Wind tunnel calculations (making my own)

[Kevin_787]

Hey guys, I'm not sure this is necessarily a physics question, so if it's not I'd appreciate it if someone could point me in the right direction; but here goes. I've been giving some thought to building a wind tunnel lately, I realize this won't be cheap on any large scale, but I just want to make sure my math is right. If my tunnel were 100ftx12x20 that'd be 24,000 cubic feet. My question is I figure that if I move 24,000 cubic feet of air per minute, that should be 100 feet per minute, or 1.13 MPH provided there are no leaks, right? Or are there factors I'm not considering here?

 

[BvU]

Your logic is impeccable .
Finding out how to move that amount(*) of air is another story.

(*) for a wind tunnel that 0.5 m/s is peanuts: crawling speed

 

[berkeman]

Welcome to the PF.

If my tunnel were 100ftx12x20

That's a pretty big wind tunnel for personal use. Have you considered using scale models and a smaller tunnel instead? Also, it may be easier/cheaper to just rent time on an existing wind tunnel. Race teams do that all of the time...

 

[russ_watters]

If my tunnel were 100ftx12x20 that'd be 24,000 cubic feet. My question is I figure that if I move 24,000 cubic feet of air per minute, that should be 100 feet per minute, or 1.13 MPH provided there are no leaks, right? Or are there factors I'm not considering here?

The way you did it works for the problem you laid out, but it won't work if the cross section isn't uniform (and it never is in real wind tunnels). You should take the flow rate and divide by the cross sectional area of the test section.

And yes; start smaller to get your feet wet. What will the purpose of this project be?

When I was in 9th grade I built one that was something like 12" diameter at the inlet and 8x10" test section. The only thing the project actually accomplished was building the tunnel and measuring the speed, but that was enough.

 

[boneh3ad]

A wind tunnel with a 12-foot x 20-foot test section would, simply put, cost many millions of dollars do to correctly, and the operations cost would be substantial as well. That's larger than many (most?) commercial wind tunnel facilities.

If your goal is to build a quality wind tunnel, though, probably the best single resource is Low-Speed Wind Tunnel Testing by Barlow, Rae, and Pope. It's not cheap, so perhaps look for a library copy or something similar.

 

[Kevin_787]

I figured that was a little large, it was mostly for ease of calculations. How large would a standard wind tunnel be? I've never been inside one. This is a mostly theoretical question (I do hope to build one in the future, I'm just trying to figure out if it will be feasible first.) I can't go too small, but I can definitely cut the test section down substantially. There really is no set purpose for it yet. I'll have to take a look into that book, it seems very helpful. I appreciate the quick responses!

 

[Tom.G]

Here is a (very) short article on the worlds biggest windtunnel. At NASA Ames Research Center in Northern California. I have visited and been inside this. You walk in and sense that the test object is perhaps 15 feet away. You continue walking and realize that you are not getting closer! Strange feeling. It definitely qualifies as HUGE.
https://www.atlasobscura.com/places/the-world-s-largest-wind-tunnel-mountain-view-california

And then there is wikipedia with a list of several windtunnels and links to/about some of them.
https://en.wikipedia.org/wiki/List_of_wind_tunnels

Cheers,
Tom

 

[boneh3ad]

Even large university tunnels only have test sections that are typically on the order of 3 feet by 3 feet. There are some that are larger, some that are smaller, but that's probably a decent size, and they generally still cost several million dollars at that size range. If you are really wanting to build one yourself, you really ought to be measuring your test section size in inches, not feet.

Second, you really ought to do a little bit more research of your own into how a wind tunnel typically operates and how they are even shaped. Saying something like "100ftx12x20" doesn't make a whole lot of sense since a wind tunnel isn't just some rectangular duct with air moving through it. The cross sections change, the contours of those changes are precisely-engineered to minimize losses and maximize flow uniformity, the bends in the tunnels (if they exist) have precisely-engineered turning vanes, and host of other implements to make sure they function as designed.

 

[russ_watters]

How large would a standard wind tunnel be?

There is no such thing as a "standard wind tunnel". They can have test sections smaller than a square foot or large enough for a full sized fighter jet.

You really should spend a few hours googling and reading up on this idea you have. The information is out there and you will do better with an hour of googling than a month of asking one naive question a day here.

 

[person_random_normal]

are there factors I'm not considering here?

the viscous effects, compressible nature of the fluid and rotational effects - that' quite a lot!

 

[Kevin_787]

There is no such thing as a "standard wind tunnel". They can have test sections smaller than a square foot or large enough for a full sized fighter jet.

You really should spend a few hours googling and reading up on this idea you have. The information is out there and you will do better with an hour of googling than a month of asking one naive question a day here.

I only asked here because I knew it would give me something else to go on, like Shreyas' answer. I never would have thought to google those things and their effects on wind tunnels. It seemed like the best starting place to me. Yes, I now realize that was a poorly worded question

 

[berkeman]

Yes, I now realize that was a poorly worded question

No worries, keep on asking questions; that's how we learn. And yes, Google can be a great resource -- I've found that starting with Google Images searches often helps me to get to where I'm going a lot faster. It's easier to skim 100 images looking for what you want to read more about, versus trying to "skim" 100 hits in a text list, IMO.