# Wind tunnels - A few questions

1. Sep 1, 2007

### jonmah

Hello everyone,
I would just like to know these few things, since I'm trying to build my own little variation of a wind tunnel for my experiment with sound.

1. How is the velocity of the wind inside the tunnel measured?
2. Following the first question, how then would I go about manipulating the wind velocity? Assuming I'm using 80mm computer fans.

I've been searching around the web for answers, but to no avail. I was hoping someone here could help me out.

2. Sep 2, 2007

### olgranpappy

I'm not sure, but maybe you could shoot in a wisp of smoke or small particles of some kind and time how long it takes to make it all the way through the tunnel. Then, since you know the length of the tunnel, you can calulate the velocity.

But, maybe you are asking how to figure out the velocity of the wind from, say, a known rotational velocity of the computer fan, or something... there is probably a way, but I don't know how...

3. Sep 2, 2007

### cesiumfrog

jonmah, can you describe the exact experiment you'd like to do?

If you're using computer fans to provide the airflow, the obvious way of manipulating it would be to vary the fan power (ideally by PWM, but a cheap "variable resistor"/"potentiometer" in series with a fan will do) or you could mechanically redirect part of the flow (or crudely suffocate it even).

As for velocity, Venturi meters, gauge pressure and Bernoulli's effect come to mind. It's probably trivial to add a crude Venturi meter to your apparatus (ie. a transparent and perforated T-extension to your main tube, with a light ball inside), but you'll need to calibrate it (probably by simultaneously timing dandelion seeds being blown the length of a tube, say).

4. Sep 2, 2007

### HallsofIvy

Surprised you couldn't find anything. The velocity of wind inside a wind tunnel is measured the same way wind velocity is measured on airplanes- a pitot tube is inserted in the tunnel (of course, on an airplane it is on the outside!). The faster the air is passing over the open tube, the lower the pressure and so the faster air is sucked out of the tube. That allows the wind speed to be measured outside the tunnel.

The wind speed is controlled, of course, by contolling the speed of the fans!

5. Sep 2, 2007

### jonmah

I'm experimenting on the speed of sound in wind. Thats why I thought of a wind tunnel, where the wind would be blowing one way and then a sound wave emitted adjacent to the direction of the wind. I'm using a setup of two microphones and a timer to calculate the speed of the sound wave.

I would like to be able to adjust the velocity of air to obtain different sets of data for analysis.

well, perhaps I wasn't looking for the right things. thanks for the advice. for a simple sort of experiment like mine, would what you suggested be practical, and more importantly, feasible?

Also, I'm considering using a simple circuit with a variable resistor to control the speed of my fans. Which is why I need to know exactly how fast it is going.

Just to let you know, I'm doing this for my Extended Essay, which is part of my IB (international baccalaureate) program. It's essentially a really long lab report (4000 words!)
Do you think this experiment is appropriate?

I would like get your feedback on this, seeing that the posters here are an experienced lot.

Thanks again!

6. Sep 2, 2007

### Astronuc

Staff Emeritus
Sure.

One thing to do is to measure the background noise of the fans and air flow, and compare that to the sound source one uses.

One could look at Doppler effects, as well as looking at the effect of sound moving with and against the wind.

One might look at the sound intensity as a function of distance or position and the effect of wind.

7. Sep 2, 2007

### cesiumfrog

Why did you choose it? Are you trying to demonstrate that the speed of sound is fixed relative to the medium?

The speed of sound is ~340 m/s. A few cheap little fans on a tube is unlikely to beat 1m/s breeze. This means your speed measurement will need precision of about 0.1%. (Assuming a metre or so of tube, seems you'll be timing a period of a few thousandths of seconds, so you'll need nearly millionth-of-second precision. Can you do that?)

8. Sep 2, 2007

### jonmah

Clearly, no. I did consider this, but a little too late, it seems.

And as for your first question, I chose because I wanted to investigate a real-life situation, ie : on top of a hill or somewhere to that effect, when you speak to another person, it's barely audible when the wind is blowing. I thought it would be an interesting experiment.

Additionally, How much does the average leaf-blower cost? We don't seem to have them here in Malaysia, as far as I know (or do, but are rarities). Would a leaf-blower generate wind of sufficient velocities? Or should I align the wind direction with the sound wave, and then experiment with how the wind speeds up the sound wave?

Quite honestly, now I'm a little intimidated by the sheer scope of such a simple experiment such as this. There are so many variables to consider, so many things that you have to work out. This is a little step into the world of engineering for me. It's terribly fun though, I must admit.

Last edited: Sep 2, 2007
9. Sep 2, 2007

### Staff: Mentor

When I was in 8th grade I built a working pito-static tube and manometer to measure airspeed using Bernoulli's equation (as HallsofIvy said). Take some rubber and some brass tubing, bend one piece of brass tubing at 90 degrees and aim it upstream, poke the other piece into the tunnel perpendicular to the airflow and connect them with a piece of flexible tubing filled with water and mounted on something that allows you to measure the difference in the height of the water. Then use Bernoulli's equation to take the velocity pressure you just measured and convert it to speed.

I did that for a year-long project in which I built a wind tunnel that was powered by a 1hp model airplane engine. I got a 90mph airspeed through a test section that was something like 6"x10"

10. Sep 2, 2007

### AlephZero

That's easy enough. Use a periodic sound signal (e.g. a sine wave), record the two mic signals digitally (e..g with a computer soundcard) then calculate the phase difference between them.

The sampling interval for 44.1KHz audio is about 23 microseconds. By averaging over many samples (also easy to do - 100,000 samples is only 2.6 sec of audio) you can get resolution much better than the sampling interval if you need it.

11. Sep 2, 2007

### AlephZero

A leaf blower sounds an expensive option. You might want to consider a vacuum cleaner (the fact that it sucks instead of blows proabably doesn't matter), or a scrap radiator cooling fan from a car (easier to control the speed than with a vacuum cleaner).

Pitot tubes don't need to be high tech devices - not if you want to measure low velocities, and you don't need to make a small one to take local airspeed measurements without disturbing the flow, anyway. As Russ said, a glass U-tube, some water, and a couple of bits of pipe should do the job.

Last edited: Sep 2, 2007
12. Oct 7, 2008

### Donny

Hello!
Is it possible to control the suction volume, so the volume can be increased and decreased on demand using computer?

Thanks.

13. Oct 7, 2008

### Staff: Mentor

Computer fans are DC and can be regulated with a rheostat.

14. Oct 7, 2008

### Donny

Can it be possible to regulate the fans not using rheostat? I hope I can use a computer for example.

15. Oct 8, 2008

### Staff: Mentor

Well, if you plug them into one of the sockets for case fans, you can regulate the rpm via software.

16. Oct 8, 2008

### Donny

Any idea of how to do that? I wonder wether that will be possible or not.
I'm not really understand that.