Showing papers by "Tsinghua University published in 1985"

Formulation of Equations of Motion for Systems Subject to Constraints

Abstract: A method for constructing equations of motion governing constrained systems is presented. The method, which is particularly useful when equations of motion have a/ready been formulated, and new equations of motion, reflecting the presence of additional constraints are needed, allow the new equations to be written as a recombination of terms comprising the original equations. An explicit form in which the new dynamical equations may be cast for the purpose of numerical integration is developed, along with special cases that demonstrate how the procedure may be simplified in two commonly occurring situations. An illustrative example from the field of robotics is presented, and several areas of application are identified.

Single-particle momentum distribution at high energies and concept of partition temperature.

Abstract: A concept of partition temperature is introduced in high-energy collisions. It is a natural mathematical consequence of the Darwin-Fowler method, and neither requires nor implies thermal equilibrium. A collision at a given incoming energy is described as an incoherent superposition of collisions with different partition temperatures. Angular distributions are then presented for ..sqrt..s = 540 GeV collisions.

Heat transfer to a particle under plasma conditions with vapor contamination from the particle

Abstract: Heat transfer to a copper particle immersed into an argon plasma is considered in this paper, including the effects of contamination of the plasma (transport coefficients) by copper vapor from the particle. Except for cases of high plasma temperatures, the vapor content in the plasma is shown to have a considerable influence on heat transfer to a nonevaporating particle, and, to a lesser extent, on heat transfer to an evaporating particle. Evaporation itself reduces heat transfer to a particle substantially as shown in a previous paper [Xi Chen and E. Pfender, Plasma Chem. Plasma Process.,2, 185 (1982)]. Comparisons of the calculated results with those based on a method suggested in the above reference show that the simplified assumptions employed, i.e., that the surface temperature is equal to the boiling point and that plasma properties based on a fixed composition are applicable, can be employed to simplify calculations for many cases. This study reveals that a considerable portion of a particle must be vaporized before a steady concentration distribution is established around the particle.

Theory of second-harmonic generation in nematic liquid crystals.

Abstract: In recent years the observation of optical second-harmonic generation (SHG) in nematic liquid crystal (NLC) MBBA [N-(4-methoxybenzylidene)-4'-n-butylaniline] raised again the question of whether NLC's possess centrosymmetry. Lyakhov et al. [Izv. Akad. Nauk SSSR 45, 917 (1981)] suggested that these observations can only be explained by considering the NLC as noncentrosymmetric and optically biaxial. In this paper it is shown phenomenologically that even with ${D}_{\ensuremath{\infty}h}$ symmetry, SHG arising from ee\ensuremath{\rightarrow}e, oo\ensuremath{\rightarrow}e, and oe\ensuremath{\rightarrow}o interactions can occur in an aligned NLC. With curvature strains SHG arising from ee\ensuremath{\rightarrow}o, ee\ensuremath{\rightarrow}e, oe\ensuremath{\rightarrow}o, oe\ensuremath{\rightarrow}e, oo\ensuremath{\rightarrow}o, and oo\ensuremath{\rightarrow}e interactions are all possible. Several experimental observations are satisfactorily explained by the present calculation. The order of magnitude of the effective susceptibilities estimated from a quadrupole-moment mechanism agrees with the values obtained by Arakelyan et al. [Mol. Cryst. Liq. Cryst. 71, 137 (1981)] and by Barnik et al. [Mol. Cryst. Liq. Cryst. 98, 1 (1983)]. One may conclude that it is possible to explain the occurrence of SHG in NLC's without having to invoke noncentrosymmetry of the system.

On eigenvalue assignment in discrete systems with fast and slow modes

Abstract: This paper deals with eigenvalue assignment in linear discrete control systems which have a two time-scale property. It has been shown that such systems can be decomposed into a fast subsystem with small eigenvalues and a slow subsystem with large eigenvalues. Separate eigenvalue assignment is attained using independent feedback gains.

