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Funnel in a vacuum

  1. Oct 24, 2009 #1
    Hi, I am new here and have a love for Physics, a passion for it you might say. I have a premise that I believe it to be true. (we will see!) I am no Physicist by any means, so the only way to describe my problem is in real basic terms so here goes......

    Lets say I have a clear tube that is 3 inch's inside diameter and three feet long. Inside this tube I have placed a funnel suspended in the middle, with the large diameter of 2 1/2 inches down to a small diameter of only 1/2 inch. I then pull a vacuum of lets say 10 inches on the end of the tube with the large end of the funnel facing the vacuum source.

    Now my thoughts: I believe that pulling this vacuum will cause the air being pulled threw the tube and around the funnel will cause a air foil, low pressure. That is the air going around the funnel will be slower than the air going through the center of the funnel...... if any at all. If this is true then the over all air flow will be faster with the funnel in the tube then without the funnel in the tube.

    Well there ya go. I am trying to test it as we speak, I am trying to pull smoke threw the tube to see the effect it has on the funnel. Have you ever tried to make smoke before! Its not easy! I have filled my garage full of smoke though and caused the smoke alarms in my house to all go off. Well I look forward to any input you may have on this. please feel free to ask questions of my idea. Thanks Dave
    Last edited: Oct 25, 2009
  2. jcsd
  3. Oct 26, 2009 #2
    Firstly I think the explanation is a bit confusing. Could you draw a picture? And what do you mean by a vacuum on 10 inches?

    I recommend thinking about this from a more intuitive point of view. You are saying that if you put an obstruction in a tube it will *increase* the rate of flow through the tube?

    I am almost certain that's not possible (there may be some super sonic nozzle cases it might be true...).

    If you make it thinner then yes the the air will go faster but you'll have a smaller cross-sectional area.

    It's also not clear where your aerofoil is coming from - you need a path difference, which will be minimal unless you have a very strange funnel.
  4. Oct 26, 2009 #3
    Here is two drawings. the first is without the part. straight tube, vacuum being pulled through the tube.

    The next drawing is with the part in the tube. same vacuum. I think that the speed of the air going through the tube will be faster and increased volume. Hope that helps explain my idea.

    Attached Files:

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    • 2.JPG
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  5. Oct 26, 2009 #4
    I really need to measure the wind speed? Instead of trying to see what is happening in the tube. (without all the crazy smoke idea) The air going around the part and through the middle should be traveling faster. Is there a change in speed?
  6. Oct 26, 2009 #5
    You are describing a venturi (venturus?). See
    I believe the overall effect will be to increase turbulence and Renolds number of the air flow, and decrease mass flow.

    [edit] sorry, it is Reynolds number.
    Bob S
    Last edited: Oct 26, 2009
  7. Oct 26, 2009 #6
    Bob, yes its a venturi. But instead of pushing air thru I am pulling the air from the large end of the funnel (venturi) Is that what you are seeing in my drawing? Thats what I am trying to accomplish. I do want to increase speed, I did not conceder turbulence, thank you. I will have to research what the "Renolds number of the air flow" is so I can understand what you are talking about (amateur remember) if you think its going to decrease mass flow are you saying it will or may increase the speed of air flow in the tube? Thanks for the website that will be a big help.
  8. Oct 26, 2009 #7
    Here is a actual pic of the part

    Attached Files:

  9. Oct 26, 2009 #8
    Definition: Reynolds number Re is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.

    Well I didn't come up with that, thats for sure! I am still trying to get my head around the sentence. I get the general idea but the math is more than I can do, at this time. Its cool though.

    I have a better way of doing my wind tunnel test. I have included some pics.

    You can see the clear tube also you can see the part (I made by the way) and also a small white tube I put into the side of the clear tube in front of the part. I tested it and with vacuum applied to the other end of the clear tube, a vacuum is being pulled through the whole tube it also pulls a vacuum on the white tube. It should pull enough smoke through the white tube and then over the part. I am able to very where I put the smoke because I can pull the white tube in and out of the clear tube. More testing to be done later. Kind of cool I think. (crude I know)

    Attached Files:

  10. Oct 26, 2009 #9


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    Staff: Mentor

    It is a little more complicated than just a venturi, but close....
    Yes - but understand that pushing or pulling makes no real difference here.
    The speed of the airflow inside the funnel will be lower, the speed of the airflow outside the funnel will be higher and the overall mass flow rate and freestream speed (speed away from the funnel) in the tube will be lower than if the obstruction wasn't there.

    Away from the funnel, all you have done is add an obstruction that blocks airflow.
    Last edited: Oct 26, 2009
  11. Oct 26, 2009 #10
    Hmmmmmmmm................ Do you think that at all speeds this will be the case? Because (I don't know for sure yet) But the speeds can be very high, just guessing right now, but could be around 80 to 150 mph or higher. You are also saying that pulling the air or pushing the air are the same, well then reversing the part in the clear tube would make no difference? Interesting........... I should also mention that at very low speeds I don't want to obstruct any air flow. Thats why the funnel is not as big as the diameter of the clear tube. It isn't until I reach very high speeds that I want to make even slight changes in speed, rate, volume.
    Last edited: Oct 26, 2009
  12. Oct 26, 2009 #11


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    Staff: Mentor

    This is true for anything below supersonic speed. You should read that wiki on the venturi effect...
    That's not what I meant. I meant pulling in one direction vs pushing in the same direction. As long as the pressure difference between inlet and outlet is the same it doesn't matter if you are blowing into one end or sucking out of the other.
    The funnel is an object placed into the airstream. It is an obstruction.
  13. Oct 27, 2009 #12
    Thank you very much. I will read the information on the venturi effect. My original idea for this, was when you put your finger over the end of a garden hose you are restricting the flow yes, but you are increasing the speed of the water. I was thinking it would be the same doing this with my funnel in the air stream.

    Its still very interesting.......I also realize now that you can't any more out of it than you put into it maybe less, nothing is free.
  14. Oct 27, 2009 #13


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    Homework Helper

    True, but although the speed is increased, the cross sectional area of that flow is much smaller and the mass flow is less because of the restriction in the flow.

    If an unpowered solid object is placed in a flow of fluid or water, that object can't increase the mass flow rate. The best the object could do is to not affect the mass flow rate if there was zero loss in the interface. In the real world, there would be losses and the mass flow would be reduced somewhat by the presence of the object.
  15. Oct 27, 2009 #14
    Thanks Jeff and everyone, I have a few more things I plan on trying and will keep you and the others updated. In the next few weeks I have plans on a test that will conclusively prove of disprove my main idea. What we have talked about here is just part of the main idea. I wish I could tell everyone now but......not just yet. I also have some ideas on Gravity, oh ya that will be fun too!
  16. Oct 27, 2009 #15
    Hmm Reynolds number is not that difficult (especially for a cylindrical pipe). The easiest way to think of it is marking the limits where the flow becomes turbulent (this is a simplification but a useful one).
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