Multi-plane visualization of an artificially initiated young turbulent spot using liquid crystals

• Structure of young turbulent spot was visualized using liquid crystal tracers.
• Temperature and parameters of young turbulent spot were measured, simultaneously.
• Turbulent spot spanwise overhang was showed in spatial and temporal characterization.
• Technique of applying liquid crystal tracers into unsteady flow was presented.

In this paper, the temperature inside an artificially generated young turbulent spot in a low free stream turbulence water tunnel were measured experimentally, using a digital particle image thermometry (DPIT) method by seeding slurry thermochromic liquid crystals to the flow. The measurements were conducted on both an elevation view at distances of 0, 10, 20 mm from the central plane, and a plan view at distances of 3, 6 mm from the flat plate at times of 0.8, 1.2, 1.6, and 2 s after the spot initiation. During the test, the Reynolds number, turbulence intensity, and temperature of the mainstream was maintained at 75,000, 0.93%, and 23.6 °C, respectively. Meanwhile, the temperature of the heated plate, considered as the heat source of the turbulent spot, was kept at 25.4 °C. The results showed that this technique provides both an instantaneous temperature distribution throughout an entire spot structure and the spot parameters such as the spot celerities, the half spreading angles, and non-dimensional spot propagation parameter simultaneously. Moreover, the mechanism of the upward heat diffusion was observed and discussed. Finally, when the obtained structures were coincided with the temperature contour of the turbulent spot footprint, the heat convection characteristics of turbulent spot were, then, further clarified.