Measuring wind in a vertical wind tunnel

merce1

for a physics project i am designing a vertical wind tunnel made out of stovepipe and powered by a leaf blower. this experiment is designed to find the terminal velocity of different shapes. We already have a way to vary the wind speed, but what we don't have is an accurate way to measure this inside the wind tunnel without buying expensive gear.

if anyone has any ideas, please post them.

TVP45

Well, you probably already know you're got a turbulent tunnel and measurement is going to be a challenge. But, assuming you can straighten some of that out, a simple anemometer is very cheap. A piece of plastic or metal free to rotate on a rod will show the air speed if (I probably should say if) you can calibrate it.

merce1

we have a setup that does straighten out the airflow, but the wind tunnel is only six inches across so we need something small and i'm not sure if a home made anemometer will do the job. if you know somewhere where i can get a small one for cheap(< $30), please post it.

russ_watters

You can build yourself a pitostatic tube and manometer for about $10 - I did it when I was in high school...

Take a piece of brass tubing and bend it at a 90 degree angle. Take another straight piece and mount the two pieces through the wall of the wind tunnel. Connect them to opposite sides of a u-tube manometer (u-shaped piece of clear tubing with a ruler behind it). Use Bernoulli's equation to calculate the velocity.

TVP45

Yes, follow Russ's advice. If you have $30 and reasonably laminar flow, the pitot tube is accurate and easy. You might check McMaster-Carr and Grainger for some low cost materials.

FredGarvin

There's no restriction to having laminar flow when using a pitot tube. If there are flow straighteners (highly recommended) then the rotational flow should be minimized. The only thing left to do is to make sure that the measurement is taken in a section with fully developed flow. That way you don't have to do a traverse and integrate to get an semi accurate measurement since a fully developed turbulent velocity profile is pretty flat.

The best way to do it is to separate the static pressure measurement from the dynamic pressure measurement. Take the dynamic measurement via the pitot tube. Do not have the static ports on the tube itself. Take the static readings via static ports on the wall of the tunnel. This does assume that you've done a decent job at flow straightening and are fully developed.

russ_watters

They actually sell the stuff itself:
http://www.mcmaster.com/#pitot-tubes/=16q343

'bout $110 for a pre-made pitostatic tube and a manometer.

mender

I have a La Crosse anemometer #EA-3010U that usually sells in the $30 range but it has a max air speed of 67 mph (30 m/s). If that is enough a quick search on e-bay will show a few suppliers.

merce1

tanks guys. mender's cheap anemometer sounds the easiest, but i should probably try the manomater as well