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Wind Tunnel Help

  1. Jun 6, 2015 #1
    Hi, this is my first post. I teach high school physics and am trying to build a simple wind tunnel. For my first attempt I used a Utilitech high capacity 20 inch fan (nominal 6800 CFM) but the resulting air speed was disappointing – only 4.8 m/s. What type of fan would be better? I was thinking of trying a high-capacity, whole-house fan.

    A second issue is that I put a funnel in front of the fan: 20 inch diameter on the large end and 12 inch on the small end. I measured the average air speed directly in front of the fan to be 4.9 m/s so that the equation of continuity shows that the speed at the small end should be about 14 m/s, but I measured it to be 5.6 m/s. I also tried to use a cylinder and found similar results: the speed dropped from 6.6 m/s in front of the fan to 4.8 m/s on the opposite end, even though nominally the speed should be the same. I thought that perhaps there is a build up of pressure to explain the lower speeds which would produce a higher density. With these low velocities I suspected the flow would be relatively incompressible, but perhaps not? But the numbers do not work out: the increase in pressure is not enough to produce a higher density unless the air temperature drops significantly – which is not reasonable. Regardless of the fan used, shouldn’t the equation of continuity still work and produce a higher air speed through a smaller opening?

    Below are photos of the wind tunnel under construction. Any help would be greatly appreciated!
    IMAG0303.jpg IMAG0305.jpg
     
  2. jcsd
  3. Jun 6, 2015 #2
    I can't answer your specific question, but I made a very similar tunnel for school pupils, mainly based on the Wright Brothers' one, which I found on the NASA web site. We just use it to show flow lines over model cars etc. It has a big flared cone at the intake, having a sectoral shape. I presume your fan is sucking air from the tunnel.
     
  4. Jun 6, 2015 #3
    Actually I used a different NASA design initially. The tube is vertical and a smaller fan on the top blowing down to measure drag on a bottle rocket. But since the fan was small, the air flow was slow (only about 3 m/s) so the drag force was small. I tried turning the tube horizontal and using a larger fan but still blowing inward, as in the initial design. I assumed the flow would be similar. I will try flipping the fan around to see what happens. Thanks!
     
  5. Jun 8, 2015 #4
    tech99 - I flipped the fan so it now sucks air into the tunnel and the air speed DID improve! It is now between 6-7 m/s. Still can't explain the problem with the equation of continuity (the air speed should be higher in the narrower portion, now it is the same as in the wider portion), but its better! Thanks!
     
  6. Jun 8, 2015 #5
    Have you got a big flared cone on the intake?
     
  7. Jun 8, 2015 #6
    Not yet ... that was my next step! Thanks.
     
  8. Jun 9, 2015 #7

    cjl

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    Chances are there's nothing wrong with the continuity equation - did you measure the airspeed at the fan exit with the cone present? The cone will present a restriction to the airflow, so the pressure differential across the fan will be larger, reducing the fan's flow rate. You might be able to improve your airspeed by adding a diffuser at the end of your wind tunnel, and possibly even adding a second fan (so you have a fan at the intake and exit). Also, you may want to add flow straighteners to your intake, in order to have good flow quality through the tunnel.

    Oh, and at these speeds, the flow will be essentially incompressible. You shouldn't start seeing compressibility effects until 90-100m/s or so.
     
  9. Jun 9, 2015 #8
    I do suspect the flow should be incompressible at these low speeds - which is why I was surprised when the equation of continuity did not work. The air speeds were always measured in the tunnel. I did also measure the air speed with the fan open and got faster speeds - more for curiosity. I did add a diffuser without effect and I have air flow straighteners (paper towel tubes about 3 inches in length glued together). Would a second fan at the opposite end complicate the operation of the first one?
     
  10. Jun 10, 2015 #9

    cjl

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    Hmm. If you measured both with the tunnel present, my best guess is that the flow is not uniform at the fan's exit, so you were measuring a higher speed than the actual average across the whole cross section. Continuity definitely holds, and you won't see any compressibility at these speeds, so something else must be going on. As for a second fan at the opposite end, It really should increase your overall flow, but by how much is difficult to say.

    Also, running some numbers as a back of the envelope calculation, I'm surprised your numbers are so low. A 20 inch diameter fan has a cross sectional area of about 2.2 square feet, so 6800CFM should provide a bit over 3000 feet per minute linear velocity, or just over 50 feet per second (about 16m/s). This should be without any sort of funnel attached. Since you're measuring such a reduced number compared to this, I would suspect that either your fan's performance curve drops rather precipitously in flowrate with additional backpressure, or your fan isn't as powerful as the spec would imply. If it's the former (a very steep flowrate/pressure curve), a second fan should improve matters a lot, since you'll be dropping the backpressure on each fan significantly.

    Out of curiosity, how are you measuring airspeed?
     
  11. Jun 10, 2015 #10
    I am using an anemometer (all that I have to measure air speed). I have wondered if that is an issue. I bought this fan with precisely the calculations you cite - expecting much better performance. I am trying the second fan to see if that helps. Thanks!
     
  12. Jun 13, 2015 #11
    Well, I added a second high capacity fan and the air speed in the central section is now 7.2 m/s which is enough to get reasonable measurements of a drag force and measure a drag coefficient of a bottle rocket, but well below what I had hoped for.
     
  13. Jun 14, 2015 #12
    Have you placed a nozzle at the inlet of the tunnel? At the very least a smooth contour at the inlet of the tunnel will help the air enter the tunnel more smoothly. Based on your pictures and the posts above I assume your fan is pulling air through the tunnel so the air is entering through the right and flowing to the left in your picture. If you are just pulling air into a tube like that there will likely be some flow separation around the circumference of your tube. A smooth inlet will help with the flow quality.

    What type of anemometer are you using? That could mean a lot of different things.

    Reducing the angle of expansion of your diffuser may help. If a diffuser expands to aggressively the flow can separate along the surface of the diffuser resulting in reduced flow quality.

    I agree with cjl that part of your "continuity problem" stems from lack of flow uniformity. Looking at your 2nd picture there is a fairly larger cap on the center of your fan creating a lot of blockage and likely resulting in very significant flow non-uniformity.
     
  14. Aug 3, 2015 #13
    Hi, I am just wondering if you have successfully make the airspeed higher than 10m/s? If so, do you have any advice? I am trying to make something similar ;)
     
  15. Aug 4, 2015 #14
    Actually, no I did not. Max speed about 7-8 m/s. Even with 2 fan on high speed.I'd suggest making the intake/outflow funnels as long as possible?
     
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