Proceedings of the 2001 International Symposium on Environmental Hydraulics Copyright © 2001, ISEH Application of Liquid Crystal Tracers for Full Field Temperature and Velocity Measurements Tomasz A. Kowalewski Polish Academy of Sciences, IPPT PAN, Swietokrzyska 21, PL 00-049 Warsaw, Poland 1. Introduction It is common in computational fluid dynamics research to validate numerical models against laboratory experiments. It is because due to the problem complexity, it is not a trivial task to determine precisely an error of the numerical results. Errors appearing due to limited accuracy of different numerical methodologies, and due to inevitable simplifications introduced in the models, are usually difficult to predict a priori. For thermally driven flow the velocity and temperature fields are primary measured quantities. Modern measuring techniques offer several methods for precise measurements. However, most of these methods are based on point measurements, leading often to imprecise or misleading conclusions. Hence, full field measurements of velocity and temperature gained great importance. With this objective in view a new experimental technique based on a computational analysis of the color and displacement of thermochromic liquid crystal tracers (TLCs) was applied to determine both the temperature and velocity fields of the flow. Small particles of the liquid crystal material suspended in the fluid ideally play a role of tracers following the flow pattern 1 . Using standard PIV technique, the local velocity of the flow can be measured by cross-correlating two sequential images. In addition these particles change color with their temperature. Hence, after proper calibration, they behave as small thermometers simultaneously monitoring local fluid temperature. Full 2-D temperature and velocity fields are determined from a pair or a longer sequence of color images taken for the selected cross-section of the flow 2 . We found that only with help of the full field measurements it was possible to identify serious discrepancies between numerical predictions and the experimental observations, even for apparently simple cases of laminar flow. With help of the experimental feedback it was possible to introduce necessary modifications to numerical codes and in such a way to improve its output in the correct direction 3 . In the following we describe our experience in applying the method to selected problems studied in our laboratory. They include modeling flow configurations in the differentially heated inclined cavity with vertical temperature gradient simulating up-slope flow as well as thermal convection under freezing surface [4,5]. The main aim of these experimental models is to generate reliable experimental database on velocity and temperature fields for specific flow [6]. Such data can serve further validation of numerical codes before they are applied to environmental problems. 2. Experimental technique The temperature visualization is based on the property of some cholesteric and chiral-nematic liquid crystal materials to refract light of selected wavelength as a function of the temperature and