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CFM to Force

  1. Oct 14, 2015 #1
    Hello, I'm new here so be gentle.
    I have a fan that produces 400CFM.
    I am attaching the fan to a 4 inch pipe in an effort to move a 2lb object down a near frictionless track.
    I'm having trouble figuring out CFM to the pressure/force that will act on the object.
    The object is approximately 2in X 4in or 8sq.inches of surface area.
    Any help is appreciated.

    If 400 CFM is not enough. What would be a general formula to equate how many CFM's I need to move an object?

  2. jcsd
  3. Oct 14, 2015 #2
    I am not sure what the 4" pipe is doing in your experiment. Please elaborate. Are you somehow directing the airflow along the pipe and placing the 2 lb object at the other end?
  4. Oct 14, 2015 #3


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    Bernoulli's equation provides the force (pressure) from exit velocity. Or you could use momentum.
  5. Oct 14, 2015 #4
    The pipe is used to concentrate the airflow and to hold the near frictionless track. also to measure pressures.
  6. Oct 14, 2015 #5
    So now the flow of air through the pipe is not 400 CFM. The argument will go like this: If you know the CFM of air hitting your target, you can calculate the velocity of flow of air coming out of the pipe. When this air hits your target, there is transfer of momentum, as russ_watters points out. This transfer of momentum per second is the force on the target. This is effectively a drag force on the target, and is given by a Stokes formula for drag. Unfortunately it is not just a simple formula, but depends on the size and shape of the target. A lot also depends on whether the flow is laminar or turbulent. So if you are looking for a simple formula that you can use straightaway, I am afraid you are in for disappointment. It may be easier to just measure the acceleration of the target down the slide, and estimate the force from that. Good luck!
  7. Oct 15, 2015 #6
    Thanks for taking the time to think about this. I needed another mind on it. I believe I will go with your suggestion about measuring the acceleration. This will also solve my problem about calculating the drag force on the object as it moves. Both can be figured with a little experimenting. Thanks!
  8. Oct 15, 2015 #7


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    Can the object be made a loose piston fit in the tube ?
  9. Oct 15, 2015 #8
    More or less...yes. It would be similar. Air flow will move around the object so there will be loss of pressure acting on the object. What that pressure is I don't know yet. I am building this project on weekends so I will possibly have more information in a month or so. My first thought when asking this question was to get a little help in with thinking about what I need to calculate as I build my Simulink (Mathworks) model. I am going to need to do some pretesting to get some initial data first. I will build the physical model in parallel with my Simulink to help verify. Thanks
  10. Oct 15, 2015 #9


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    Just to state what is probably obvious: a frictionless surface will allow ANY air motion to move ANY mass horizontally. This of course implies that your problem statement is incomplete. ( An "almost" frictionless surface will "almost" allow any air motion to move "almost" any mass...) (see "static friction" vs "moving friction") The other probably obvious thing is that depending on your pipe and fan design, there will be some (negligible, slight, or significant, IDK) reduction in the air velocity due to it 'hitting' the object (call it "blowback" :smile: ) Most textbooks on this type of problem (predicting the force on a "wing" or "hull" or "sail") will have a few simple geometric shapes together with their force "fudge factors". I've also seen "rule of thumb" calculators on-line for the pressure drop over a wall perpendicular to the wind, fwiw.
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