Experiments in a turbine cascade for the validation of turbulence and transition models

Abstract

This thesis presents a detailed investigation of the secondary flow and boundary layers in a large scale, linear cascade of high pressure turbine rotor blades. The puropose of the data is to provide a suitable test case to aid the design and validation of the turbulence and transition models used in computational fluid dynamics. Hot-wire measurements have been made on a number of axial planes upstream, within and downstream of the blades to give both the mean flow conditions and all six components of Reynolds stress. Suitable inlet conditions have been defined at one axial chord upstream of the blade leading edge where the velocity and turbulence have been measured in both the freestream and endwall boundary layer. The turbulence dissipation rate has also been measured in order to define fully the inlet flow, a quantity that is usually missing in other data. Measurements through the blade show that the turbulence generation associated with the secondary flows is considerable and that all three shear stress components are significant. Intermittency measurements close to the endwall and blade surfaces show that the boundary layers are mostly laminar or transitional. The new endwall boundary layer, that forms behind the separation line, was found to be initially laminar. On the suction surface transition occurs over the latter part of the blade and on the pressure surface the accelerating flow causes relaminarisation. A number of calculations using a mixing length and high and low Reynolds number k-ϵ calculations show that reasonable overall results may be obtained. The lack, or failure, of transition modelling caused profile losses to be generally overpredicted and there was little evidence that the more sophisticated models produced better results. No model accurately predicted the individual turbulence quantities largely due to the inadequacy of the Boussinesq assumption for this type of flow. Good transition modelling appears to be more important than turbulence modelling in terms of the overall results.