WaTurSheD involves experimental campaigns which combine a suit of techniques in ways not done before, so a large amount of experimental methods development and preliminary tests have been necessary. We have successfully developed a method whereby the detailed time-resolved motion of an area of a water surface can be measured with high accuracy, while the flow velocity field directly underneath the surface is acquired simultaneously. This has given us unprecedented experimental access to the coupling between the water surface and the turbulent mixing below. The first major experiment of WaTurSheD was recently performed with this technique, a large step towards one main objective.
Very recently, the acquisition stage of the second and so far greatest experimental campaign was concluded, where three state-of-the-art measurement techniques were used simultaneously to study the full wave-turbulence-current system: the velocity field in a vertical plane beneath the wavy surface, the mixing in of a fluorescent agent which represents transported gas-saturated water, and infrared imaging of the surface to visualise the turbulence at the surface itself. We can now directly observe how the background turbulence is drastically changed by incoming waves, creating long cylinder-like eddies. We can literally watch how these eddies grab hold of the water at the surface and pulls it into the deep, replacing it with new water, precisely the process whereby gas and heat enter the real-world ocean. Extracting the results from the raw data will take some time, but will provide an unprecedented look into the processes that mix the upper ocean, at a level of detail, control and completeness not achieved before. The result of more than a year of planning and testing, this experiment is a major milestone and will give insights– and almost certainly new questions – which will guide the final stages of the project.
When flowing water, such as a river, is viewed from above, the surface is not quite flat but full of a multitude of patterns, imprints of the turbulent flow below. A goal is to understand what these imprints tell us about the mixing underneath. Using numerical simulation data from a collaborator we have discovered how long, thin "scars" on the water surface signify a particular kind of vortex underneath, giving us a far more complete intuitive picture. We eagerly await the data from the newest experiments, where we can study the phenomenon experimentally.
