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This thesis is concerned with the investigation of boundary layer transition phenomena in relation to turbomachinery flows. The present study involved the experimental investigation of the flow field around flat plates with various leading edge configurations. This study has also included some theoretical predictions using computational fluid dynamics. The experimental investigation involved a variety of free stream environments which have attempted to simulate typical turbomachinery flows. Three flat plate configurations with a sharp, a semi-circular and a C4 leading edge shapes have been employed. The effect of free stream turbulence intensity over a range of free stream Reynolds numbers has been examined. Surface flow visualisation techniques have been applied to a total of 36 different experimental conditions in order to define the transitional characteristics of the boundary layer. This flow visualisation method was found to be appropriate for the large range of test cases involved and especially for the flat plate with the cylindrical leading edge. For the cylindrical leading edge configuration, a separation bubble has been detected in the vicinity of the leading edge, under all tested conditions. Hot-wire investigation of the boundary layer has been undertaken over the flat plate with the C4 leading edge, which has been regarded as the most relevant configuration for turbomachinery applications. This method provided high frequency velocity data which have been analysed in order to obtain information about the spatial distribution of integral parameters of the boundary layer, such as the mean velocity, the turbulence intensity and the skewness and flatness factors. Boundary layer spectra have also been acquired. Statistical analysis of this data has been employed in order to obtain the intermittency distribution. The present results have been found to be in good agreement with existing transition correlations. Calculations of the flow over flat plates with leading edge shapes similar to the experimental configuration have been undertaken using a two-dimensional elliptic Navier-Stokes solver. A solution of the flow field around a semi-circular and an elliptical leading edge has been obtained. Low Reynolds number k-Ɛ modelling has been applied in order to model the transitional characteristics of the boundary layer flow very near the wall. The use of the Nagano-Hishida version of the low-Reynolds number turbulence model led to predictions of an early start and end of transition.

This thesis is divided into six chapters. The first chapter provides a brief introduction concerning the behaviour of brittle materials. It also contains the justification for the undertaking of the study as well as a brief description of the method of approach adopted, and thesis layout. Chapter two provides a critical review of the current literature available at present in the failure prediction brittle materials. Both theoretical and experimental studies are discussed and the relevance to the present work is justified. Chapter three deals with the numerical analyses adopted within the thesis. Five different failure criteria were utilized in the initial analysis of the results presented. Among them, the empirical model using the Principle of Independent Action satisfactorily represents the biaxial fracture behaviour of brittle materials in both tension-tension and tension-compression quadrants. Its validity has never been tested before. Various statistical fracture models were used to analyze the failure of brittle materials under multiaxial states of stress, the experimental failure data for simple tension being a starting point for their calculation. It was shown that the Energy Density theory led to a better agreement with the experiments than any other well-known fracture criterion. The study investigates methods of evaluating the Weibull parameters which were crucial in the failure prediction of brittle materials. Monte Carlo simulation techniques are also presented as a method of evaluating the data ranking for the failure probability of brittle materials. Chapter four is devoted to the description of experimental techniques adopted in the study, using specially designed rigs. Six different tests were conducted to evaluate the performance of brittle materials in static loading and also to enable comparisons with the theoretical predictions. Attention was given to specimen casting, loading frames, alignment, measurement techniques and other relevant parameters. The use of the linear elastic fracture mechanics method to predict the behaviour of cracks in bodies, which are subjected to steady stresses, is discussed. The compliance function for the three-point notch bend specimen is presented in addition to the determination of the fracture toughness of Herculite LX plaster. The work was also supplemented by the use of scanning electron microscopy (SEM) to failure analysis of plaster material. This is an extremely important tool in the study of brittle materials since the dimensions of small defects and fracture features on individual grains are often pertinent information to the failure analysis. Chapter five details the analysis of the theoretical results as well as the experimental findings. Based upon the previously mentioned approaches, a comparison was made between theoretically predicted and experimentally observed data. The comparison indicates that discrepancies exist between the observed and predicted results, the reasons for the discrepancies have been justified in this work. Chapter six provides a brief summary of conclusions derived from the complete study, together with recommendations for future work.