Let's say I have duct work shown as below. I have air moving from left to right with high velocity.There are two elbows and there is a damper of the same kind just before each of them and there is another one in the straight line which is closed, they are represented in the image below. Having the identical dampers, the flow rate of air downstream of the second damper is significantly higher than the first one. I have some ideas why, but still I am not sure about the path of the air near the second elbow.

Can it be because as the air will follow the least resistance path, and as this path is closed zith the damper, so there will be high turbulent flow around this elbow and most of the air will go through this elbow, or is it physically wrong?

enter image description here

  • $\begingroup$ Interference of the first damper to the flow can cause the second to get preferential airflow. Modelling this with CFD or by careful experiment may show what happens. Smoke wands and cotton tufts get used in wind tunnel experiments, as well as using pitot tubes to measure pressure values. Mocking up the installation with transparent pipes may lead to hours of fun :) $\endgroup$
    – Solar Mike
    Jun 4, 2023 at 18:09
  • $\begingroup$ But why it would cause the second one having preferential flow? I mean, ideally, should not both of them have equal amount of flow considering they are identical? Yes I am trying with Ansys & CFD atm. :) $\endgroup$ Jun 4, 2023 at 18:16
  • $\begingroup$ Streamlines, pressure drop, turbulence, interference all to look at. $\endgroup$
    – Solar Mike
    Jun 4, 2023 at 18:54

1 Answer 1


Near the inline damper, we have stagnation pressure which is the sum of the static pressure of the flow and its dynamic pressure.

$$ P_{stagnation}= P_{static}+ P_{dynamic}$$

This high-pressure flow zone is much higher in intensity before it is reduced by the fast discharge flow at the first damper on the first elbow, then the lower pressure will discharge at a lower rate at the second damper.


Obviously, my answer was based on my wrong assumption and was wrong. Now that I understand the situation, I think one explanation is that:

  • Near the end of the duct and the first damper, we have higher pressure but choked flow. Because the flow is similar to that of escape flow from an orifice on a pressurized tank. In choked flow, the discharge reaches a plateau after certain pressure.

  • But the flow through the second damper can be free flow, depending on the size and geometry of the elbow. So it can possibly discharge the flow fast and create a continuous exit path only constrained by the atmospheric pressure, similar to what you propose.

  • $\begingroup$ OP observes a higher flowrate for the 2nd damper - which is not what you explain. $\endgroup$
    – Solar Mike
    Jun 4, 2023 at 18:07
  • $\begingroup$ Yes exactly, the flow measured is actually higher in the second elbow $\endgroup$ Jun 4, 2023 at 18:16

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