Institut für Hydromechanik

Direct Numerical Simulation of gas transfer through the air-water interface in a turbulent flow environment

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The present numerical work aims at improving the understanding of the gas transfer mechanisms across the air-water interface in turbulent flow environment. One important example of such a process is the absorption of oxygen from the atmosphere into rivers or lakes. For oxygen as well as other gases having low solubility, the transfer process is controlled by the hydrodynamic conditions on the iquid side concentrated within an extremely thin boundary layer (typically 10 − 100µm) which makes accurate measurements difficult. Even advanced optical techniques face limitations in resolving the minute details of the mass exchange process. The present direct numerical simulations intend to resolve all details of the process and shall in turn complement the existing experimental results. Two different turbulent forcing mechanisms are studied, namely bottom-shear (e.g. river flow) and buoyant-convection (e.g. turbulence caused by surface cooling in lakes).






(a)Bottom-shear case, Sc=32

(b) Buoyant-convection case, Sc=500.

Figure 1: DNS results visualizing the gas transfer process with a) bottom-shear and b) buoyant-convective induced turbulence. As in the experiments, the 3D-DNS results show that the transfer mechanism with buoyant-convective instability is dominated by the continuous movement of sinking and rising plumes, while the continuous motion of eddy structures approaching the interface from below is the main mechanism controlling the gas transfer in shear-free far-field homogenous turbulent environment.


[1] Herlina H. and Wissink J.G. Direct numerical simulation of turbulent scalar transport across a flat surface. J. Fluid Mech., 744:217-249, 2014 [Link]