Near-wall velocity structures that drive turbulent transport from a line source at the wall

This work involves numerical experiments that were conducted in a turbulent channel flow using direct numerical simulation in conjunction with the tracking of thermal markers released from a single instantaneous line source at the wall of the channel. The simulations were conducted at Re-tau = 300 and for different Prandtl number fluids (Pr = 0.1, 0.7, 6, 20, and 50), in order to investigate the physical mechanism of turbulent transport and the flow structures that promote turbulent transport. The behavior of these heat markers was observed in 12 locations ranging from 1 to 12 half-channel heights downstream from the source. In each of these locations the flow structures that carried thermal markers towards the channel center (i.e., those that transfer heat away from the wall) and flow structures that carried thermal markers towards the channel wall (i.e., those that transfer heat towards the wall) were identified and characterized as a function of the Prandtl number. The characteristic time and length scales were obtained based on the calculation of correlation coefficients for the markers that were captured in each of the downstream locations. The average width of the heat-transferring plumes that have positive vertical velocity fluctuations is larger than the plumes with negative vertical velocity. Even though areas where heat markers are present extend for a long distance downstream, consistent with a sheet-like behavior of thermal plumes, it is found that the eddies associated with these thermal structures are considerably shorter and a synergism among these short eddies, both close and farther away from the wall, is needed in order to transport heat. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3689194]