The presence of high and low temperature streaks close to the wall, similar to the high and low Reynolds stress streaks for the velocity field, has been observed for low Pr fluids in turbulent channel flow [4, 5]. These previous studies have found that the velocity and the temperature streaks show a strong resemblance to each other. In fact, turbulence producing streaks are correlated to flow structures that produce temperature fluctuations. The streaks play a very important role in the generation of temperature fluctuations, and in the mechanism of turbulent heat transfer, because they are responsible for moving fluid of different temperature to different regions of the flow. The effect of the Prandtl number on the temperature streaky structures has not been explored, however, even though it can be quite significant within the viscous wall sublayer. The presentation will focus on medium and high Pr fluids in plane channel and plane Couette flows, using the data from DNS/LST method. The instantaneous velocity field and thermal fields can be visualized with this method, and the correlation between the flow structure and the thermal field structure as a function of Pr will be discussed. The connection between the turbulent flow structure close to the wall and the mechanism of turbulent heat transport in wall turbulence will be explored. As the Pr increases, turbulent heat transfer is more and more the result of strong fluid motions either away or towards the channel wall. These structures are spaced several hundred wall length units from each other. For low Pr, however, smaller flow structures contribute to the transport of heat. These results can have implications for the development of a model for the prediction of the turbulent Prandtl number in wall turbulence as a function of the molecular Prandtl number.
 B.M. Mitrovic, P.M. Le, D.V. Papavassiliou, On the Prandtl or Schmidt number dependence of the turbulence heat or mass transfer coefficient, Chemical Engineering Science 59(3) (2004) 543-555.
 P.M. Le, D.V. Papavassiliou, Turbulent dispersion from elevated sources in channel and Couette flow, AIChE Journal 51(9) (2005) 2402-2414.
 P.M. Le, D.V. Papavassiliou, Turbulent heat transfer in plane Couette flow, Journal of Heat Transfer – Transactions of ASME 128 (2006) 53-62.
 H. Kawamura, H. Abe, Y. Matsuo, DNS of turbulent heat transfer in channel flow with respect to Reynolds and Prandtl number effects, International Journal of Heat and Fluid Flow 20 (1999) 196-207.
 H. Abe, H. Kawamura, Y. Matsuo, Direct numerical simulation of a fully developed turbulent channel flow with respect to the Reynolds number dependence, Hournal of Fluids Engineering – Transactions of ASME 123 (2001) 382-393.