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.
Just looking at the math quick, it looks like both of them would operate in the 106-107 Reynold's number range, assuming an 1 m characteristic length and a typical GA plane. I've done way too many engineering problems today (Fundamentals of Engineering exam today). Anyway, in those kind of areas, I would expect the flow to behave similarly. They would be well above low Reynold's number effects, and neither would have compressibility concerns.
chase!! chase!!9 years ago
Well, I decided I wasn't happy with my estimation before so I redid all the calculations. I used an orca and a Cessna 172, anyway, the whale ends up with a Reynold's number of 1.3x106 and the airplane is 5.8*106, for our intent, those are equivalent.

...I have too much time on my hands.
You're right! I screwed up the order of magnitude on the viscosity of water (centipoise versus Pa-s, kind of a big difference) when I was thinking about this. While we're on this, have you ever seen something in nature with wingtip gates?
Off the top of my head, no. But that has a lot to do with how animals in nature generate lift. The flapping motion is very different than the uniform flow over an airfoil. It's also a very interesting fluid flow problem, especially with insects where the Reynold's number is low and we don't really understand it. I am a fan of winglets and fences though.
trebuchet03 (author)  chase!!9 years ago
holy crap! you responded to that before I even posted!!! But yes, those same L's on the leading portion of the wing are the very same thing. Before I read about this, I read about their use to decrease stall speeds (which I guess makes sense because it keeps the flow attached a little longer)...
Yeah, they were the first thing I thought of when I heard bumps+lowered drag. It's easy to post fast when you know exactly where your reference is (Aircraft Design: A Conceptual Approach [3rd] by Raymer). I'm just happy that there's been two aero related threads in the past few days.

How much fluid flow work have you done in your classes?
trebuchet03 (author)  chase!!9 years ago
Haha, yeah your response looked like it was towards the paper I posted (but re-reading Eric's post explains what happened :P). I'm in the first semester of my third year... so I'm basically done with the tortuous classes designed to weed people out -- and into the just plain tortuous classes :P I am just getting into focused subjects - I'm currently in enrolled in a fluids class :P I have been also involved with a human powered vehicle senior design project (my avatar) - so I've gotten some extra exposure from the fairing team :P Today (for this measurements class), we did a lab using a flow bench, pitot tube and pressure transducer. The end goal is to show the flow over a cylinder and where separation occurs.
trebuchet03 (author)  ewilhelm9 years ago
I came across this... it looks like the abstract and introduction of whatever paper that was published in 2004.

If only I had access to a machine shop :P
Yeah, that was the paper! Access to a 3D printer would be even better.
trebuchet03 (author)  ewilhelm9 years ago
heh, well that has been my problem.... my school has an awesome machine shop (at least well equipped) ... There's at least 3 CNC's and a horizontal bandsaw big enough to cut any other machine in there in two :P But I've never seen that shop used - there's no class to learn the equipment - I've come to the conclusion that they sprinkle metal shavings and coolant on the machinery to make it look used :/
I've found that most university shop managers love to tell you the right way to do something. So, if you show up with the right attitude (you already know how to be safe, but really want their opinion on the best way to make something), you'll soon be welcome to work there on your own.
trebuchet03 (author)  ewilhelm9 years ago
Until recently, there has been no shop manager :/ I was recently told that this year someone was hired - but I have had no luck finding who the person is nor have I seen him/her... I really don't mean to sound pessimistic (I've really tried to get in there) - its just a bit of a let down to have a machine shop and not being able to find a way in :P
trebuchet03 (author)  ewilhelm9 years ago
I'd think the attack angle can be changed. I really haven't taken a close look at the apparatus, but I did notice some sort of black colored rigging over the tank with an arm that extended into it....

Hrmm... that sounds like reverse golf ball dimples... It sounds somewhat similar to the vortex generators you may see on some cars. I didn't think they would work, until I saw this paper:

I've read about the vortex generators on cars. I know that a lot of high performance cars will put little fins or bumps above the rear window to create a vortex, which in turn flows lower onto the rear of the car creating more downforce. That paper explains what I saw in a car magazine. They did a review of the Mitsubishi Lancer Evo and found that the top end model had significantly faster lap times, even though it didn't have much more power. The increased performance was mostly credited to the vortex generators; little things that make a big difference.
trebuchet03 (author)  intoon9 years ago
Big difference -- but it doesn't change that they're damn ugly in my opinion :P Heh, I'll probably end up making some and putting them on my roof -- just because I'm a tight arse about efficiency :P
zachninme9 years ago
I think it would be interesting to see a mouse in there. The computer kind... of course! Or maybe even a small pumpkin! :-P
chase!!9 years ago
Do you have a sting for the tunnel or are you just doing visualizations?
trebuchet03 (author)  chase!!9 years ago
I believe there is -- I really haven't looked at it too closely. The instructions say it is for visualizations - so I guess its just for fun. Just curious, where are you from? I know the head TA person f(that sets up the labs) is named Chase - just wondering :P
If it's just for visualizations, and you don't have to pump the dye into the test object like my dye pictures below, I'd say do a baseball, I want to see the flow over the seams. And how the roughness of the leather affects the flow. But you'd have to do some Reynold's number matching for that to be like an air flow. I'm from Minnesota. I'm an AeroE, but I'm definitely not a TA. Sorry to disappoint.
trebuchet03 (author)  chase!!9 years ago
No worries :) I'm in Orlando, FL studying Mech. Engineering. But give me a big tub of water a a room full of measuring equipment and I'm a happy camper :P
chase!! chase!!9 years ago
As long as I'm here, I might as well post some pictures from the last time I was using a water tunnel. The first picture is a symetric airfoil (15.5% t/c) at 0 degrees angle of attack with hydrogen bubbles flowing over it. Second picture is the bottom of a cambered airfoil (also 15.5% t/c) at -10 degrees angle of attack. Note the vortices coming off the back 1/3 of the airfoil. Last picture is of circular cylinder. What's interesting here is the Karmen Vortex shedding where the vortices will shed on alternate sides of the cylinder at a frequency specific to the flow.
hydro_bubbles.jpgCambered_-10_deg.jpgUntitled Image.jpg
saul9 years ago
I'm with Eric. I want you to either do the humpback fin, or to do a line (string) under tension. I actually have some fancy string in mind... interested?
trebuchet03 (author)  saul9 years ago
String eh? Fancy string? I might just be interested :P I really don't have a foil (nor do I have two identical ones) - so that might be difficult to do :/ Would this happen to be the fancy stress detecting smart string I read about on the squid-labs website?
leevonk9 years ago
a swimming fish or a plasticized fish.