http://dspace.cityu.edu.hk:80/handle/2031/753 2013-05-01T03:06:36Z 2013-05-01T03:06:36Z Yip, Lee Kei ( 葉莉姬) http://dspace.cityu.edu.hk:80/handle/2031/6654 2012-08-07T07:48:47Z 2011-01-01T00:00:00Z

Title: Boundary slippage effect on hydrodynamic lubrication Authors: Yip, Lee Kei ( 葉莉姬) Abstract: It is well aware that efficiency of machines and engines can be increased by lubrication. Contact parts or components in relative motion in a well-lubricated machine can be totally separated by a hydrodynamic lubricating film which prevents direct solid contact and reduces friction. However, viscous friction still exists and its magnitude is proportional to the viscosity of the lubricant. Simply speaking, using thinner lubricating oil can reduce the magnitude of viscous friction, but it also reduces the thickness of the lubricating film, i.e. lubricating effect is affected. Thus, the means of using thinner oil to reduce lubricating friction is only limited and not practical. Recently, an idea for reducing lubricating friction on the basis of boundary slippage has been proposed. If the lubricant can slide on or move with a different speed of the solid surface, friction can thus be reduced. To better use the boundary slip phenomenon, this work aims to find out its effects on hydrodynamic lubrication for various operation conditions through experimental and theoretical studies. The experimental study started with a typical hydrodynamic system - squeeze film. The set up included two horizontal parallel planes that were submerged in a specimen lubricant. The lubricant was squeezed in the vertical direction. The load and the corresponding relative displacement between two parallel planes were captured. Experiments were carried out with various pairs of parallel planes. The lower plane was fixed (untreated) while the upper plane was modified with different surface treatments for various interfacial adhesion between the solid and the liquid. By comparing the experimental measurements with the hydrodynamic lubrication theory derived based on no-slip boundary conditions, genuine differences were obtained which proved the existence of boundary slippery. Based on the experimental and theoretical comparison, the degree of slip was inferred. To more accurately and systemically capture the parametric effects on boundary slip, shear film experiments were carried out with an optical slider test rig. The hydrodynamic lubricating film was generated with a rotating transparent disc and a stationary slider of adjustable tilted angles. The affinity of the specimen oil to the bounding surfaces, as quantified with contact angle, was systematically varied by the amount surfactant added to the same base oil such that a wide range of contact angles, from 3 to 52 degrees, was generated. The variation of film thickness against rotational speeds and different loads were detected by optical interferometry, which provides submicron measuring accuracy. Comparing the experimental results of load and film thickness to the hydrodynamic lubrication model with the critical shear stress criterion of slippage, the degree of slip can be illustrated and the critical shear stress can be inferred. In squeeze film experiments, the load carrying capacity was decreased with increasing the contact angle. The critical shear stress, which was inferred from the comparison of the experimental and theoretical results, was found having a negative effect on the carrying load. Moreover, it was found the load carrying capacity varied linearly with the squeeze velocity and this linear relationship was generally applied to specimen oils of different interfacial affinity. In the slider test, the contact angle was found increased with the amount of surfactant added. The experimental results illustrated that the film thickness was decreased with increasing the contact angle for a given load and speed. On the other hand, the increased contact angle would promote a reduction in the critical shear stress as well as the work of adhesion. Hence, lowering the wettability of an oil-solid system (larger contact angle) would promote slip, which, in turn, reduces the film formation capacity. Moreover, it was found that the critical shear stress is a linear function of shear rate. Lastly, the identified significant parameters that affect boundary slippage include the contact angle (or wettability), the critical shear stress, the shear rate, the speed and the work of adhesion. Notes: CityU Call Number: TJ1073.5 .Y56 2011; xiv, 124, [18] leaves : ill. 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 119-124)

2011-01-01T00:00:00Z Zhong, Guixian (鍾貴賢) http://dspace.cityu.edu.hk:80/handle/2031/6276 2011-05-25T01:20:13Z 2010-01-01T00:00:00Z

Title: Designing efficient mechanisms for mergers & acquisitions and studying impacts of such events on stock prices Authors: Zhong, Guixian (鍾貴賢) Abstract: Mechanism Design studies solution concepts for a class of private information games. Such class of games is usually solved by a mechanism which motivates each agent to disclose their true information while at the same time punishes the liar. Economic transactions take place in markets, within firms and under a host of other institutional arrangements. Some markets are free of government intervention while some are under controlled by government. Mechanism design theory provides a coherent framework for analyzing this great variety of institutions, or "allocation mechanisms", with a focus on the problems associated with incentives and private information. My work focuses on modeling a particular economic game situation - the negotiation of Merger and Acquisition among firms, and then applying mechanism design theory on it so as to achieve a particular economic goal - maximizing Social Surplus. Several important propositions were obtained, and we choose to test these propositions in Information-based Industries by Event Study Methodology. Supportive empirical results were also obtained. Therefore, we conclude that Mechanism Design is a workable approach to Merger and Acquisition problems. More researches on this topic are deserved. Notes: CityU Call Number: HD2746.5 .Z45 2010; xi, 94 leaves 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references (leaves 68-75)

2010-01-01T00:00:00Z So, Chung Yin (蘇頌燕) http://dspace.cityu.edu.hk:80/handle/2031/6272 2011-05-25T01:20:04Z 2009-01-01T00:00:00Z

