Let's say I have a free jet of air leaving a pipe into the atmosphere. I know that the static gauge pressure at the pipe exit is equal to the atmospheric. But what about the static gauge pressure 10 meters away if the air is still traveling as a free jet? Is it still atmospheric?


1 Answer 1


I'm confused. Bernoulli's eqn. says the static pressure inside the jet should be less than atmospheric. As you go further out and the jet slows down, then it should approach atmospheric pressure. The pressure gradient between the atmosphere outside and the low pressure inside the jet leads to air getting sucked into the jet (entrainment).

  • $\begingroup$ I'm not saying 10 meters radially across the jet. I'm asking if we put another point that is 10 meters away from the opening of the pipe (where the pressure is atmospheric) along the streamline. $\endgroup$ Commented Oct 28, 2011 at 17:56
  • $\begingroup$ 10 meters away, the jet should have slowed considerably, so it should be close to atmospheric pressure according to Bernoulli. $\endgroup$
    – user1631
    Commented Oct 28, 2011 at 18:40
  • $\begingroup$ Bernoulli's equation says no such thing. Why would the static pressure be less? The ambient air and jet have radically different stagnation pressures, so it would be improper to use Bernoulli's Equation across the streamlines of the jet and those of the ambient atmosphere. Secondly, as long as the jet is subsonic it will exit at zero gauge pressure due to expansion/compression waves propagating into the reservoir and throttling the flow. Look into pressure-thrust in gas turbine vs. rocket engines for more on this score... $\endgroup$
    – Bryson S.
    Commented May 27, 2014 at 5:40

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