Gas-transfer across the turbulent air-water interface of a surface water body is a relevant process in the environmental fluid mechanics area. The movement through this interface of oxygen, carbon dioxide, nitrogen and toxic chemicals can greatly affect water quality levels. In this chapter the gas-transfer of sparingly soluble gas, such as oxygen, carbon dioxide and many environmental contaminants, across the free surface of rivers and streams was discussed in details in terms of experimental measurements and observations, predictive models and numerical simulations. The transfer of these substances across the air-water interface is controlled by the processes occurring in a thin region below the interface. Also, in open channel flows turbulence is mostly generated at the channel bottom wall and is then self-transported towards the free surface. Hence, this condition leads to define the air-water interface as unsheared or shear-free. Both experimental and numerical studies as well as theoretical analysis have pointed out the role played by turbulence characteristics into the gas-transfer process. Turbulent structures produced in the bed region move upward to the free surface and interact with it producing a renewal of the near-surface layers of flow, which controls the gas-transfer process. Although the classic analysis leading to the surface renewal theory by Higbie and Danckwerts can be considered as an adequate general picture of the process, considerable efforts are currently produced to understand how turbulent coherent structures affect surface-renewal. Hence, conceptual models proposed to describe this process and to predict its rate KL have tried to relate it to both global and local properties of turbulence. The models based on global properties, such as large-eddy and small-eddy models, relate KL to the Schmidt number and the turbulent Reynolds number of the flow, which is defined with the aid of the integral length scale and some velocity scale. They basically differ on the range of scales which is assumed to control the gas-transfer process, that is large scale or small scale. However, to solve this conflict, it was proposed that both scales would be involved in the process and their relative importance would depend on the value of turbulent Reynolds number. More recently, models based on local properties of turbulence, that is interfacial turbulence characteristics, were proposed. Basic concept of these models is the surface divergence, that is β parameter, which is the vertical velocity gradient. This parameter is related to the horizontal velocity fluctuations. Recent numerical simulations were able to resolve both velocity and concentration fields near the air-water interface. Numerical results have pointed out that positive and negative values of β correspond to large-structures reaching the interface or moving downward from it, carrying low and high concentration. Despite these important advances in gas-transfer understanding and modelling, many efforts are still needed to achieve a complete knowledge of this process. First, even if recent developments in experimental techniques are encouraging, they still require improvements to made measurements very close to the air-water interface, in the uppermost layers of the flow, which control the transfer of the gas across the free surface. Also, it is very important that detailed measurements of concentration field would be linked with simultaneous measurements of near-surface velocity field. Second, even if numerical methods have provided detailed and precise determination of velocity and concentration fields very near to the air-water interface pointing out relevant features of the interaction between turbulence and gas-transfer process, these methods should be extended to higher both Schmidt and Reynolds numbers to encompass typical conditions existing in streams and rivers. Also, the influence of the turbulent anisotropy on the relationship between gas-transfer rate and Reynolds number should be further investigated. Finally, future modelling efforts should be addressed to take into account the role of all the turbulent scales in the gas-transfer process.

Gas-transfer at unsheared free surfaces

GUALTIERI, CARLO;PULCI DORIA, GUELFO
2008

Abstract

Gas-transfer across the turbulent air-water interface of a surface water body is a relevant process in the environmental fluid mechanics area. The movement through this interface of oxygen, carbon dioxide, nitrogen and toxic chemicals can greatly affect water quality levels. In this chapter the gas-transfer of sparingly soluble gas, such as oxygen, carbon dioxide and many environmental contaminants, across the free surface of rivers and streams was discussed in details in terms of experimental measurements and observations, predictive models and numerical simulations. The transfer of these substances across the air-water interface is controlled by the processes occurring in a thin region below the interface. Also, in open channel flows turbulence is mostly generated at the channel bottom wall and is then self-transported towards the free surface. Hence, this condition leads to define the air-water interface as unsheared or shear-free. Both experimental and numerical studies as well as theoretical analysis have pointed out the role played by turbulence characteristics into the gas-transfer process. Turbulent structures produced in the bed region move upward to the free surface and interact with it producing a renewal of the near-surface layers of flow, which controls the gas-transfer process. Although the classic analysis leading to the surface renewal theory by Higbie and Danckwerts can be considered as an adequate general picture of the process, considerable efforts are currently produced to understand how turbulent coherent structures affect surface-renewal. Hence, conceptual models proposed to describe this process and to predict its rate KL have tried to relate it to both global and local properties of turbulence. The models based on global properties, such as large-eddy and small-eddy models, relate KL to the Schmidt number and the turbulent Reynolds number of the flow, which is defined with the aid of the integral length scale and some velocity scale. They basically differ on the range of scales which is assumed to control the gas-transfer process, that is large scale or small scale. However, to solve this conflict, it was proposed that both scales would be involved in the process and their relative importance would depend on the value of turbulent Reynolds number. More recently, models based on local properties of turbulence, that is interfacial turbulence characteristics, were proposed. Basic concept of these models is the surface divergence, that is β parameter, which is the vertical velocity gradient. This parameter is related to the horizontal velocity fluctuations. Recent numerical simulations were able to resolve both velocity and concentration fields near the air-water interface. Numerical results have pointed out that positive and negative values of β correspond to large-structures reaching the interface or moving downward from it, carrying low and high concentration. Despite these important advances in gas-transfer understanding and modelling, many efforts are still needed to achieve a complete knowledge of this process. First, even if recent developments in experimental techniques are encouraging, they still require improvements to made measurements very close to the air-water interface, in the uppermost layers of the flow, which control the transfer of the gas across the free surface. Also, it is very important that detailed measurements of concentration field would be linked with simultaneous measurements of near-surface velocity field. Second, even if numerical methods have provided detailed and precise determination of velocity and concentration fields very near to the air-water interface pointing out relevant features of the interaction between turbulence and gas-transfer process, these methods should be extended to higher both Schmidt and Reynolds numbers to encompass typical conditions existing in streams and rivers. Also, the influence of the turbulent anisotropy on the relationship between gas-transfer rate and Reynolds number should be further investigated. Finally, future modelling efforts should be addressed to take into account the role of all the turbulent scales in the gas-transfer process.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11588/203283
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