As I wrote a few blogs ago, it was a rude awakening Charlie Flagg and I got when we put to sea on the Norröna in early January 2006. The ADCP was producing no useful data. It quickly became evident what the problem was, and we knew then and there it was going to be a major challenge: bubbles from the huge bow thruster openings blanketed the hull wiping out the acoustics. As I also wrote, Charlie saved the project with his plan to build a fairing that would deflect the bubbles around the ADCP aperture. Later on I contracted with a student to explore the possibility of creating a bubble-free spot with the help of chines. See Charlie’s website (listed below) for a detailed discussion of all those activities. These include the construction of a permanent fairing, using chines to create upwelling ahead of the ADCP, and the use of magnetically attached cameras to get a better grasp of the bubble issue.
In the fall of 2022 I was a guest at the marine robotics group at the Royal Institute of Technology in Stockholm. My discussions of the bubble issue led me to propose a capstone project for two senior students. The first one was to carry forward Charlie’s camera idea by making it more streamlined and programmable, and readily deployable to explore and identify areas free of bubbles. The other project was to continue the numerical modeling studies about fairings and chines, the objective being to evaluate the pros and cons of both, i.e., how to create bubble-free windows or spots in the hull with as little drag as possible. These proposals didn’t fly then, but much has happened since then, and both projects are well underway now. This is welcome news about an incredibly important issue for ocean observing science. Why do I say this?
The answer is simple. The day will come when the oceanographic community begins to realize what an incredible asset the merchant marine fleet is for ocean observation. It will ask how we can make these platforms as effective as possible. The single most important issue facing those of us interested in the water column is how to create bubble-free spots in the hull for acoustic and optical instrumentation, and for future telemetry and communication needs. That is what the first of the two proposals was about: cameras to delineate possible bubble-free areas in the hull, most likely up near the bow. Such a camera has now been developed and deployed on several vessels. This is a wonderful development. We are going to learn a lot with this new tool. The second project seeks to evaluate the effectiveness of two approaches for creating bubble-free spots in the hull. One is the use of fairings, hydrodynamically smooth structures that extend a distance under the hull so that instruments in it sit below the bubbles streaming along the hull. The other is to use chines - vertical rails that gradually increase in separation forcing a divergent flow along the hull and so brings up clear water from below. In short, fairings extend our reach below the hull; chines bring clear water up to the hull. The camera will give us essential information on the thickness of the bubble layer and thus guide us on which approach may work best for a given application. This is an ongoing research initiative of enormous importance because armed with this knowledge we will be able to deploy and operate instrumentation on vessels far more confidently than we have in the past. Exciting times!
http://po.msrc.sunysb.edu/Norrona/