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Continuity equation and air flow

  1. Jun 20, 2013 #1
    Although continuity equation is often part of fluid mechanics, does it have an application in air flow? For example, lets assume we have a frictionless air duct where air is introduced at a constant velocity and temperature. If the air duct varies in dimensions will the flow rate at the end point be equal at all points along the duct?
  2. jcsd
  3. Jun 20, 2013 #2


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    The mass of air into a duct must equal mass of air flowing out of the duct. What happens in between depends on friction losses, velocity of the air, etc. Gas dynamics is the discipline to study, especially if compressibility effects are suspected of occurring. If the flow velocity is below about 0.3M, then the air flow can be treated as incompressible and treated with the regular equations of fluid mechanics.
  4. Jun 20, 2013 #3
    Perfectly answered, thank you.
  5. Jun 20, 2013 #4


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    I'd be careful saying incompressible equations are the "regular equations of fluid mechanics." Really the regular equations are the continuity, Navier-Stokes and energy equations plus an equation of state for any continuous fluid. The equations for incompressible flow are just a simplification of those, so I would argue that the equations for a compressible flow are the "regular equations."

    Just silly semantics, I know. I'll drop it now. :-)
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