Diabetic retinopathy is an eye disease that causes blindness amongst many people with diabetes if left untreated. When the eye is affected, various changes in the blood vessels occur. To be able to observe these changes over time, images of the back of the eye called retinograms are acquired. Automated analysis of retinograms becomes important as the number of people afflicted with diabetes increases worldwide. A diabetic retinopathy detection system should be able to analyse retinograms, interpret them and record the changes over time. The methods associated with blood vessel detection in retinograms have a common drawback which is that small vessels that have low contrast are normally missed out during detection. Evaluation of these methods is commonly carried out using a technique which has the drawback of having a bias towards the detection of thick vessels. In this thesis a retinal vessel detection method is proposed which is capable of detecting blood vessels of different width, length and orientation in the back of the eye. The state-of-the-art method proves to be comparable to the existing methods applied to the same images used in this project. When applied for segmenting the whole vessel network, it achieves an area under the ROC curve (Az) of 0.960(±0.0021)599 compared to the best result of 0.9722 obtained via another method. When applied for vessel centreline detection, it achieves an Az of 0.977(±0.0013)66 which is higher than the best method with an Az of 0.967(±0.0017). These results are obtained using an evaluation method proposed in this project that eliminates the drawback of current evaluation methods hence removes the bias towards the detection of thick vessels.

This work investigated a numerical approach to the search of a maximum heat transfer rate density (the overall heat transfer dissipated per unit of volume) from a two-dimensional laminar multiscale array of cylinders in cross-flow under an applied fixed pressure drop and subject to the constraint of fixed volume. It was furthermore assumed that the flow field was steady state and incompressible. The configuration had two degrees of freedom in the stationary state, that is, the spacing between the cylinders and the diameter of the smaller cylinders. The angular velocity of the cylinders was in the range 0 ≤ ϖ, ≤ 0.1. Two cylinders of different diameters were used, in the first case, the cylinders were aligned along a plane which lay on their centrelines. In the second case, the cylinder leading edge was aligned along the plane that received the incoming fluid at the same time. The diameter of the smaller cylinder was fixed at the optimal diameter obtained when the cylinders were stationary. Tests were conducted for co-rotating and counterrotating cylinders. The results were also compared with results obtained in the open literature and the trend was found to be the same. Results showed that the heat transfer from a rotating array of cylinders was enhanced in certain cases and this was observed for both directions of rotation from an array which was aligned on the centreline. For rotating cylinders with the same leading edge, there is heat transfer suppression and hence the effect of rotation on the maximum heat transfer rate density is insignificant. This research is important in further understanding of heat transfer from rotating cylinders, which can be applied to applications ranging from contact cylinder dryers in the chemical processes industry and rotating cylinder electrodes to devices used for roller hearth furnaces. / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / unrestricted