Visualization of Thermal Structure of Turbulent Spot under Adverse Pressure Gradients Using Liquid Crystals

Abstract

The pressure gradient affects the stretching and retracting of a streamwise length of the transition zone of a fluid flow over a flat plate. Furthermore, the turbulent spot is directly controlled in size by the pressure gradient. The details were investigated of the evolution of the thermal footprint (the area of heat transfer underneath a turbulent spot) using thermochromic liquid crystals under various adverse pressure gradients. The experiment was carried out over an isothermal flat plate in a low turbulence water tunnel with a turbulent intensity of 0.925%. Information was obtained on the development of the temperature, the heat transfer coefficient and the heat flux contours of the footprint as it propagated downstream. All spot parameters in this study, achieved from the use of the developed image processing code were slightly different to those reported by others. The heat transfer characteristics of the footprint were deduced by an “implicit” heat balance approach. Moreover, the characteristics of the turbulence breaking down under the influence of adverse pressure gradients were presented by representative structures of the turbulent spot footprint. Thus, these results could be applied as qualitative and quantitative data into numerically predictive formulas, used for the flow under a boundary layer transition, induced by an adverse pressure gradient.