Element Importance and System Failure Frequency of a 2-State System

Abstract: This paper redefines three measures of element s-importance for 2-state systems regardless of s-coherence, presents the calculation methods of these measures, and gives a new method to calculate the system time-specific failure and recovery frequencies. A poor point in the literature in extending methods for evaluating the s-importance of elements of s-coherent system to non-coherent system is pointed out. An example illustrates these definitions and calculation methods.

Shape detection using range data

Abstract: The original algorithm of Hough transform was for detecting and finding straight lines in two dimensional images. Since then, it has been extended to detect and locate special planar curves such as circles, parabolas, etc. Recently, generalized Hough transform has been applied for recognizing and locating three dimensional objects using range data. Our work is based on the same principle by decomposing the overall parameter set of nine elements representing a quadric into three decoupled parameter subsets: scalar parameter, translational parameter, and orientational parameters. The use of Hough transform on each decoupled subset in an independent but sequential manner rather than the whole enables us to reduce the computational cost and storage requirements by orders of magnitude as compared to all previous published applications.

Recoil implantation and interface mixing: A computer simulation study

Abstract: The process of recoil implantation and interface mixing of a bilayered target (Sb/Si) is investigated by using a Monte Carlo code, TCIS. Particular attention is paid to the movements of the atoms participating m the mixing. In order to obtain some statistical knowledge of the mixing process, 1000 independent incident events have been simulated for the case of an Sb(150 A)/Si target bombarded by 100 keV Ar + . The effects of the incident ion species, the film thickness and of an interface potential are considered.

A New Heuristic Search Technique-Algorithm SA

Abstract: In this paper, we present a new heuristic searching algorithm by introducing the statistical inference method on the basis of algorithm A (or A*). It is called algorithm SA. In a simplified search space, a uniform m-ary tree, we obtain the following result. Using algorithm SA, a goal node can be found with probability one, and its mean complexity is O(N·ln N) where N is the depth at which the goal is located.

Further studies of ion mixing in binary metal systems

Abstract: Using free energy-composition diagram, a simple model is proposed for the formation of amorphous alloys by ion mixing of metal layers. The basis of the model is the limited atomic mobility in such samples after ion mixing at a suitably low temperature. The model explains the formation of amorphous alloys that have been reported previously and those obtained in this study in the Zr-Ru and Ti-Au systems by ion mixing. These include phases with compositions in both two-phase and single-phase regions of the equilibrium phase diagram. In the Ni-Mo system, an unusual phase transition was observed by X-ray diffraction photos, i.e. an amorphous phase was formed after room temperature aging of an ion induced metastable crystalline phase (h.c.p. structure). Post-irradiation annealing of some ion mixed Ni-Mo amorphous alloys were performed at various temperatures. A schematic free energy diagram is proposed according to the phase evolution in the annealed samples upon annealing, and is used to discuss the ion induced phenomena in this system.

A New Numerical Technique for Computing Surface Elastic Deformation Caused by a Given Normal Pressure Distribution

Abstract: This paper presents a new numerical method for computing the elastic normal surface displacement field caused by a given normal pressure distribution. The pressure function is approximated by a piecewise biquadratic polynomial on the whole domain analyzed, and the deformation of every node is expressed as a linear combination of the nodal pressures whose coefficients can be combined into a deformation matrix. Consequently, the iterative calculation of elastic deformation is simplified and the amount of work is greatly reduced. It has been proved, in addition, that the numerical accuracy of the new method is higher than that of some others.

The limit analysis of cylindrical shell with hemispherical ends

Abstract: The present paper is concerned with the limit analysis of a thin cylindrical shell with hemispherical ends under internal pressure. Seven cases of collapse mechanisms for various geometry parameters are examined and the complete solution for each mechanism is obtained. The range of validity for each mechanism is also determined. It is proved that the solutions obtained from the seven mechanisms are complete.

Effect of pressure on heat transfer to a particle exposed to a thermal plasma

Abstract: This study is concerned with the effect of reduced pressure on heat transfer to particulate matter injected into a thermal plasma. Reduced pressures in conjunction with the high temperature in a thermal plasma lead to a strong increase of the mean free path in the plasma, and therefore, the Knudsen effect plays an important role in this situation. Based on the heat-conduction-potential jump approach, calculated data are presented for small spherical particles, typical for applications in plasma processing (for example, plasma spraying) and in plasma chemistry. The Knudsen effect severely reduces the heat flux to a particle, even for larger particles (up to 100 μm) at reduced pressures. For small particles (<10 μm) this effect is even felt at atmospheric pressure.

