Backdrop: Designing a dust extraction system (LEV) with a branched junction.

What principles allow for the calculation of volume flow rate and pressure at the inlets if the volume flow rate and pressure is known at the outlet using a compressible fluid?

I am familiar with Continuity and Bernoulli equations using incompressible fluids but clearly in this state they are inadequate here.

Alternatively is anyone aware of simple approximations for such calculations?

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1 Answer 1


Though the answer to the respective principles behind compressible fluid flow through a pipe still interests me, it transpires that one can treat air as incompressible at speeds below the velocity of sound.

As such continuity (mass conservation), momentum and Bernoulli equations are all applicable in this situation.

  • $\begingroup$ You also have to consider compressibility if the air is being forced through a narrow aperture under high pressure. Basically any time the pressure difference is a substantial fraction of one atmosphere. But in your case the flow will be incompressible, yes. $\endgroup$
    – N. Virgo
    Apr 11, 2013 at 0:35
  • 1
    $\begingroup$ The subject you need to read up on is Gas Dynamics, and I would recommend Zuckrow & Hoffman as a great reference. $\endgroup$
    – Bryson S.
    Jul 8, 2014 at 20:03

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