Surface attached vortices as a proxy for gas transfer in free-surface turbulent flow
- 1NTNU, Energy and Process Engineering, Energy and Process Engineering, Trondheim, Norway
- 2Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway
- 3SINTEF Energy Research, 7465 Trondheim, Norway
- 4Department of Mechanical Engineering and Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55455, USA
Gas transfer between ocean and atmosphere is largely governed by the turbulence in the topmost centimetres beneath the free surface. It has been frequently observed that areas of strong positive divergence of the surface-tangential velocity field correspond to efficient surface renewal and consequently increased transfer of gas across the interface. Patches of strong positive surface divergence occur in the form of intermittent upwelling events visible as ``boils'' on the surface.
It has been qualitatively observed that surface-attached ``bathtub'' vortices tend to appear at the edges of upwelling boils, hence a correlation between the density of these long-lived vortices and average surface divergence might be expected. Surface-attached vortices leave imprints on the surface which are particularly simple to detect: they live for a long time compared to turbulence turn-over, and their imprints are in the form of nearly circular dimples.
From direct numerical simulations, we show that a very high correlation exists between the time-dependent number of surface-attached vortices and the mean square of the surface divergence. A correlation coefficient of over 0.9 is found, where peaks in the number of vortices occur a little time after the peak in surface divergence, approximately half of the integral timescale of the bulk turbulence.
We use a newly developed method whereby the surface-attached vortices are identified with high precision and accuracy from their surface imprint only. Thus, observation of surface dimples can act as a proxy for surface divergence, with the prospect of remote sensing of gas transfer rate.
How to cite: Babiker, O., Bjerkebæk, I., Xuan, A., Shen, L., and Ellingsen, S. A.: Surface attached vortices as a proxy for gas transfer in free-surface turbulent flow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15444, https://doi.org/10.5194/egusphere-egu23-15444, 2023.
