This study investigates an effect of density stratification on turbulence structures and turbulent heat transfer in a turbulent flow with a free surface. A direct numerical simulation (DNS) technique is applied for this purpose to fully developed turbulence in an open channel. Both positive and negative temperature differences between the bottom wall and the free surface are introduced to impose both stable and unstable density stratification.
Turbulence at a gas-liquid interface has received extensive attention in the last several decades. This is motivated by geophysical interests for better understanding of turbulent heat and mass exchange across an atmosphere-ocean interfaces. It is crucial to understand exchange rate of CO2 and other global-warming gases across the interface, which is considered an important component in the prediction of global warming scenario. Also, many industrial processes in chemical and mechanical engineering, metallurgy and material science involve turbulence with the gas-liquid interface. Such apparatus is often used for separation of gases, hence, turbulent heat and mass transfer at the interface is critically important for optimum designing and controlling the apparatus.
Transport phenomena into turbulent flows across the interface have been investigated experimentally. Komori et al. (1989, 1990) measured the gas transfer rates at the free surface in the open-channel, and they were correlated by the surface-renewal frequency following an idea of the surface-renewal (Dankwerts 1951). Rashidi et al. (1991) carried out similar laboratory experiments on turbulence measurements and presented their result of empirical relation between the gas transfer coefficient and hydrodynamics in the open-channel. In addition to the experimental technique, a direct numerical simulation (DNS) technique has been introduced for exact evaluation of turbulent heat and mass fluxes at the interface (Pan & Banerjee 1995; Nagaosa 1999; Handler et al. 1999; Nagaosa & Handler 2003) in the last decade.
