Gas-transfer at unsheared free-surfaces

Transport processes through the gas-liquid interfaces are of paramount importance in environmental fluid mechanics. In environmental systems gaseous substances may be directly exchanged between air and water in either direction across the air-water interface. Gas fluxes being transferred can be upward to the air or downward to the water depending on the substances involved. Nowadays, despite the significant theoretical, laboratory, field and numerical studies, research efforts have not yet achieved a complete understanding of gas-transfer process. In this chapter gas-transfer is introduced. First it is explained how the physicochemical characteristics of the substance being transferred affect gas-transfer process and they can control which phase governs the process. Second the interaction between turbulence in the bulk liquid and the air-water interface a for a substance being controlled by the liquid phase. Third, gas-transfer in open channel flows is presented in detail discussing the literature conceptual models for the prediction of gas-transfer rate. Then, recent efforts about the numerical simulation of gas-transfer process at an unsheared interface are described. The chapter ends pointing out the challenges to be faced and the needs for future research about gas-transfer.