Field Report: 2015-03-23 Flow Sensor “drag fins” tested

Our deployments last year saw only modest instrument response in slower systems, especially those where the water was flowing slower than 1 cm/second. Most of the deep saline circulation fell into this category, and we really wanted better data from those caves. So I came up with a add-on attachement for the flow meters, hoping to dramatically increase their surface area without affecting buoyancy that much.

Officially this was an introduction to beach facies mapping, but it looks more like geosci Kung Fu to me.

Technically, this was an introduction to mapping beach deposits, but to me it looked like geo-scientist Kung-Fu

I had a couple of these new fins on this trip and I asked my wife, who was busy leading the Northwestern University earth science students around the peninsula, when I might sneak away from the group for a few hours to see if they actually worked. She suggested instead that we do an actual deployment, using the opportunity to expose the undergrads to the aspects of real underwater fieldwork.  I was instinctively cautious about this idea, having seen a fair number of tech demos go wrong over the years, but I have also come to realize that Trish’s enthusiasm is an unstoppable force, so we added the dive to a schedule that was already bursting.

The new "parallel" anchor rig make the differences between the instruments obvious...

The new “parallel” anchor rig made it easy to see differences in instrument response from the last deployment. It’s hard to achieve consistent results with all the changes I make from one generation of underwater housings to the next.

With all the other activities on the go, it was mid- afternoon before we actually donned our gear, while answering questions from the students about the double tanks, and doing little demos of the other cave-diving kit. Then we waddled off to the waters edge, festooned with the mesh bags of loggers, cables, and other bits that accompany a typical deployment. I’m not saying we looked bad, but it was probably clear to the students that we weren’t going to peal off and reveal a freshly pressed tuxedo after the dive 😉  Once in the water, we had a short swim along an open channel to the cave entrance with a gaggle of snorkeling students following our progress at the surface. One of our primary lights started acting a bit flaky on the way and we had another impromptu Q&A session floating among the mangroves as one student paddled back to fetch our spare.  A bother, but it did put a point on what we had said earlier about all the redundant equipment we were carrying. When the extra light arrived, we started the dive, and I made a note to myself to take more photos than usual for the debrief that would occur at the end of the day.

Here I am adjusting the ballast on one of the flow meters before deploying it.

Here I adjust the ballast mass on one of the flow meters before installation (with the new drag fin in the foreground)

Once on site, Trish set our mesh bags up in a little work area, and I swam out for the usual round of inspections. North? check. Epoxy OK? check. Vortex shedding? etc. Once that was in the dive notes, I began the one-by-one exchange of the old units.  The indicator LED’s pipped right on schedule, telling us that we had no epoxy failures this time round. Once all the flow sensors had been replaced, I took a few photos and noted that the unit closest to the main line was not being deflected as much as the other sensors. I then added the new drag fin to that heavier unit.  I also had a pressure sensor to install, and while I switched that out I could see that the sensor with the new drag fin was now almost horizontal compared to the other sensors:

I don’t know about you, but I am calling that a success.  In faster systems the fin might clip the high end, although the cross sectional area now changes quite a bit as the unit approaches 90 degrees. Any approximation with drag on a sphere has also gone out the window, but I already knew that empirical testing was going to be necessary to get point velocities. As I refine this idea, I will come up with different sizes, and integrate the baffles more elegantly with the ballast mass adjustment. Wheels are already turning in my head with possibilities.


As part of the extra video we captured for the students, I recorded a short clip of our exit from the cave. With the water at high flow, there was significant mixing at the fresh/salt water interface, producing an optical consistency similar to salad dressing. This is limited to the mixing zone region, and you can see this when I placed the camera below the level of the interface where it obtains clear visibility again.  While cave divers run into this kind of thing frequently, it’s probably something that regular divers don’t experience very often. So I thought I would post the clip just to show people what it was like:

<— Click here to continue reading the story—>