DNS of turbulent heat transfer in channel flow with low to medium-high Prandtl number fluid

DNS of turbulent heat transfer in channel flow with respect to Reynolds and Prandtl number effects

Surface heat-flux fluctuations in a turbulent channel flow up to Reτ=1020 with Pr=0.025 and 0.71

The structure of a turbulent flowfield along a riblet surface was investigated with the aid of a three-dimensional particle tracking velocimetry. The statistics of all three velocity components were measured and compared with those above a smooth wall. Under a drag-reducing condition, all of the turbulent velocity fluctuations and the Reynolds shear stress were decreased near the riblet surface, although the flow characteristics in most of the flowfield were quite similar to those above the smooth wall. It was also found that the redistribution mechanism of the turbulent kinetic energy from the streamwise component to the spanwise one was considerably suppressed in the region above the riblet valley. On the other hand, under a neutral drag condition, a cross-stream secondary flow was apparent near the ribs. This fluid motion should enhance the turbulent momentum transport and deteriorate the drag-reducing effect of the riblet.

Turbulent Drag Reduction Mechanism Above a Riblet Surface

Use of direct numerical simulation to study the effect of Prandtl number on temperature fields

A direct numerical simulation of the fully developed thermal field in a two-dimensional turbulent channel flow was carried out. The simulation was made at a molecular Prandtl number of Pr = 0.025 to investigate low Prandtl number effects on the transport mechanism in wall turbulence. The computation was executed on about 1.6 × 106 grid points by using a spectral method. Mean field parameters as well as various thermal turbulence statistics including rms temperature fluctuations, turbulent heat fluxes and turbulent Prandtl numbers were obtained. Each term in the budget equations of temperature variance, its dissipation rate and turbulent heat fluxes was also calculated in order to establish a data base of convective heat transfer for thermal turbulence modeling. The results were compared mainly with those at Pr = 0.71 obtained by Kasagi et al. (1992). In addition, the computed thermal fields were visualized to investigate the role of the near-wall quasi-coherent structures in the turbulent transport mechanism.

N. Kasagi

Papers

Direct Numerical Simulation of Low Prandtl Number Thermal Field in a Turbulent Channel Flow