Quantitative Imaging of CO2 Transfer at an Unsheared Free Surface
We present laboratory measurements of simultaneous velocity and concentration fields for the transfer of CO2 across a free surface. The interface is subject to the effects of free shear turbulence generated far beneath the surface, exhibiting low mean flow and excellent homogeneity. From measurements of the spatio-temporal mass flux we examine coherent structures below the free surface, as well as one-point statistics to better understand the fundamental physics of turbulent transport at a free surface in the absence of mean shear. We observe surface penetration events caused by bulk fluid impacting the interface from below, as well as downwelling events in which the near-surface fluid is injected into the bulk in narrow filaments. Both types of events contribute to the turbulent mass flux, and we measure that downwelling events are responsible for at least as much mass transfer as the upwellings on which existing models are based. Our measurements indicate that the dominant length and time scales are different for upwellings and downwellings; the quantification of these will be important to modeling efforts.
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