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Fluid model question

  1. Mar 22, 2005 #1
    I have a specific question about fluid analysis that is bugging me. I am no expert in this field but very interested in it.

    Lets's say we want to model a flow where air expands out of an air reservoir across a valve into a cylinder and applies a pressure force on an airtight piston, which moves, allowing the air to further expand. Assume we know the inertial and frictional forces which resist the movement of the piston as well as the initial pressure in the reservoir before the opening of the valve. Also assume that the valve opens instantaneously. We would like to obtain (by numerical analysis) the functions of the piston's velocity as well as the pressure just behind with respect to time.

    How is such a moving (dynamic) boundary, as it shows up at the piston, usually treated? At the same time, the solid boundary at the parts of the valve, which are fixed in space, should also be included.

    Does anyone know how such problems could be solved using CFD methods or some other method?
  2. jcsd
  3. Mar 22, 2005 #2


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    Hi SV. From your three recent posts it would seem you're devising a gun of some sort. Can you explain what you need this for? What you're asking for here is simple enough to calculate.
  4. Mar 23, 2005 #3


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    I'm not sure if I'm understanding you properly, but it seems like a situation similar to an Otto Cycle motor, where you have a high pressure generated moving a piston.

    Firstly, you would need to get your volume as a funtion of piston travel, which should be easy enough. Then use whatever method you want to calculate pressure as a function of a volume. Your force is then pressure times area of the piston, then minus any frictional forces.
  5. Mar 23, 2005 #4


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    It depends on how accurate you want to be. You could code it in a CFD program, but I think it is a bit exagerated for your purposes. You don't need the details of the flow, do you?.

    Therefore is appropriated an Integral Formulation. What do you know about it?. Try to employ the integral Navier-Stokes equations particularized for inviscid and compressible flow, retaining the unsteady terms of volumetric variation. These terms will represent your moving boundary. Employ mass conservation and Energy to work out the pressure P(t).
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