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Ducted fan CFD

  1. Oct 29, 2014 #1
    Hello,

    I was hoping someone on here could suggest some affordable software to model the flow produced by a ducted fan. I saw someone using Solidworks flow simulation, which is convenient because I can use SW to model the fan AND get a rough estimate of the thrust produced. I'm not looking for something super accurate and complex, but I AM hoping for something that gives me a rough indication of thrust produced, and torque required to turn the fan at the simulated RPM.

    As a side note, I majored in aero, but had to make a quick career change shortly after, so I've forgotten a lot. Even so, I don't remember ever discussing propellers or fans in any of my classes. Is it safe to assume a very rough estimate can be calculated by:
    1. obtaining angle of incidence of a section of a blade against the air, obtained by adding the vector of incoming air (velocity of plane through air) with the rotational velocity of the section of the blade
    2. calculating the produced forces using air foil calculations
    3. adding produced forces along the length of the blade using 1&2
    4. multipying by # of blades
    It seems to me it wouldn't be too far off if the prop consisted of only 2 blades, the interference from one should not affect the other too much. However, if we're talking about a fan from like a Boeing engine, where its not uncommon to see blades practically overlapping, then I would assume at some RPM, the flow from blade 0 interferes with blade 1, and cascades all the way to blade N and back onto blade 0. In such a case you would have to iterate several times until the change in interference is negligible, and THEN calculate the forces. I would imagine omitting this effect would drastically change the results of the calculations. I don't even know where to approach the issue of one blade interfering with another.

    I'd be grateful for any suggested software or literature.

    Thanks,

    Misha
     
  2. jcsd
  3. Nov 3, 2014 #2
    Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
     
  4. Nov 3, 2014 #3

    rcgldr

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    Even with just 2 blades, an induced flow is produced ahead of the ducted fan, most noticable in a static situation (where the airs initial velocity relative to the fan is zero). In general, the flow accelerates towards the fan as its pressure decreases from ambient to below ambient, then as the air flow across the plane of the fan, the speed stays about the same, and the pressure increases (called pressure jump). This is where the work is performed by the fan. Then the air continues to accelerate as its pressure decreases from above ambient back to ambient. The thust is related to the pressure jump and the effective swept area of the fan.
     
  5. Nov 5, 2014 #4
    You pretty much have the write idea for a basic analysis of this problem. This approach is called blade element theory. Typically when using this method for propellers you divide the blade up into several sections and at each section you determine the local flow angle which is a function of the local twist of the blade, the incoming velocity, the rpm and the affects of the rest of the blade. You probably learned about lifting line theory when you majored in aero and that is the basic idea here. Once you have the lift on the various sections of a given blade you then integrate to determine the total force on each blade. Not exactly sure how you take into account the duct and its influence on the tip vortex. It might actually make the problem simpler because it might be possible to ignore the 3D effects that a wing tip typically produces. I am sure that there has been other work on this problem.

    I am not a prop or ducted fan expert but I would guess that even with 2 blades you probably need to account for the interference to get a good estimate, but for your purposes it may be sufficient to ignore this. You would probably have to do some research on this method to be sure.
     
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