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

In the present study, direct numerical simulation of turbulent heat transfer in a channel flow has been carried out in order to investigate the characteristics of surface heat-flux fluctuations. The Reynolds numbers based on the friction velocity and the channel half width are 180, 395, 640 and 1020, and the molecular Prandtl numbers are 0.025 and 0.71. A local peak for Pr=0.71 and large peaks for Pr=0.025 appear in the spanwise-wavenumber power spectra at low wavenumbers, and these peaks become more significant with increasing Reynolds number. This suggests that the effect of large-scale structures extends even to the surface heat-flux fluctuations, and increases with increasing Reynolds number. In addition, it was found that the surface heat-flux fluctuations for Pr=0.71 are mostly similar to the streamwise wall shear-stress fluctuations, while a noticeable dissimilarity can be seen in the large positive and negative fluctuations.