the absolute calibration coefficient for the SeaWiFS measurements of each of the two samples is not required either. Instead, the result of the ratioing technique is based on the linearity of the instrument's response to the intensity of the input light. The calibration requires knowledge, however, of the reflectance of the onboard diffuser at the start of the SeaWiFS mission and the response of the instrument bands, in digital numbers, for measurements of the diffuser at that time.     

Variational method for the solution of the problems of tomography of the stressed state of solids

We study the problem of reconstruction of the stressed state of piecewise-homogenous solids on the basis of the data of measuring of the effects of interaction of external physical fields with the stress field. The general mathematical model includes three main elements: a model of stressed state, the data of nondestructive testing obtained by one or several methods, and a model of interaction of the sounding radiation with the stress field. Within the framework of this model, the problem of reconstruction of the stressed state is reduced to the problem of minimization of a quadratic functional taking into account both the experimental data obtained by different physical methods and the conditions imposed on the surface of the body and the interfaces of its components of different nature. This approach enables one to perform the numerical realization of both direct and inverse problems on the basis of a single algorithm. We establish the analytic structure of the beam integrals of the stress field determined according to the results of polarization-optical measurements. On this basis, we apply the proposed method to the solution of problems of integral photoelasticity and present an example of the applicability of our method to the investigation of the stress concentration near the interface of bodies of different nature. Pidstryhach Institute of Applied Problems in Mechanics and Mathematics, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 5, pp. 23–32, September–October, 1999.     

汽车空调旋叶式压缩机排气阀片的振动特性

排气阀片是汽车空调旋叶式压缩机中的关键零件,是压缩机主要振动噪声源之一。通过对排气阀片结构运动分析,建立了阀片振动的数学模型,并求解了压缩机排气阀片的固有频率及强迫振动。利用UG NX Nastran模态计算,确定了排气阀片的固有频率和振型。测试结果证明,压缩机的外壳振动总加速度、噪声和排气脉动都低于美国通用汽车公司“GMW标准”的规定上限,证明排气阀片工作的工作状态是可靠的。但研究结果表明,阀片振动的极限位移同限位板高度比较接近,因此,提高限位板的高度或者限制阀片的振幅以进一步控制系统的排气脉动和噪声仍然具有一定的空间。分析结果对排气阀片乃至压缩机整体的振动分析与控制以及故障诊断具有参考价值。     

元代肖形印的图文融合策略与设计启示

目的 通过对元代肖形印图文融合特征的分析归纳,总结其图文组织关系与印面经营策略,在完成元代肖形印图形传统文化价值挖掘与保护的基础上,尝试为传统视觉经典的当代设计转化继承带来有益思考。方法 从元代肖形印图文融合的历史文化背景入手,分析其出现的缘由与特征,并在此基础上梳理探讨元代肖形印的图文融合策略,及其对当代设计的关联启示。结论 与先秦及汉代肖形印不同,图文融合是元代肖形印的显著特征,其具有多样化的适形、多层次意指和多元素碰撞的核心构建逻辑。文字的图形化、雅文化的大众普适及符号凝练的求同存异,则分别是元代肖形印对图文融合需求、市民文化需求和形式法则需求的具体回应,这便成为现代设计对元代肖形印图形继承转化的有效切入点。     

A modified impulsive force and pressure sensor intended forwaveform reconstruction purposes

The correction of the effect of dispersion on the accuracy of processing in the Hopkinson bar type mechanical transducer enables extension of its application in measurements of impulsive force and pressure of short duration onto the dispersive operating range. In the experimental methods of correction, two-point measurement of the strain inside the bar is usually applied. This way, the dispersive characteristic of the bar is determined and also the process of reconstructing waveforms is performed. In the paper, a sensor of impulsive force and pressure with one-point measurement of the strain inside the bar is presented, in which the incident wave and the first reflected one, generated by the input function, are utilized. Temporal restrictions of the input function and spatial restrictions of mounting strain gauges on the bar, preventing the superposition of the incident and the first reflected pulses in the strain gauges, are given. The nonsuperposition of these pulses is the condition of the sensor functioning correctly. The method of using the sensor for reconstruction of impulsive mechanical quantities, based on the frequency-domain deconvolution, is demonstrated. The advantage of the described sensor is the reduction in the complexity of the amplification circuit and the output signal processing circuit and thus an easier achievement of the required processing accuracy     