An Algorithm for One and Half Layer Channel Routing

Abstract: Channel routing is one of the key problems in the automatic layout design of LSI chips This paper presents an efficient routing algorithm for one-and-half layer channel model which is based on single layer metal mask and fixed polysilicon crossunders in CMOS gate array The algorithm makes parallel horizontal routing in each zone by means of ordering and prediction The nets contend for crossunders in a greedy approach This results in higher probability of routing success and less crossunders occupied (equally less via holes) Furthermore, by inserting interactive information at the same time of execution if necessary, the router provides more chances of 100% routing success

BEM calculation for magnetic shielding with steel sheets

Abstract: The Boundary Element Method (BEM) is adapted to the solution of two-dimensional and axisymmetrical nonlinear shielding problems. The high vector-field accuracy attainable permits studies of extremely uniform nuclear magnetic resonance (NMR) fields to be made as well as less demanding shielding calculations. A technique is described in which nonlinearities are replaced by virtual sources in an explicit iteration scheme. All unknowns are on boundaries only and matrix size is not increased due to interior nonlinearities. An example is given. Suggestions for efficiency improvements are made.

A weighted technique in heuristic search

Abstract: As shown in [1], we examine search as a statistic sampling process. Based on some statistical inference method the probability that a subtree in search tree contains the goal can be decided. Thus some weight is in tentionaly added to the evaluation function of those nodes which are unlikely in the solution path so that the search will concentrate on the most promising path. It results in a new weighted algorithm-WSA. Tn a uniform m-ary tree, we show that a goal can be found by WSA in the polynomial time, although the computational complexity of A (or A*) may be O(e) for searching the same space. Where N is the depth at which the goal is located.

Eigenproblems of radially inhomogeneous optical fibers from the scalar formulation

Abstract: A unified method based on finite difference is proposed to find cutoff frequencies and propagation constants of optical fibers with arbitrary refractive-index profiles in the scalar form. This method is shown to be efficient in terms of accuracy and computation cost.

Comparison of Wear Resistance of Ion Implanted Surgical Ti-6Al-4V with Other Alloys

Abstract: The wear resistance of N-ion implanted titanium, N-ion implanted Ti-6Al-4V alloy, ion plated Ti-6A1-4V, ion nitrided Ti-6Al-4V and surgical Co-Cr-Mo alloy were studied. All were studied under similar conditions, including dry wear and wear with lubricants of distilled water, saline solution, and human plasma. The results favor use of ion implanted Ti-6Al-4V alloy.

Improvement of side-mode suppression ratio of a single-mode SCC semiconductor laser

Abstract: A new way to improve the single-longitudinal-mode spectrum of short-coupled-cavity (SCC) semiconductor lasers is reported. It is found that an antireflective coating on the end facet of the laser diode in the SCC cavity is necessary for high-quality single-mode operation. Based on the theoretical analyses, a new type of miniature SCC laser has been constructed and demonstrated. Single-mode output with 35–40 dB side-mode suppression ratio has been obtained under CW and 150 MHz modulation conditions.

Study on the abnormal signal in the saturation absorption spectroscopy of sodium atoms

Abstract: The abnormal saturation absorption signal of sodium atom D1 line has been observed in different experimental conditions. The explanation has been presented quantitatively using the velocity-selective optical pumping model.

Gyroscopic Mode Synthesis in the Dynamic Analysis of a Multi-Shaft Rotor-Bearing System

Abstract: A mode synthesis method used in the dynamic analysis of multi-shaft rotor-bearing system has been developed in this paper. By introducing the idea of connecting springs and dampers, and using gyroscopic modes instead of complex modes in the mode synthesis, this method differs not only from classical component mode synthesis method, but also from complex mode synthesis. Several numerical examples show the advantages of this method.Copyright © 1985 by ASME