I am setting up a test-bed for an experiment. Here's what I have.

  • A circular car exhaust like this Image

  • The gas comes out of the exhaust at a velocity, x.

  • We need to lower the gas velocity to X/3


  • The 'reducer' should exert very less backpressure on the flow

What possible directions should I look into, if you know any pre-filters, baffles, geometries that can help me reduce the flow rate in minimum size, that'd be awesome.

  • 1
    $\begingroup$ increase diameter of the pipe to sqrt(3) times the current one. $\endgroup$ Sep 6, 2017 at 14:05
  • 1
    $\begingroup$ @ratchetfreak please explain the calculation $\endgroup$ Sep 6, 2017 at 14:07
  • $\begingroup$ volume/time must remain the same (to avoid adding backpressure) but longitudinal distance/time must drop by 3. => cross section area must increase by factor 3. $\endgroup$ Sep 6, 2017 at 14:13
  • $\begingroup$ For a diameter change which has the lowest loss then the included angle should be approx 6 degrees... $\endgroup$
    – Solar Mike
    Sep 6, 2017 at 14:30
  • $\begingroup$ If you want to slow the gas stream down, you have to take some kinetic energy out of it. Either you can increase the pressure, or reduce the temperature. Sorry, but anything else is likely to be an attempt at doing magic, not engineering. $\endgroup$
    – alephzero
    Sep 6, 2017 at 21:10

1 Answer 1


Try using a diffusor. But to give you an equation which helps your problem, we need more information about your air flow. Most important is: Air temperature, velocity, heat capacity ratio of your gas (for air approximately around 1.4).

This helps to calculate the Mach-number. If it is lower than 0.3 you can just use the Bernoulli equation to get your result. Otherwise the easiest way is to look up gas dynamic tables.


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