Energy relations in the thermomechanical fracture of materials

This work is devoted to the theoretical substantiation of the thermodynamic equation of the limiting state deduced earlier. The analysis of the energy state of the ideal crystal lattice in the process of its deformation and fracture demonstrates the existence of the ultimate value of the ratio of mechanical energy to thermal energy under thermomechanical loading. This observation specifies the relationship between the eleasticity modulus, heat capacity, the coefficient of linear thermal expansion, and the work of fracture of the material. This relationship is consistent with the well-known energy, deformation, and power equations of the limiting state. We also analyze the characteristics of interatomic bonds in the process of fracture and present the results of numerical calculation of the ratio of mechanical energy to thermal energy for different potentials of interaction.     

Influence of Hydrogen on the Equilibrium of a Dislocation Submicrocrack in α-Iron

Numerical methods are used to analyze the influence of hydrogen on the fracture of steels with bcc lattice by the mechanism of microcleavage. The behavior of submicrocracks at the head of a dislocation pileup is studied under uniaxial tension. In the case of delivery of hydrogen by merged dislocations of the pileup into the submicrocrack, it loses stability due to the adsorption decrease in the specific surface energy earlier than it becomes possible under the action of the pressure of hydrogen in the crack. We deduce the dependences of the fracture stress on the specific surface energy and the amount of hydrogen delivered by dislocations into the submicrocrack. The influence of hydrogen condensed on dislocations on the formation of submicrocracks is significant. We establish the relationship between the amount of hydrogen condensed on dislocations, the concentration of diffusing hydrogen in the metal, and rapid degradation of mechanical properties of steel even for small concentrations of dissolved hydrogen.     

Debonding along the interface of an elliptic rigid inclusion

The two-dimensional problem of a crack lying along the interface of an elliptic rigid inclusion embedded in an infinite elastic matrix is theoretically studied. Based on the complex variable method of Muskhelishvili, closed form solutions of the stresses and displacements around the crack are obtained when both general biaxial loads at infinity and uniform normal internal pressure are applied. These solutions are then combined with the virtual work argument of Griffith to give a criterion of the crack extension, namely the growth of the debonding of the interface. The critical applied loads are expressed explicitly by a function of four parameters; the size, the ratio of the length of the minor axis to that of the major axis of the inclusion, the angle subtended by the crack arc and the polar angle of the middle point of the crack arc. It is shown that when the applied load is only a simple tension or only an internal pressure the critical load is inversely proportional to the square-root of the size of the inclusion. The variations of the critical load with the angle subtended by the crack arc and with the ratio of the length of the semi-axes are graphically shown and discussed.     

Tunable photonic bandgap in a one-dimensional superconducting-dielectric superlattice

The transmittance of one-dimensional photonic crystals consisting of superconductor and lossless dielectric has been systematically studied through the transfer-matrix method. Obviously, the shift of the photonic bandgap (PBG) becomes more noticeable by adjusting the thicknesses of the dielectric layers than that of superconductor layers. Furthermore, the number of PBGs can be controlled by varying the thicknesses of dielectric layers. Compared to the thicknesses of the dielectric layers, the width of the PBGs is more sensitive to the thicknesses of the superconductor layers. However, the width of the first PBG promptly varies when the thicknesses of the dielectric layers increase from 0 to 40?nm. If the contribution of the normal conducting electrons of the superconductor is nonnegligible, the temperature of the superconductor has no influence on the width of the PBGs. Moreover, the damp coefficient does not affect the PBGs under low-temperature conditions.