Title: The effects of different design factors on comprehension performance and subjective evaluation for reading Chinese on LED display Authors: So, Chung Yin (蘇頌燕) Abstract: Dynamic display systems are widely used in information communicating technology. Most of these display systems employ cathode ray tubes, liquid crystal displays, gas plasma displays, or light-emitting diodes (LED) as the output device. LED display simply acts as a display output unit like a computer screen, which provides a means of delivering messages or advertisements to customers or people. In order to overcome the limitations of screen size of the display units, numerous means of presenting moving materials on dynamic display have been invented. The advanced display technology offers a selection of colors, display methods, display rates, and highlighting methods, which have improved the presentation of textual information. The text display method, character type, text display direction, text color, and other features can be freely chosen by designers or operators. Thus, the different combinations of the selected factors would affect the readability of electronic text. However, all the previous findings on optimal dynamic text display and readability of electronic text were focused on small screens, such as cellular phones, pagers, and desktop phones. There has been an obvious lack of consideration of reading situations in which the displays are located at a distance from the observers, especially when reading Chinese characters. Therefore, there is a need to understand the effects of the display factors on the readability of electronic text in order to design better display interfaces on large displays written in Chinese. Three experiments were conducted in this research on investigating the effects of display factors on comprehension performance and subject preference for reading Chinese characters on LED displays. In each experiment, objective performance measures and subjective evaluation measures were collected. The objective performance measure consisted of the comprehension score, which denoted the accuracy of subject responses to a set of multiple-choice questions in a comprehension task following the reading passage. The preference rating was a subjective evaluation of the testing display format given by the readers reflecting individual preference, which should also be considered when designing an optimal display method. The subjective evaluation questionnaire investigated the subjects' preferences towards a reading interface by having them evaluate reading comfort, reading ease, eyestrain, musculoskeletal strain, mental demand, usability, and overall preference. Experiment 1 was performed by examining the effects of display method, display text rate, and observation angle on comprehension performance and preference on LED displays. The results indicated that a display rate of 160 characters per min (cpm) was found to be superior to 240 cpm and 320 cpm in terms of higher comprehension scores and better subjective evaluations. The interaction between display method and text display rate had a significant effect on reading comprehension. Observation angle and display method were found to be non-significant for both comprehension performance and subjective evaluation. Experiment 2 investigated the effects of display method, number of message lines, and display color on comprehension performance and preference for LED displays. The results showed that green text was superior, with higher comprehension scores than amber and red. The condition with the RSVP display method resulted in higher comprehension scores than the scrolling with pauses method. However the scrolling with pauses condition had better subjective evaluation ratings than the RSVP method. A multi-line display was found to be superior to a single-line display, yielding higher comprehension scores and subjective evaluations. Experiment 3 was conducted to evaluate the effects of text direction and highlighting methods of color coding and font format on comprehension performance and preference for LED displays. The results indicated that subjects achieved better comprehension in the highlighting conditions with color coding (red) than those without color coding. It was also noted that regular format with color coding (red) resulted in the best comprehension. Font format and text direction did not significantly affect comprehension or subjective evaluations. Furthermore, the validity of highlighting was also examined and the results showed that the condition with 100% validity of highlighting was found to result in better comprehension performance than other validity levels and conditions with no highlighting. The three experiments successfully revealed the design factors that would affect the readability of electronic text. According to readability and preference ratings of the subjects in different conditions, the optimal display method, color, highlighting method, and its validity for comprehending the delivered messages were investigated. These results provide useful design recommendations for using LED displays for advertising and delivering messages to the public. In summary, this dissertation 1) investigated the effects of display factors on comprehension performance and subject preference for reading Chinese characters on LED displays, 2) investigated the comprehension performance under the conditions of different highlighting validities and highlighting methods, 3) established general ergonomics guidelines applicable to various display configurations for determining the optimum dynamic text display methods, and 4) provided information for formulating further research plans for determining the optimum dynamic text display methods on LED display boards. Notes: CityU Call Number: TK7882.I6 S6 2009; x, 118 leaves : ill. 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2009.; Includes bibliographical references (leaves 90-96)

2009-01-01T00:00:00Z Luo, Binhui (羅斌暉) http://dspace.cityu.edu.hk:80/handle/2031/6269 2011-05-25T01:19:58Z 2010-01-01T00:00:00Z

Title: Preparation of hydrophobic surface on steel by patterning using laser ablation process Authors: Luo, Binhui (羅斌暉) Abstract: Currently there is an increasing demand for the application of hydrophobic surface in industrial and biological processes. It has been found that the contact angle with liquid, closely related to hydrophobicity of a solid surface, is largely determined by micro-geometrical structure and chemistry of the said surface. In this investigation, the hydrophobicity of steel surface was achieved by implementing micro-patterns on substrate using laser ablation process. For the patterning, a short pulse excimer laser with a wavelength of 248nm was used to etch the substrate surface to form different controlled patterns. Based on the models proposed by Wenzel and Cassie-Baxter, the depth of etched grooves and the clearance between two grooves were optimized. The water contact angle of the patterned surface was increased to about 130°, compared to 68.5° found from a plain smooth steel surface (Ra ≤ 0.01 μm). As a preliminary investigation, an amorphous carbon (a-C) coating was deposited on the patterned surface. It shows that the contact angle was increased further by about 10°-20° on a patterned surface. At the same time, the deformation in local surface induced by laser heating was considered. On the patterned surface, the melted substrate was observed to have formed slopes and piled to be ridges along the sides of grooves. With an assumption of overhangs which could suspend the free liquid surface in indentations on a hydrophilic substrate, the relationship between the contact angle and pattern parameters was also discussed. The different geometrical models of the surface profiles were employed to approximate the real laser patterned surface to forecast the contact angle with better accuracy. The real contact area between solid and liquid was calculated. The effect of ridges was also considered when the clearance between two grooves was smaller than 50μm. The original Cassie-Baxter equation was modified with appropriate geometrical models to calculate the contact angle successfully. Notes: CityU Call Number: TA1715 .L86 2010; 2, 70 leaves : ill. 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references (leaves 66-70)

2010-01-01T00:00:00Z