摘要:

A theoretical analysis for the entrance length of Darcy's flow (i.e.Re≤ 1) into porous media filled in impermeable channels was carried out in the present study. Three channels with different geometries were considered. The entrance lengths were found to be independent of the properties of fluid and porous medium and the Reynolds number as long as the flow is in Darcy's regime. The entrance lengthLeis 1.5R0and 0.75Hfor porous media filled in tubes with radiusR0and 2‐D channels with widthH, respectively. For 2‐D channel flow with jump disturbance at the inlet, the entrance lengths for different disturbance factorsζare obtained and presented. The present results indicate that neglecting this entrance effect could probably result in an overestimation of heat transfer in porous media with lengths in the order of one diameter or one channel width.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677225

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An approach to the nonlinear control of flexible robot arms based on the differential geometric nonlinear control theory and the linear quadratic regulator (LQR) theory is considered using only the actuators at the joints. The control is separated into two parts: one derived by the differential geometric structure algorithm is a global external linearization and decoupling nonlinear feedback law for nominal trajectory tracking, and the other synthesized by combined LQR and servocompensator techniques is a robust linear time‐invariant stabilization and error correcting feedback control designed for active damping of elastic vibrations and for robustness to uncertainty in system parameters. By suitable selection of output variables, the suggested control design methodology can be used for either joint space control or task space control. For illustrations, a weightless three‐link PUMA type robot arm with a special flexible third link is tested via simulation.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677226

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Based on linear fracture mechanics concepts and random process theory, several stochastic models of fatigue crack propagation have been proposed in recent years. One of the models randomizes the fatigue crack propagation equation by employing a random pulse train and applying a stochastic average technique to treat the dynamic fatigue crack propagation as a Markov process. The probabilistic structure of a Markov diffusion process is governed by a Fokker‐Planck‐Kolmogorov equation. Therefore, in the present paper, the probability distribution of crack size at any given time and the probability distribution function of the random time at which a given crack size is reached are treated as solutions of the Fokker‐Planck‐Kolmogorov equation associated with the Markov process. Analytical solutions are found for these quantities and numerical examples are given. The results are compared with some experimentally obtained data.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677227

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Based on the concept of causality, a physically reasonable form of the parameter matrix of the random part of multivariate time‐series models is proposed. This general form can be simplified into the full‐matrix form used by Matalas and the lower triangular form assumed by Young. Application conditions for these two simplified forms are identified. Furthermore, a technique of decomposing the Gramian matrix to obtain the solution for the parameter matrix of the general form is presented. This technique is simple and permits a direct solution for the parameter matrix. This technique can also be used to obtain the direct solution for a parameter matrix of full‐matrix form.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677228

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The growth of CdTe epitaxial films on both (100) and (111) GaAs was studied in this work. Our results indicate that the growth rate on (111) GaAs is about twice of that on corresponding (100) GaAs substrates under the same growth conditions. These results, as well as those from the study of the effect of input molar ratios of organometallic compounds, can be fitted reasonably well by our previous model. The kinetic rate constant derived from these data has an activation energy of 70 kcal/mole, whose order of magnitude is consistent with our assumption that the decomposition of diethyltelluride (DETe) molecules is the rate‐limiting step in this MOCVD process. In addition to growth rates, we also find significant difference in morphologies for (100) and (111) surfaces, which exhibit pyramids on (100) surface and are very often smooth and featureless on a (111) CdTe surfaces.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677229

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The behavior of a hybrid ALOHA/TDMA protocol with a buffer at each of the user terminals is studied. An approximate expression for the average gueueing delay is derived as a function of the system input parameters (i.e. the number of users, the input traffic, the transmission and retransmission probabilities). In comparison with Kamal's model, the analysis carried out in the paper reveals that the reduction in the average queueing delay is quite significant, especially under heavy traffic conditions.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677230

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Radio‐frequency signals may be picked up from the environment and interfere with a low‐frequency circuit. A computer‐aided method of predicting the resulting effects of this interference on the junction‐field‐effect transistors is presented in this paper. The method presented provides a convenient and simple means of analyzing the conducted‐susceptibility problem in a low‐frequency circuit. In our analysis, the parasitic components of the package are taken into account. The variation of the equivalent input‐noise voltage with the gate‐bias voltage and the decrease of the rectification factor with the increase of frequency are illustrated. The predicted results show good agreement with the experimental data.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677231

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Numerical predictions of flow over a two‐dimensional backward‐facing step are made in the laminar flow regime. Solutions are carried out by solving the Navier‐Stokes equations in Hemholtz formulation. The wall vorticity is formulated through the use of the circulation theorem. The unsteady features of the flow development and the effects of Reynolds number on the flow structures are discussed in detail from the variations of streamline patterns, velocities, vorticities, and reattachment and detachment lengths. The current predictions are compared with the available numerical and experimental results. It is found that the comparisons are in reasonable agreement and provide detailed information about the unsteady separation phenomenon for this particular flow field.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677232

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The problem of determining a “good” sequence that minimizes total tardiness for a set of jobs with unequal importance on a single machine is addressed in this paper. The term unequal importance is interpreted here to mean that there is one job in the set of jobs which is not allowed to be tardy. To find an optimal solution for this kind of non‐linear and NP‐complete problem, it is necessary to rely on the concepts of combinatorial optimization. Commonly used methods include dynamic programming and branch and bound. These enumerative methods are computationally inefficient, and no easy method has yet been found. This paper applies the algorithm of Ref. [15] as the basis for developing a heuristic algorithm to obtain a sequence for a set of jobs. A comparative evaluation of the proposed algorithm with the dynamic programming method is provided in the later sections of this paper.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677233

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A convenient method to determine the thickness of barium stearate Langmuir‐Blodgett (LB) film using the interference of white light at perpendicular incident is described. This method permits a great sensitivity in the small increments of film thickness measurement. Film thicknessdis given byd=mλ/4n. Data are compared with the thickness from the interference of monochromic at perpendicular incident and the monomole‐cular film on water surface.

ISSN:0253-3839    

DOI:10.1080/02533839.1990.9677234

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor