A study of wake boundary Layer transition using liquid crystals

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This was a joint project between the Whittle Laboratory and the Osney Laboratory of Oxford Univeristy. The experiments were performed in a low free stream turbulence water tunnel. The tunnel was configured so that there was an insignificant variation in the free stream velocity along the plate as measured by using miniature current flowmeter. The velocity profile at several locations has been measured by using hot wire probe. The profile is close to the Blasius profile for a laminar boundary layer under zero pressure gradient. More details of the experimental facility can be found in Zhong et al. (1997a). Figure 1 shows in detail the test section and heated test plate. Measurements were carried out in the Plexiglass Perspex test section, which is rectangular in cross section and is 430 mm wide by 320 mm high.

Figure 1

Encapsulated cholesteric liquid crystals were used to visualise the process of wake-induced transition on the test surface. The liquid crystal material displays colour over a narrow temperature range of, about 5 deg. C. It is red at 18 deg. C and passes through the visible spectrum becoming blue at 22 deg. C. The water temperature was about 17 deg. C for a typical experiment. During the experiment, the test surface is heated until it appears a uniform blue colour under the developing laminar boundary layer. As the turbulent events convect downstream, augmented heat transfer from the surface of the heated plate to these turbulent events increases and the surface cools locally. This causes the liquid crystals to change colour. This colour change not only makes the footprint of these turbulent events visible, but also provides information on the onset and extent of the turbulent spots as they convect downstream. The method has been used successfully by Zhong et al. (1997b) to study the thermal footprint of single artificial turbulent spot. Information obtained on spot characteristics were consistent with those reported by other researchers using different experimental approaches. The internal structure of the spot was revealed together with details on its growth in the spanwise and streamwise directions as it convects downstream.

Figure 2 (a) An artificially created turbulent spot and
(b) wake-induced turbulent spots

Figure 2a is an image of a single artificial spot obtained from Zhong et al. (1997b). Figure 2b is from the present series of experiments and shows consecutive frames of wake-induced events (generated by 4.75 mm diameter bar) as they propagate downstream in a laminar boundary layer. By comparing Fig. 2a and Fig. 2b, it can be concluded that wake-induced transition takes place via the formation of turbulent spots under the influence of the wake. This means that, at the initial stage of wake-induced transition, the boundary layer flow is transitional instead of fully turbulent in the spanwise sense. However, as levels of turbulence in the wake increase, the number of spots across the span increases and the spots are more dense. Consequently the time it takes for the individual spot in the transitional band to merge into each other and for the boundary layer to become fully turbulent is shorter. If the turbulence intensity in the wake is very high, the assumption that the wake is a production source of turbulent spots so dense that the spots immediately form a turbulent strip is reasonable as in the model proposed by Mayle and Dullenkopf (1990). The Reynolds number based on the plate length was kept constant at the value of 300,000.

Papers by Chawalit Kittichaikarn (Oxford) , Shan Zhong (Manchester), Peter Ireland (Oxford), Howard Hodson and others are available in the download section. Papers by Roger Dambach and Howard Hodson are available in the download section. Press here to down load papers and PhD Theses.

Here are some (large - 2MB) movies of the turbulent spots moving through the boundary layer.

Turbulent Spot in Adverse Pressure Gradient (2MB)
Turbulent Spot in Zero Pressure Gradient (2MB)
Turbulent Spot in Favourable Pressure Gradient (2MB)

Howard Hodson and Peter Ireland

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