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No description available.

No description available.

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An analysis of a commercial vehicle drum brake fitted with a typical asbestos free friction material has been investigated by the finite element method. The "GAPFRIC" concept has been extended to model in three dimensions the frictional interface between brake linings and drum. This approach incorporates an accurate representation of the brake itself to include such features as the brake drum stiffener and mounting flange. The brake shoe representations include the actual web and platform thus eliminating the need for shoe stiffness approximations to a curved beam of uniform section as used in previous two dimensional work. The mechanical "GAPFRIC" analysis is combined with a thermal analysis of the brake to form a brake analysis package. The package is fully automatic; the output from each stage of the analysis is post processed and the results used to modify the original data file. Variations in physical properties exist between new and used friction material and these are incorporated in a five phase idealisation of the friction pair. Modifications to allow for the change of coefficient of friction with temperature were made by means of tables within the coding. In addition friction material wear was included in the analyses using an empirically derived wear criteria. Analyses were completed to investigate the effect of combined axial and circumferential distortions on temperatures at the friction interface, interfacial pressure distributions and subsequent brake performance. Predicted results show that high temperatures are reached at certain regions on the rubbing path and the temperatures may fluctuate during a brake application. Pressure variations are seen to exist both around and across the surface of the linings. The coupling of pressure and temperature variations combined with frictional changes over the lining produce changes that result in the frictional drag per unit area tending to be reasonably constant over the interface between drum and lining. The predicted values of brake torque and brake factor f rom the three dimensional analyses have been compared with results derived f rom the earlier two dimensional brake analysis and validated by comparison with measured results from a brake mounted on a dynamometer. Similarly predicted brake drum and lining temperatures were compared with measured values and some reasonable trends established. The work itself presents a better physical description of the behaviour at the friction surface during braking to improve the determination of brake drum performance.

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The effects of air trapped in soils at zero or small negative values of matric potential were studied in relation to saturated and unsaturated hydraulic conductivity, the water release characteristic, and root growth. A reliable method was developed for measuring the trapped air porosity of 'saturated' soils. Using this method, the surface layer of three soils of contrasting texture, in a relatively undisturbed state, was found to have similar amounts of trapped air (5 to 7% of total soil volulme). After digging, the amount of air trapped by ponding water on the surface increased to a value of about 10%. Values of trapped air porosity below a stationary water-table were very low (1%) and this can be attributed to opportunity which the trapped air has to go into solution and diffuse to the water-table. The effects of trapped air on the water release characteristic were investigated in both field and laboratory studies using the neutron-probe and both the neutron-probe and the gamma-probe to measure the water content in the field soil and in a tank packed with sand in the laboratory, respectively. The presence of trapped air had an effect on the field water release characteristic. This effect was more pronounced in dug soil than in undisturbed soil. In the laboratory, the amount of trapped air differed according to the method of wetting, less trapped air occurred when wetting was from the bottom upwards compared to wetting by ponding water from the top. Amounts of trapped air decreased with increasing depth of sand. The water release characteristic was found to depend not only on the method of wetting but also on the previous history of wetting. In a comparison between a tension-table water release and the water release characteristic measured in a sand tank, they were found to be closely similar except at zero and small negative values of matric potential, where effects due to varying amounts of trapped air with depth and wetting history caused differences. Unsaturated hydraulic conductivity measured in the field was affected by the presence of trapped air but only at potentials close to zero. In the laboratory, the values of saturated hydraulic conductivity measured in the sand tank and the values calculated from the water release characteristic using Marshall's theory agreed to within the limits of experimental accuracy. Unsaturated hydraulic conductivity values were found to be not only dependent on the method of wetting but also on the previous history of wetting. The results suggested that once dry sand has been wetted and allowed to drain, that rewetting leads to trapping of air in large pores which reduces the flow during the next drainage period. There was an agreement between unsaturated hydraulic conductivity variation with matric potential obtained from the instantaneous profile method in the sand tank and that using Marshall's theory, however, it is not to be expected that such an agreement will be found for most soils which are not comparable to the relatively homogenous sand used here. In a laboratory experiment with winter barley, the relationship between the presence of trapped air, soil aeration and root growth was studied at 9 and 15°C. The techniques of measuring redox potential with bare platinum electrodes, measuring exygen flux with the same platinum electrodes, and measuring the concentrations of O₂, CO₂ and N₂O in samples extracted through a hypodermic needle gave results which were consistent with one another and with observed root growth rates. Root extension rate was much slower in the saturated soil than in the freely-drained soil. At both temperatures, root growth during 'daytime' (the time of illumination) was about 3 times greater than at night. At 15°C, there was a greater rate of root growth in soil cores which had been flooded rather than vacuum saturated which was related to a slightly greater measured rate of oxygen flux from air trapped by the flooding procedure.

The following thesis investigates the performance and economics of a Pneumatic Water Engine capable of extracting energy from differential heads of water in the two to three metre range. Initial concepts are discussed and a system configuration is physically modelled at a laboratory scale. Outline designs using a variety of materials are developed and these provide a basis for the estimation of a probable capital cost using standard Civil Engineering methods. The proposed system is mathematically modelled using a lumped mass approach to the complex hydrodynamics. The resultant differential equations are solved by means of a variable Runge Kutta numerical analysis. The model includes the thermodynamic aspects of the system's compressible airflow. A computer program has been developed from the mathematical model and Is utilized in a series of parametric studies. An economic assessment based upon both the average power output achieved during the parametric studies and the probable capital cost of the system is presented, together with an estimate of the cost per kilowatt-hour of the electricity produced. This assessment takes into account maintenance costs, expected value of the energy produced and the possible effects of both Water Abstraction Charges and Local Authority Rating. In addition to the parametric studies a final, more rigorous optimization of the system involving a number of the many interacting variables has been undertaken. This optimization is achieved via Cumulative Evolutionary Design techniques involving the use of Genetic Algorithms. An optimal design of the chamber shape is achieved in the same manner.

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