What is it about?
This study focused on local gas transfer in an accelerated open-channel flow and conducted a theoretical expansion of the streamwise development of the Concentration boundary layer thickness (CBT) related to the gas transfer velocity. In addition, the CBT was evaluated using a local DO profile measured with an extra-fine needle-probe. The mean velocity and turbulence were measured via a PIV system including a high-speed camera and an LLS. The newly proposed theoretical model and the measurement results enable us to understand how the CBT is formed and how it varies in the streamwise direction as influenced by the mean velocity acceleration and the production of free-surface turbulence.
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Why is it important?
It is important to determine the transport mechanisms of dissolved gases through free surfaces in open channels. Even though many physical and phenomenological models have been proposed, not much is known about the local distribution of the gas transfer velocity. Thus, this study formulated a theoretical equation for developing a concentration boundary layer in an open-channel flow.
Perspectives
There have not been any detailed discussions in previous works on what drives the CBT in accelerated open-channel zones. By introducing the mathematical expression of the streamwise variation of CBT, this study not only suggested turbulence diffusion, but the mean flow gradient also significantly contributed to the gas transfer. The theoretically introduced terms can be obtained by the accurate measurement of CBT with a sub-millimeter-scale in a meter-scale laboratory flume. The obtained results and findings here could not be achieved before.
Michio Sanjou
Kyoto Daigaku
Read the Original
This page is a summary of: Local gas transfer rate through the free surface in spatially accelerated open-channel turbulence, Physics of Fluids, October 2020, American Institute of Physics,
DOI: 10.1063/5.0021098.
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