using a water tunnel or tow tank to test aero?

[StraightSix]

Anyone here done this?

Thinking of building a setup to play around a bit, since I can't get to Bonneville for a while I might as well do some scale testing

[Malcolm UK]

Starightsix - the answer looks as a negative on any one reaching this site using water as the aero test mediium.  The only claim that I have seen was by car maker BMW in press advertising, so there is no technical information from that route.

Most modern aero people will suggest that you sit at a key board and use CFD programs to refine the shape you have in mind - but as most of these are for race cars, the speed envelope may not be right.  But the Flatfire streamliner team did get some support using this technology and they top out at 300 mph+.

With the density of water the Reynolds equations might be more easily balanced at scale.  Best suggestion is to make a circulating tank where the velocity of the water can be varied and try the concept.  The test section will need straight flow without eddies and currents which may be a major challenge to achieve.  

Should you get results that are repeateable and accurate you will soon see a host of copy cat designs and designers saying that they have used the method for years!

Malcolm

[John Burk]

StraightSix - Whatever the medium , a simple way to experiment with streamlining changes is to use the principal of the balance scale . Mounting  2 models on a paralelogram , the one with the least drag drag will pivot foreward .

[gazza414]

Scaling of the models needs to be precise for exact evaluation ...

[smitty2]

I did some work along these lines when I was in college. We were studying Magnetohydrodynamics, and needed a way to test different venture shapes. Since we were working in a water medium, and using full scale shapes there wasn't a problem with scaling or density. Our problem was getting a straight flow of water through the tunnel.
 The walls of the tunnel must be mirror smooth, and the test subject must be far enough away from the tunnel walls so it won't be affected by eddies and torrents produced by the boundary layer effect on the tunnel walls. For a 6 inch test subject we ended up with a tunnel 3 feet wide by 12 feet long. A system of vanes shaped like a honey comb were used at the front and rear of the tunnel to "straighten" the flow of water. This whole "Cluster !@#$%" was about 20 feet in circumference, and was pumped by a large (10" dia) muffin fan type pump on the opposite side of the "Apparatus".
 So if you can get a good straight flow of water through this thing, then scaling and density problems should be the least of your worries.

 Hope this helped some....    Smitty

[Glen]

Back in the late 50s and early 60s we had a water tunnel at Douglas air craft Co. I only got to see it in operation one time as the new wind tunnel had been completed and was in use. The water tunnel was interesting to see the water flow and turblance down stream of the test specimans. Smoke in the wind tunnel worked well and was easier to photograpf with the high speed cameras. Again I am telling my age, but wouldn't trade it for anything. I have seen many great things in the engineering world.
Glen

[Richard Thomason]

Check with the university of british columbia in Vancouver B C. They have a water tank that seems to work very well in translating to the real world.rht

[4sfed]

Check with the university of british columbia in Vancouver B C. They have a water tank that seems to work very well in translating to the real world.rht

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Another reference is SAE Paper 860091 "Aerodynamic Drag Studies on Rolling Vehicles by Underwater Tow Testing".  It's a good tool, especially for open-wheel vehicles when you don't have a moving ground plane in your wind tunnel.

Jim