Most Direct Numerical Simulations (DNS) for turbulent flow and heat transfer are performed under the assumption of negligible buoyancy forces. Therefore, the temperature is treated as a passive scalar, and the velocityfield is independent on the temperature distribution. This simplification is not applicable in many situations of practical interest, where relevant buoyancy
forces are present. In this study, we investigate, by means of DNS, a weakly turbulent, fully developed horizontal channel flow under unstable thermal stratification at two different Grashof numbers. Both the Reynolds number, based on shear velocity and channel half-height, and the Prandtl number are set at constant values of 150 and 0.71, respectively. It is found that an increase of the Grashof number leads to a monotonic increase of the Nusselt number, while the drag force initiallydecr eases, and eventually increases at higher values of the Grashof number.
