Water Tunnel Testing

So, my measurements professor got us permission to fill and fire up the water tunnel in the measurements lab.

For those that don't know, a water tunnel (like the one below), pumps water from one end of a tank to another. The water goes through a conditioning filter (to make the flow nice and even), and then it passes through a tunnel (that blueish area in the picture). You can then add dye or solids to the water to see how a fluid flows over the object.

We're allowed to bring in one item (per person) to put through the tunnel. The tunnel is a little more than a foot wide. What would you like to see in there? After the lab, I'll post up pictures (and possibly video) :)

Keep in mind that the object should be heavier than water (e.g. no balloons :P).

See the photo results here!

Picture of Water Tunnel Testing
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palmchese9 years ago
How bout some sodium? Who's with me?
ha i'm with you there
drcrash9 years ago
The most useful thing I can think of is testing several variants of Kline-Fogelman airfoils, which have a sharp step in them that entrains a vortex. The vortex prevents separation (and hard stall) at high angles of attack.

Partial testing decades ago led to disillusionment with K-F airfoils, some of it wwapparently premature because the wrong variants were tested.

Some of the disillusionment is probably warranted, becuase K-F airfoils do work better at the Reynolds numbers applicable to small (model) aircraft than to full-size airplane wings.

Recently there's been a resurgence of interest among RC plane guys, for whom the Reynolds numbers are fine. Small aircraft in that range are increasingly important for "real" stuff, too though, because with modern electronics, you can build useful autonomous and remotely-piloted vehicles.

(People increasingly use them for aerial photography, fire-fighting reconnaissance, police reconnaissance, searching for lost folks in the boonies, etc.)

There are several ongoing, long threads about K-F airfoils over on www.rcgroups.com, with people gradually getting a better handle on what works and what doesn't by building and flying models. (And a very few very limited 2D adaptive mesh CFD simulations that don't capture the 3D dynamics of an entrained vortex rolling outward toward the wingtips.)

(Dick Kline has shown up over there, and so has another guy who built and flew a human-carrying KF ultralight, as well as a crude home wind tunnel, many years ago.)

If you're interested, some of the more aerodynamically savvy people over there could give you guidance as to some useful airfoils to test and how to test them.

Some of the relevant threads:

Unfortunately, those threads are long, concurrent, and not very focused, with little gems scattered here and there. If you're interested, I can give some people over there a heads up and maybe figure out how to get you up to speed on KF airfoil issues & what would be good to test.

(Actually, I can think of a couple of things right off, but I'm sure others over there can think of a couple of equally good things, and refine the parameter ranges.)

I think a fan would serve as a nice thing to put in.As the blades speed increases in becomes more water-dynamic
put in a magnet thats been crusted with un-removable ferro-fluid.. i really gotta see that..
Water channel fluid isn't magnetic. What would be the point of putting in magnetic stuff?
trebuchet03 (author)  chase!!9 years ago
Maybe because of the goofy bumps it forms? I went to a different lab section today that was doing this very same lab today... I'll post some pictures and video up (unfortunately, my camera's shutter speed isn't very fast :/).
ewilhelm9 years ago
I read a paper some time ago that measured the stall-reducing properties of the bumps on the leading edge of whale fins. Apparently those fairly large bumps have all sorts of wonderful properties. Can you vary AoA of the object in the tank? Take two identical foil sections and add bumps (say spheres of diameter equal to the foil depth?) all along the leading edge. Vary the AoA and see if the flow stay attached better to one versus the other.
I haven't read that paper so I'm just speculating on the purpose of the bumps, but it's common on airplane wings to attach little L brackets to the wing. They serve as vortex generators that reenergize the air with minimal increases in drag. Basically they cause the high energy air near the boundary layer to be pulled into the lower energy areas. Anyway, I suspect that the bumps on whale fins are doing the same thing.
I think you're right on about the L brackets keeping the flow attached. While I haven't done the math, I'm pretty sure the whale bumps operate in a vastly different flow regieme, which is why they're so cool.
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