Institute of Fundamental Technological Research

The temporal evolution of a thermal pattern observed on a heated wall by infrared camera is correlated with the propagation velocity of the thermal perturbations calculated by DNS. In the experiment the propagation velocity was measured by using PIV-based analysis of infrared images of the thermal pattern on the wall. To verify the experimental technique of image analysis, a sequence of synthetic images, simulating thermal patterns on the wall, was generated from the DNS solution, and the convective velocity was evaluated. It was found that the convective velocity of thermal structures obtained by PIV-based analysis of the experimental and synthetic images was in relatively good agreement with that calculated from the DNS solution. The present study confirmed that for a high Prandtl number fluid (water) the propagation velocity of the thermal perturbations is only about half of the convective velocity of the velocity perturbations. It was also found that the convection velocity observed for hot spots is distinctly lower than that for the cold spots.

This paper deals with measurements of convective velocity of large-scale thermal structures, using the thin foil technique and infrared thermography to visualize the thermal pattern on the wall. An image correlation method is proposed to track the displacement of the observed thermal pattern. The idea of the method is similar to that of particle image velocimetry, but the thermal patterns on the heated wall are used, rather than tracing particles. On this basis, the thermal patterns created by the coherent structures of turbulent channel flow are examined. Particular attention is paid to the determination of the optimal parameters of image acquisition, including spatial and temporal separation. An attempt is made to relate momentum and scalar transport analyses by considering the propagation velocity of large-scale temperature structures. The proposed technique appears to be an attractive alternative for non-intrusive analysis of turbulent flow, especially, where opaqueness of channel walls excludes the use of optical methods.