Showing papers by "Beihang University published in 1995"

Dynamic user optimal traffic assignment model for many to one travel demand

Abstract: A freeway or expressway corridor where all vehicles travel to the same destination such as the city centre is considered in this article, similar to the morning commute problem. A continuous time optimal control model that deals with the dynamic user optimal assignment for multiple origins and single destination is proposed. The splitting rates of traffic flows at each network node are defined as the control variables in this model. The optimality conditions are proved to be equivalent to the dynamic user optimal principle or user equilibrium of instantaneous travel cost. In order not to solve the complicated two-point boundary-value problem with substantial computational times for obtaining the optimal control solution, a steady state-costate solution algorithm is developed that generates an approximate solution to the network optimal control problem. This algorithm exploits advantage of the embedded network structure of the problem and would be computationally efficient. A numerical example with two peak period traffic demands which was drawn from the road network problem between Hong Kong and several adjacent cities of inland China is used to demonstrate the performance of the proposed algorithm.

Two-level multipoint constraint approximation concept for structural optimization

Abstract: A two-level multipoint approximation concept is proposed. Based upon the values and the first-order derivatives of the critical constraint functions at the points obtained in the procedure of optimization, explicit functions approximating the primal constraint functions have been created. The nonlinearities of the approximate functions are controlled to be near those of the constraint functions in their expansion domains. Based on the principle above, the first-level sequence of explicitly approximate problems used to solve the primarily structural optimization problem are constructed. Each of them is approximated again by the second-level sequence of approximate problems, which are formed by using the linear Taylor series expansion and then solved efficiently with dual theory. Typical numerical examples including optimum design for trusses and frames are solved to illustrate the power of the present method. The computational results show that the method is very efficient and no intermediate/generalized design variable is required to be selected. It testifies to the adaptability and generality of the method for complex problems.

A combined algorithm for solving and calibrating the stochastic traffic assignment model

Abstract: This paper studies Fisk's stochastic user equilibrium (SUE) assignment model in which travellers are assumed not to have perfect knowledge of the traffic system. It is shown that the stochastic properties of this model completely depend on the calibration parameter. After giving the searching method of an 'efficient paths' set, a modified combined algorithm for solving and calibrating Fisk's stochastic traffic assignment model is developed. In this algorithm, the path enumeration is achieved automatically in the preliminary phase and renewed in the iterative processes, and the number of alternative paths generated can be controlled by a predetermined function. Two numerical examples are presented to illustrate the model's properties and the algorithm's efficiencies.

Study of the shock motion in a hypersonic shock system/turbulent boundary layer interaction

Abstract: Wall pressure fluctuations and surface heat transfer signals have been measured in the hypersonic turbulent boundary layer over a number of compression-corner models. The distributions of the separation shock oscillation frequencies and periods have been calculated using a conditional sampling algorithm. In all cases the oscillation frequency distributions are of broad band, but the most probable frequencies are low. The VITA method is used for deducing large scale disturbances at the wall in the incoming boundary layer and the separated flow region. The results at present showed the existence of coherent structures in the two regions. The zero-cross frequencies of the large scale structures in the two regions are of the same order as that of the separation shock oscillation. The average amplitude of the large scale structures in the separated region is much higher than that in the incoming boundary layer. The length scale of the separation shock motion region is found to increase with the disturbance strength. The results show that the shock oscillation is of inherent nature in the shock wave/turbulent boundary layer interaction with separation. The shock oscillation is considered to be the consequence of the coherent structures in the separated region.

Mixed-Mode Delamination Behavior of Carbon/Epoxy Composites:

Abstract: The mixed-mode bending (MMB) test was employed in an investigation of delamination fracture behavior in static and fatigue loading for carbon/epoxy composites 1300/M10 and HTA/6376. As specimens were loaded at different ratios of alL for different mixed-mode ratios, both investigated composites exhibited brittle unstable or brittle stable behavior of crack growth. Mixed-mode delamination fracture toughness was determined at a IL = 0.5. Fracture surfaces of MMB specimens were examined using scanning electron microscopy to distinguish fracture features. Stable crack growth was observed over the entire range of crack lengths investigated in the cyclic tests. A consistent relation of crack growth per cycle to the cyclic strain energy release rate was obtained. The trend of reduction in crack growth rate data with diminution in cyclic strain energy release rate suggests the presence of a threshold strain energy release rate.

