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Slowtwitch Forums: Triathlon Forum:
Alternative to wind tunnel?

vitus979

Mar 26, 04 11:55

Post #1 of 9 (175 views)

Alternative to wind tunnel?	Can't Post

You know this forum is in sad shape when it's so far off-topic that there are more swimming threads than there are threads about aero brake calipers. Are you guys all feeling OK?

Anyway, I was wondering if anyone has ever used a water flume as an alternative to a wind tunnel. Seems like it would be a cheap, if imperfect, way of "aero" testing things like that new Giro helmet. Or is there some scientific reason this wouldn't work?

"People think it must be fun to be a super genius, but they don't realize how hard it is to put up with all the idiots in the world."

Tom Demerly

Mar 26, 04 11:57

Post #2 of 9 (170 views)

Re: Alternative to wind tunnel? [vitus979] [In reply to]	Can't Post

Hmmm, very interesting question.

cerveloguy

Mar 26, 04 11:57

Post #3 of 9 (170 views)

Re: Alternative to wind tunnel? [vitus979] [In reply to]	Can't Post

I really like Kraig Willet's article on frontal area as a poor man's wind tunnel

http://www.biketechreview.com/projects.htm

tom

Mar 26, 04 11:58

Post #4 of 9 (168 views)

Re: Alternative to wind tunnel? [vitus979] [In reply to]	Can't Post

Just blowing smoke (pardon the pun) but the different viscosity and density of water would keep the results from accurately translating into watts or mph. You could probably judge products relative to other products but even that would be skewed.

vitus979

Mar 26, 04 12:45

Post #5 of 9 (141 views)

Re: Alternative to wind tunnel? [tom] [In reply to]	Can't Post

Agreed that the results wouldn't be perfect. ( Then again, from what I gather, neither are wind tunnel tests. )

But it seems like it might at least be relatively easy way to compare two frames, or two different disc wheels, for ex.

Aren't there a couple of real rocket scientists on the forum? What about it?

"People think it must be fun to be a super genius, but they don't realize how hard it is to put up with all the idiots in the world."

john

Mar 26, 04 12:51

Post #6 of 9 (128 views)

Re: Alternative to wind tunnel? [vitus979] [In reply to]	Can't Post

Doesn't work. One fluid is compressible, the other isn't.

vitus979

Mar 26, 04 12:55

Post #7 of 9 (122 views)

Re: Alternative to wind tunnel? [john] [In reply to]	Can't Post

Damn. I was just making plans to quit my day job. Might've even sponsored TSH:

"Cheap Aero Testing By Acme HydroFlumes. We're All Wet!"

"People think it must be fun to be a super genius, but they don't realize how hard it is to put up with all the idiots in the world."

agret

Mar 26, 04 14:42

Post #8 of 9 (84 views)

Re: Alternative to wind tunnel? [john] [In reply to]	Can't Post

In Reply To:

Doesn't work. One fluid is compressible, the other isn't.

Lots of people have tried to replace water by gas or the other way round, mostly because of time scale issues. Physics, Civil eng., Biology, Geology...

But it doesn't really work, mostly because of the compressibility issue.

asgelle

Mar 26, 04 15:29

Post #9 of 9 (58 views)

Re: Alternative to wind tunnel? [vitus979] [In reply to]	Can't Post

It's done all the time. Scaling by Reynolds number (density*velocity*length/viscosity) allows scale models to be used when the real item is too large. It's just that for cycling components there are plenty of wind tunnels to allow full-scale testing so there's no need to use a water tunnel. If you did, you would not only have to make sure that you had scaled correctly, but also that you didn't introduce unintended wall or free-surface effects. The higher viscosity of water means the effects of boundaries will propogate further into the flow.

As to the role of compressibility, compressible effects scale with the Mach number (Ma) and only begin to become significant at Ma~0.3. That's about 200 mph, so you decide how important that is.