AbstractAn experimental investigation of the development of the boundary layer in a simulated turbine blade flow is described.
A velocity distribution typical of a "squared-off" turbine blade was reproduced in an open return wind tunnel. Measurements were made in the boundary layer formed on a flat, polished aluminium plate using standard hot wire instrumentation. An automatic data acquisition and control system was based around an Amstrad PC1640 16-bit microcomputer. By using assembly language subroutines, very fast sampling rates could be achieved which allowed a detailed digital representation of the raw signal to be captured. The relatively large RAM capacity of the Amstrad PC1640 allowed samples of sufficient duration to be stored. The mean flow variables, such as the mean velocity and RMS, were calculated by software thus reducing the requirement for external instrumentation. Assembly language subroutines were again necessary to process the large quantities of data involved within acceptable timescales.
An algorithm was developed to discriminate between laminar and turbulent flow. This was used to conditionally sample the signal in the transition region and hence provide mean laminar and turbulent velocity profiles in the transition region. It also provided a measurement of the intermittency. Boundary layer profiles were measured under the imposed velocity distribution at various levels of freestream turbulence giving a range of normal and separation bubble type transitions. The concept of statistical similarity of transition regions was observed to remain intact for severe adverse pressure gradients, including cases where separation bubbles were present during the early stages of transition.
An original correlation for the spot formation rate parameter, N, was developed using the adverse pressure gradient data of Gostelow (1989b). This correlation accounted for the combined effects of freestream turbulence and adverse pressure gradient. The correlation was found to give better prediction of the transition length in turbine blade flows than currently used methods.
The conditionally sampled velocity profiles indicate that intermittent laminar separation can occur in the early part of the transition region.
|Date of Award||Aug 1990|