A New Type of Hydrostatic/Hydrodynamic Gas Journal Bearing and its Optimization for Maximum Stability

Abstract: The orifice annular and shallow pocket restricted hybrid gas journal bearing is a new type of gas bearing which has good high-speed stability performance. In this paper, the stability of this bearing with three shallow pockets is studied theoretically, and the optimization for its maximum stability is carried out by use of the Complex Method. Some useful conclusions are obtained. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Lahaina, Hawaii, October 16–20, 1994

Coherent Systems in Profust Reliability Theory

Abstract: Profust reliability theory is based on the probability assumption and the fuzzy-state assumption. In an attempt to provide a uniform foundation for profust reliability theory, in this paper we introduce the concept of coherent systems and distinguish two classes of systems: closely coherent systems and loosely coherent systems. To formulate the relationships between system reliabilities and component reliabilities, we present a number of general results, results concerning convexity, and results concerning unimodality. It is argued that for a coherent system, component reliability improvements don’t certainly enhance the system reliability.

An evidence of stress-induced α2 → γ transformation in a γ-TiAl-based alloy

Abstract: Two phase {gamma}-TiAl-based alloys composed of ({gamma} + {alpha}{sub 2}-Ti{sub 3}Al) are currently under extensive investigation because their mechanical properties are superior to either single phase {alpha}{sub 2} or {gamma} alloys. The alloys usually have a duplex microstructure consisting of equiaxed {gamma}-grains with various volume fractions of lamellar colonies in them. The lamellar colony, which consists of alternating layers of ordered {gamma} and {alpha}{sub 2} plates with the orientation relationship {l_brace}111{r_brace}{sub {gamma}}//{l_brace}0001{r_brace}{sub {alpha}{sub 2}}, {gamma}// {sub {alpha}{sub 2}}, is formed either by a phase transformation {alpha} {r_arrow} {alpha}{sub 2} and then {gamma} precipitates from the {alpha}{sub 2} matrix, or by {alpha} {r_arrow} {alpha}/{gamma} {r_arrow} {alpha}{sub 2}/{gamma} upon cooling. The microstructure of the colony has been studied in detail by many investigators. It has been suggested that the precipitation of the {gamma} phase essentially involves a HCP {r_arrow} FCC structure change together with long-range transport of atoms to obtain the composition of the two phases. The structure change can be brought about if a/3 Shockley partials travel on alternate basal planes of the HCP phase. Thus, the {gamma} nuclei are essentially basal stacking faults in the {alpha}{sub 2} phase. This means that gliding of a/3 partial appears tomore » play an important role in the transformation. If it is so, it would be expected that an external stress would probably enhance the process. More recently, Gao et al. have claimed that stress-induced phase transformation may occur in two-phase TiAl-based alloys during deformation at room temperature, which is related to a stage on stress-strain curves. In this note, the authors are briefly reporting on evidence of the stress-induced {alpha}{sub 2} {r_arrow} {gamma} transformation in a {gamma}-TiAl-based alloy.« less

Business process reengineering: a case study

Abstract: Recently, many companies in China try their best to reform their own management systems from original central planning economic systems into market economic systems. Some design reengineering to win competition in the market. The paper gives a case study for business process reengineering in a large aircraft manufacturing company in China. What they do are as followings: 1. To set up a new planning and scheduling system. The most important discipline is Just In Time. 2. The company change the production planning organizations from four levels into one level. 3. The production planning department work out the four kinds of schedules. 4. To set up a new personnel management system. 5. To redesign the assembly process. i.e., to improved the assembly equipment, reduced the positions from nineteen to eight. 6. To design and implement the computer information systems for supporting all these redesigning. There expert systems and group decision support systems are now running in the computer networks.

Computer-aided operational dimensioning for process planning

Abstract: One of the important steps of process planning is to determine the tolerance and mean size for each working dimension. This paper presents a computer-aided operational diminsioning systems which can determine tolerances systematically and calculate mean sizes of working dimensions automatically for a process plan. The system is based on the tolerance charting technique. Firstly, a tree theoretic representation for a tolerance chart is described in this paper. Three trees can be generated from blue print dimensions, stock removals and working dimensions of a tolerance chart. The algorithms for generating the trees are provided, and these trees can be used to identify dimensional chains and solid stock removals in a tolerance chart. Then a linear programming model is built to determine the tolerance of each working dimension. Finally, a mathematical model of linear equations for calculating mean working dimensions is formulated from the dimensional chains.

Calorimetric investigation of a Cu-Zn-Al alloy with two way shape memory

Abstract: One way an two way shape memory alloy of Cu-21%zn-6%Al (wt%) were studied by differential scanning calorimetry. The profile of endothermic and exothermic peaks of one way shape memory alloy are different from those of two way shape memory alloy. The former show smooth peaks while the other show bumpy peaks. Besides, the heat of martensitic transformation of two way shape memory alloy is about 25% less than that of one way shape memory alloy.

