This is an exploratory research project, a collaboration with my brother, Dr. Tyler MacCready, of Ocean Lab. The drifters are simple surface floats that follow the water motion, but becasue of the X-shape of their drag vanes, they also rotate with the local vertical vorticity of the water on approximately 1 m scales. We measure the position (and speed) using onboard GPS, and measure vorticity with an onboard compass. An arduino (or smaller) board controls sampling and writes data every second.
Here is a drifter folded up, with the electronics removed.
All three of the vorticity drifters with their vanes extended.
Here is a vorticity drifter deployed in Dana Passage in South Puget Sound, during a strong flood tide on 3/30/2014. Thanks to Allen Pleus for expert skippering!
Data from the simultaneous deplyment of three vorticity drifters in a tight formation in Dana Passage on 3/30/2014. They were deployed for about 40 minutes during a strong flood tide, moving about 2 km from upper right to lower left. The tracks are slightly offset so that you can see all three. The vorticity signal appears robust, and consistent between drifters (although we expect small-scale differences). The striking thing to me in this data was how persistent the sign of vorticity is following a drifter track - keeping the same sign for 5-10 minutes in what appears to the eye to be a very turbulent environment.
**For an amazing visualization of this data by Rob Fatland of Microsoft Research Connections, using WorldWide Telescope, follow this link. This is best viewed in a Windows operating system, and you need to install the WWT software (free and well worth it!).**
Here is more data from an earlier deployment of a single vorticity drifter in Hammersly Inlet. The vorticity time series for the two deployments is shown in the lower plot. One theoretical track I am exploring is to try to link the cross-track eddy vorticity flux to the large scale conversion of energy by the Reynolds stress. This involves calculating a smooth track and then cross-track eddy velocities relative to it.
These measurements are motivated by the incredible surface features of turbulence asociated with tidal flow over rough topography in the inland waters of the Salish Sea. The straits and headlands are known to be hotspots for the conversion of tidal energy to turbulence. Often this involves "cutting" the flow - creating shear layers through flow separation, and creating sheets of strong vorticity. These are the defining feature of this energy conversion, yet we almost never measure the small scale vorticity directly (need to ref. Sanford paper). There are some examples where larger scale vorticity is measured using groups of drifters or intensive ADCP surveys (ref. McCabe and others).
The next goal is to deply a larger swarm of these drifters to start to quantify the surface vorticity field over a bigger region. Here is the incredible field of surface features visible during a strong flood through Cattle Pass in the San Juan Islands. Photo by Rocky Geyer.