Evan A. Variano
Title: Dynamics of heat and carbon dioxide exchange across a turbulent air-water interface
The transport of mass and heat across the air-water interface is a small-scale process central to global climate dynamics and a variety of engineered and natural systems. I present laboratory experiments using quantitative imaging that reveal the physical process by which turbulent flow enhances the transfer rate. Understanding the details of this process will permit accurate accounting for oxygen and carbon dioxide budgets in natural and engineered water bodies.
These experiments are performed in a new type of turbulent mixing tank, designed to provide homogeneous turbulence at high Reynolds numbers. The novel results from this study rely on a quantitative imaging of the carbon dioxide concentration field. This is accomplished by developing a method for controlling and calibrating a pH-sensitive fluorescent dye for use as a carbon dioxide tracer. Combining this technique with particle image velocimetry (PIV), we achieve the first direct measurements of the instantaneous flux fields. From this I compute the wavenumber and frequency spectra of turbulent flux, a novel result of central importance to the sub-grid scale models useful for predicting gas transfer from coarsely-gridded observations and simulations. Flux statistics and image sequences give a broader understanding of the turbulent flux dynamics. Future work will use similar techniques to illuminate other environmentally important mixing and transport processes.