Non-intrusive and simultaneous measurements of the Lagrangian dynamics and the fluid temperature field applied to the study of natural convection*

Natural convection continues to be an interesting research topic since a variety of fundamental questions are not fully addressed, and new and exciting experimental techniques offer opportunities to cast further light on the topic. In this study, the three-dimensional convection is generated in octagonal water tank with diameter of 33cm and aspect ratio of 0.753, with heating from the bottom and cooling from the top. The nonintrusive measurement of velocity and acceleration are performed with our Lagrangian Particle Tracking system. Simultaneously, color signals of thermochromic liquid crystals (TLCs) are recorded to perform temperature measurements in the illumination plane. The main reason why TLCs are used for temperature measurement is that they display a definite color at different temperatures. Calibration, which correlates temperature and colors of TLCs, is the essential step of this methodology. The calibration process presents the challenge of correlating 3 channel color information (RGB) with a scalar, temperature. To overcome this challenge, proper orthogonal decomposition (POD) is applied to the RGB data. As a result, statistics of velocity, acceleration and temperature measurements are obtained for different Rayleigh numbers.