Help with a project - Air speed, compressed?

1. Feb 2, 2012

bpears

1.If you have a wind tunnel, a tubing that goes from a large opening, to a smaller opening, with the larger opening facing the wind, will the wind speed be more out the smaller end since its being compressed?

2.Is there a way to calculate/guesstimate the difference in wind speed by the volume or something (you know, a way without a anemometer or something?)

For the sake of explanation: lets say the large opening is 3 times larger than the smaller opposite end. And that the wind speed is 20mph. (Or what ever you prefer for a good example.)

2. Feb 2, 2012

bpears

Or would that not make compression?

3. Feb 2, 2012

Staff: Mentor

Welcome to PF!

At that low speed, there will be no compression. You can simply ratio the areas of the openings.

4. Feb 2, 2012

bpears

So if it were 30mph in, and the ratio was 1:10, then it would be 300mph out?

What speed into tube, would cause compression, and then would the compression make the output speed more than the size in/out ratio?

5. Feb 2, 2012

Staff: Mentor

At about 220mph input or output, compression starts to be an issue.

6. Feb 2, 2012

bpears

Well, I actually want compression if it will increase the wind speed out the other end. So would that change the equation with compression? And did I give a correct example of the ratio you describe?

Thanks for your help, I've been looking all over online for something about this, but couldnt find anything.

7. Feb 3, 2012

bpears

Ok, so I see what you're saying kind of now. I found the equation of continuity.
But are you supposed to square the ratio before multiplying to the input speed?

Here's the equation I have:
V1A1=V2A2.

But I think that is for no change in density? Is change in density meaning compression?

For density, I found this:
p1V1A1=p2V2A2.

I'm having trouble understanding how to use the equations to solve for the other velocity, if someone could work out an example? Or would I just multiply by the ratio like Russ was saying (output size ratio 1:10, 30mph in=300mph out?)?

Last edited: Feb 3, 2012
8. Feb 3, 2012

rcgldr

Your example is using a tube open at both ends, so the air in the larger tube is nearly stopped (not flowing at 30 mph), due to the restriction at the transition into the smaller tube.

Last edited: Feb 3, 2012
9. Feb 3, 2012

bpears

How could I have a higher output speed?

10. Feb 3, 2012

rcgldr

You would need to enclose the air flow from the fan, such as a blow dryer or a leaf blower.