Transition to turbulence in the boundary layer of turbomachinery blading
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.