[Univ of Cambridge][Dept of Engineering]

A study of wake boundary Layer transition using liquid crystals


Wakes shed by upstream blade rows are known to cause boundary layer transition in both the compressor and turbine stages of axial flow gas turbines. This transition process is believed to take place via discrete zones of turbulence known as turbulent spots which occur in an otherwise laminar boundary layer. However, the process of transition over the blade surface cannot, at present, be reliably predicted. This is due to a lack of information on where and when these turbulent spots form and how they grow and merge as they convect downstream to form the turbulent boundary layer.

The following pages present detailed experimental information on the process of boundary layer transition induced by a bar generated wake travelling over a zero pressure gradient laminar boundary layer on a flat plate. The Reynolds number was 300,000. The peak turbulence intensity within the wakes varied from 3 to 6 % by using different bar of different diameters. An encapsulated cholesteric liquid crystals coating has been employed on a heated flat plate to reveal detailed information over the full surface. The information includes the thermal characteristics, the spot onset and formation rate. Data were obtained at high resolution on a grid of 30,000 points. The results were compared to intermittency plots and time-distance diagrams obtained by using surface-mounted thin film gauges and found to be similar. The data were also consistent with well established correlations and other published data from the literature for existing wake-induced transition models.

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Howard Hodson and Peter Ireland


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