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Measurement of Local Gas Transfer Velocity in Open-Channel Turbulence

Author(s): Michio Sanjou; Takaaki Okamoto

Linked Author(s): Michio Sanjou, Takaaki OKAMOTO

Keywords: Gas transfer; Open-channel turbulence; Measurement technique

Abstract: It is extremely important to accurately measure and predict oxygen gas transport beneath free surfaces in open-channel flows to properly manage water bodies such as natural rivers, filtration ponds in purification plant, and raceway tanks in aquaculture. Many previous studies have proposed challenging techniques for measuring the reaeration coefficient, for example, the gas tracer method. Even though many physical and phenomenological models have been proposed, not much is known about the local distribution of the gas transfer velocity. Most previous studies have addressed uniform flow conditions, and therefore not much is known about the relationship between the local gas transfer velocity and the longitudinal variations in open-channel streams such as natural rivers. The present work is applicable to local-scale based environmental management in natural rivers. The gas transfer velocity or reaeration coefficient are formulated for many rivers. However, most of them are macro-scale based empirical models used for large-scale river segment. It is very important to evaluate locally the aquatic environment of rivers, because a river is generally composed of various topographies such as meandering, sedimentation / scoring, riffles and pools. The hydrodynamics and flow pattern are driven significantly by the local topography. Of course, such a non-uniform flow field has a peculiar mass exchange and related specific aquatic ecosystem. When the relationship between the velocity distribution and the gas transfer velocity is found locally, a reasonable prediction of river environment will be possible. These backgrounds motivate the present study. Thus, the free-surface velocity component and the concentration of dissolved oxygen were measured in a spatially accelerated open-channel flow. In particular, the streamwise profile of concentration boundary layer thickness (CBT) was reasonably evaluated using an extra-fine needle-type dissolved oxygen probe, and the local gas transfer rate was experimentally obtained. The theoretical work suggests that the local gas transfer is controlled by two significant terms: the streamwise gradient of the mean velocity and the relative intensity of turbulent diffusion. By focusing on accelerated open-channel flows with bottom-situated wedges, the formation mechanism of the concentration boundary layer was explained by a comparison with the theory. The comparison of the theoretical model and measurement data indicated that the contributions of both the mean velocity and turbulence diffusion are comparable and significant in the acceleration zone. The thickness of the oxygen concentration boundary layer began to decrease a bit downstream of the entrance, corresponding to the variation of the free-surface turbulence factors. The inflectional point of the concentration thickness appeared downstream of the acceleration zone and continued to decrease at the exit related to relaminarization. These interesting features are explained by the streamwise profiles of the terms in the theory.


Year: 2022

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