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Often in Physics classes we solve problems involving fluid flows in pipes. Usually they are solved by a combination of Bernoulli's law and continuity law, which states that the same volume of fluid has to flow per time in every part of the pipe (assuming incompressibility).

But why does it make sense to define velocities like this? In these problems, we usually assume the velocity depends only on horizontal position, not vertical position inside the pipe. In reality, we know that there is a velocity gradient along the cross section of the pipe, which is related to viscosity.

So in the above picture for example, what are $v_1$ and $v_2$? Are they some kind of average velocities? Or do we have to assume, if we want to apply the continuity equation in the form $A_1v_1=A_2v_2$, that the velocity is the same across the cross section? If that is the case, when is it reasonable to assume this?

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    $\begingroup$ Bernoulli's equation only applies to an inviscid fluid. Many introductory treatments of fluid mechanics ignore viscosity. $\endgroup$
    – d_b
    Sep 2, 2022 at 0:15
  • $\begingroup$ Yes, in the case of $A_1 v_1 = A_2 v_2$ we do use velocities averaged over the cross sections $1$ and $2$ correspondingly as given by $v = \frac{1}{A} \int_{\sigma} \vec v d \vec \sigma$ $\endgroup$ Jan 13, 2023 at 19:09

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There will almost always be atoms or molecules moving around in a fluid, so in that sense you could say that it is an average velocity. Continuity holds true as the flow rate is the same, even though there can be some turbulence in the fluid.

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You said it yourself , when there is no viscosity.. also the velocity you talk about is about the flux of fluid through the cross section area not of individual particles.

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  • $\begingroup$ It is stated everywhere that velocity is the speed of the flow at that point. What do you mean it is about the flux? $\endgroup$
    – S. Rotos
    Sep 20, 2019 at 19:05

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