Integration of inspection planning system and CMM via DMIS

Abstract: INSPEX is a computer-aided knowledge based inspection planning system for preparing inspection plans from CAD toleranced component models of rotational components. To achieve the complete automation of the inspection process, INSPEX has been integrated with CMMs (computer controlled coordinate measuring machines) via DMIS (Dimensional Measuring Interface Specification), where DMIS has been as a data exchange standard. The inspection procedures produced by INSPEX can be automatically converted into DMIS data format file by the DMIS pre-processor described in this paper, which can be fed into any CMMs with a DMIS interface for executing the inspection procedures.

A special bending crack tip finite element

Abstract: The general expression of the Williams expansion around a bending crack tip is presented. Based on this a special circular bending crack tip finite element of an arbitrary high order is developed, and the element stiffness matrix is explicitly given, which is especially convenient for engineering analyses. Numerical examples are presented and compared with previous results to demonstrate the efficiency and accuracy of the special element.

Rapid graphitization of a pulsed laser remelted ductile cast iron during multipass overlap melting

Abstract: As-cast ductile cast iron with an as-machined shiny metal surface was remelted with a high-power (1 kW) pulsed Nd:YAG laser using both single- and multipass overlap melt tracks. Changes in the microstructure of the underlying laser melted track caused by the transient overlap heating during multipass overlap remelting process were studied. The rapidly solidified metastable ledeburite structure of the underlying laser melted track was found to be rapidly graphitized during overlap remelting. The graphitized zone consists of a fully graphitized zone containing extremely fine graphite nodules and a partially graphitized zone containing extremely fine graphite nodules and undissolved cementite. The overlap ratios of the melt tracks were shown to have no noticeable influence on either the graphitized microstructure and the size of the graphitization zones. This newly observed rapid graphitization phenomenon is preliminarily discussed in terms of the microstructural characteristics of the rapidly solidified ductile iron and the unique heating behavior of pulsed laser beam to material.

Navier-Stokes analysis of a circulation control airfoil

Abstract: The two-dimensional, compressible, mass-averaged Navier-Stokes equations are used to investigate flows about a typical circulation control airfoil The governing equations are solved using the implicit approximate-factorization algorithm of Beam-Warming with a modified algebraic eddy viscosity model Results are compared with experimental data, and excellent agreement is obtained The effects of different jet momentum coefficients and angles of attack on the flow are studied The mechanism of genenating large lift by circulation control is discussed

New microstructure-property approach : quenching/tempering treatment in gamma TiAl alloy

Abstract: Gamma based TiAl alloys are currently seen as potential materials for high temperature structural applications due to their high specific strength, stiffness at elevated temperatures, and good creep and oxidation resistance. However, their poor ductility at ambient temperature and its poor formability prevents their practical applications. In this paper, a detailed investigation of microstructural evolution and mechanical properties of a tempered massive gamma TiAl alloy has been undertaken. The isothermally forged alloy was also investigated to determine which provides better mechanical properties.

The Relationship between the Product of Load and Sliding Speed with Friction Temperature and Sliding Wear of a 52100 Steel

Abstract: Base on the tests of friction and wear of AISI 52100 steel under the condition of dry sliding, conclusions can be drawn as follos: 1. 1. The wear volume and friction temperature increase with increasing normal load and sliding speed. 2. 2. The wear volume increases with increasing friction temperature, especially when Tb ecxeed a certain value, the wear volume increases greatly. 3. 3. Pν = 100N.m/s can be regarded as the critical condition of the transition from mild wear to severe wear during the dry sliding wear of steel, Tb can also be regarded as the transition temperature of wear mechanism. 4. 4.The normalized wear rate W = 13 × 10 −8 can be also regarded as the critical value of the transition from mild wear to severe wear during the dry sliding wear of of the steel.

A mathematic-physical approach to the satisfiability problem

Abstract: A one-to-one and onto mapping between the set of conjunctive normal forms and a subset of the potential functions of static electric fields is given; it has been further proved that a conjunctive normal form is satisfiable if and only if there exists a zero point for its corresponding potential function A particle is always moving in the direction of gradient descent in the field which is the fastest decreasing direction of potential of the particle Thus, if a conjunctive normal form is satisfiable, the gradient method for its corresponding potential function becomes a fast algorithm to solve its satisfiability problem

Annealing laser-melted ductile iron by pulsed Nd:YAG laser radiation

Abstract: Laser-melted ductile iron was rapidly annealed by pulsed Nd:YAG laser beam irradiation. The cementite-graphite phase transformation occurred rapidly during the short duration pulsed laser beam scanning. As a result, novel iron-base alloys containing either ultra-fine nodular graphite particles or both rapidly solidified cementile and ultra-fine graphite particles have been fabricated. The unusually accelerated cementite-graphite phase transformation and the production of the ultra-fine novel microstructure were predominantly due to the micro-annealing effect of the cyclic temperature fluctuations induced by the pulsed laser beam radiation.