Showing papers by "Hong Kong Polytechnic University published in 1988"

Phenomena of vortex shedding and flow interference of three cylinders in different equilateral arrangements

Abstract: This paper describes how the flows around three equal circular cylinders arranged in an equilateral-triangular manner interact at different angles of incidence and spacing ratios. Some vortex-shedding-frequency data evaluated from flow visualisation experiments conducted at Reynolds numbers of 2.1 x 10E3 and 3.5 x 10E3, based on the diameter of a single cylinder, using a dye-injection technique, are presented. In order to provide additional insight to the understanding of the flow structure around this particular cylinder array, some photographs indicating the typical flow patterns for various arrangements are also presented. The investigation indicates that the flows interact in a complex fashion for spacing ratios smaller than 2.29 and it also reveals that, at this range of spacing ratios, bistable flow characteristic exists. Moreover, for spacing ratios approximately smaller than 4.65 there always exists an angle at which the vortex shedding behind an upstream cylinder is suppressed by a nearest downstream cylinder. This angle is found not to remain constant but increases as the spacing ratio increases. For illustration and comparisons, some numerical results obtained from the application of the surface-vorticity method have also been presented.

Inelastic post-buckling behavior of tubular struts

Abstract: A numerical procedure for tracing the inelastic post-buckling load-deflection path of tubular struts is proposed. The analysis accounts simultaneously for both the geometrical and material nonlinearities with special emphasis placed on the influence of strain-unloading. An iterative numerical arc-length technique is employed in the solution procedure, but once strain-unloading is detected, iterations are suppressed and the procedure converted to a pure incremental method with arc-length constraint only. Predictions of the maximum loads and of the post-buckling load-deflecion paths using the proposed approach compared very well with other published analytical results and with tests. The approach is simple and may be applied to general thin-walled cross sections and include cyclic loading.

Instrumental measurement and control of colour

Abstract: In spite of the fact that the basic method of colour measurement was established more than harf a centuy ago, it is only during the last fifteen years that the application of colour measurement is particularly substantial and significant because of the advancement of computer technology, better understanding of colour science, and the revolution in equipment design. This has brought colour science from a scientific exercise in the research laboratory to a widely used industrial and commercial tool. The present state of the art of colour measurement is such that an analogy can be made to the measurement of linear dimensions at one time by the relatively crude yardstick, but now replaced by the precisely scaled micrometer. Thus with colour measurement now, colour can be measured with a precision unmatched by the human eye. To-day, both the colorant industry and colour dependent commerce (industries such as dyes, textiles, pigments, cosmetics, coatings, paper, plastics etc.) are taking advantage of such techniques since the economic advantages are quite substantial. Furthermore, the techniques involved are, to a large extent, no longer trade secrets. The literature abounds with practical theories and concepts which can be applied directly in the workplace by competent colourists. It is expected that more applications of colour measurement for similar general usage are being developed. This paper is designed to introduce the fundamental concept of colour science and various types of colour measuring instruments and their selection. The value of these colour measuring instruments for practical use in industry and commerce is discussed. Finally, the sources of the technical information and assistance are given.

Modelling and microcomputer control of a nonlinear pneumatic servomechanism

Abstract: A mathematical model is proposed for a low-pressure experimental pneumatic servomechanism which exhibits hysteresis characteristics at the torque motor and the servo spool valve assembly, nonlinear mass flow through the spool valve opening, and friction at piston and rod-end seals. Systems parameters are determined experimentally and correlated empirically. A single-board microcomputer is used for both servomechanism control and data acquisition purposes. The model predicts the experimental responses of three control algorithms with good accuracy. In particular, the response of PD with hysteresis compensation control shows no overshoot and minimal steady-state error when a step change of reference input is applied.

Effects of road pricing on system performance

Abstract: This paper investigates the effects of road pricing on system performance and describes two facts about flow pattern in a transportation system. The first, illustrated by an example of Braess paradox, is that adding a new link to the network does not necessarily minimise the total travel time. The second is that the introduction of appropriate toll pricing may reduce not only the total network time, but also the travel time for each individual traveller. It follows with the investigations of different system objectives and different pricing policies (only toll pricing and distance-based pricing are considered), and shows how they affect the system performance and flow pattern. Lastly, three different assignment methods are compared and an equilibrium assignment method is proposed for network design.

A Network Switching Expert System For AC Railway Electrification

Abstract: This paper reports the development of a PROLOG expert system utilising computer graphics for on-line monitoring and control of the 50-Hz single- phase overhead current collection catenary system currently in operation in Hong Kong. Running as an on-line consultant on a personal computer, this expert system allows operating personels to perform pseudo network switching on each of the six full graphics pictures representing the whole of the catenary line system. This displays may be zoomed in or out to show either details or overall layouts. Users are able to examine from graphics displays effects of their own choice of switching sequence. Legal switching brings about changes in display configurations and/or colours to relcord Circuit- breakers change of ON/OFF status, or relocation of supply points. The,expert system also checks against illegal switching which causes violation of interlocking rules, isolation of catenary sections from the supply, and/or phase-to-phase short-circuit of the utility system. In addition, this paper reports further work to develop a methodology of evaluating voltages across cross-overs which run between parallel catenary lines. A cross-over with excessive voltages across it should not be used to divert a train due to the risk of damaging insulators across the cross-over. An accurate evaluation of cross-over voltages involves the solution of a large set of simultaneous differential equations governing train time-tables, controls, movements as well as power consumptions. This paper would however show how cross-over voltages are derived on the basis of approximate relationships relating these quantities. Finally, the expert system provides a user-friendly environment and a knowledge acquisition sub-system to enable editing of a knowledge base to be made with little or no assistance from a knowledge engineer.