This is the first question I am asking on this forum after the end of 2017 and happy new year for all.

I have a small question on steady flow. When the streamlines are been closer the velocity increases. What is the reason for that?

Is it an affect of repulsion forces? I searched it on google but there was not a real answer. So could you please help me.

  • $\begingroup$ Streamline is a mathematical construction and not a physical object. $\endgroup$
    – Deep
    Jan 1, 2018 at 5:03

1 Answer 1


It’s a conservation of mass effect. The volumetric flow rate of the fluid bounded by the two streamlines is constant. So if the streamlines get closer together, the velocity has to increase to preserve the volumetric flow rate.

  • $\begingroup$ For the velocity there should be a force. As it before it is the affect of pressure change. But then I feel how the pressure change occurs? I can't see a real reason for that. $\endgroup$ Jan 1, 2018 at 2:01
  • $\begingroup$ This has nothing to do with streamlines. What you are asking is “what causes the velocity and pressure of a fluid to change?” The Bernoulli equation provides an energy balance in a fluid flow, and it also possible to write down a force balance on the flow. $\endgroup$ Jan 1, 2018 at 2:26

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