Quick question about Bernoulli principle and resistance?

[scilearner]

Ok I studied bernoulli and I can see how fluid would travel faster in narrowed regions but in most examples I see when something is narrowed the flow is reduced. Is this because of resistance? Also when there is resistance speed in the narrowed section is reduced right? I like to know what do they mean by this friction. Simply I'm asking in most occasion when something is narrowed the fluid flows slower due to resistance. It might be faster than non narrowed part but it is slow because of resistance(I mean if there was no resistance it would be faster) Is that right?

[npts2020]

In a perfect system flow rate is not reduced, thus the increase in speed of flow to maintain the same rate. However, in a real system you have reduction in flow rate due to greater friction and turbulence.

[scilearner]

In a perfect system flow rate is not reduced, thus the increase in speed of flow to maintain the same rate. However, in a real system you have reduction in flow rate due to greater friction and turbulence.

 

Thanks for the response Can you elaborate on friction and turbulence in this case. Is the friction created by the higher pressure in the non narrowed region or is it because of the difficulty to move through the walls of the pipe, in a narrowed region.

[Bob_for_short]

The wall friction depends on the fluid velocity. A narrow section is "equivalent" to locally higher wall friction. As well, after a narrow section the flow may become turbulent - with additional wall friction due to locally higher velocities. According to Bernoulli, the velocity should decrease but it happens only in sufficiently smooth diffusors with small angles and at relatively small velocities. A turbulent flow may begin at high Reynolds numbers even in a straight pipe.

[scilearner]

Thanks a lot for the help Bob. I have this feeling that fluid particles nicely travel in straight line in narrowed regions and has less collisions with walls? So friction loss mostly occurs in the non narrowed region due to the restriction. Is this wrong?

[Bob_for_short]

I have this feeling that fluid particles nicely travel in straight line in narrowed regions and has less collisions with walls?

If you mean molecules, then no. Their motion is chaotic with some average velocity along the pipe axis. If you speak of fluid micro-volumes, then yes, but the friction is determined with the average relative fluid-wall tangent velocity. So in a narrower section the friction is higher. Even solid bodies have friction when move along each other being in a close contact.

[npts2020]

Thanks a lot for the help Bob. I have this feeling that fluid particles nicely travel in straight line in narrowed regions and has less collisions with walls? So friction loss mostly occurs in the non narrowed region due to the restriction. Is this wrong?

 

The narrower the pipe the more collisions on the inside surface of the pipe of molecules per volume of fluid therefore more friction according to that Reynolds Number Bob was talking about. You have higher volume per surface area the larger the diameter of the pipe and less friction because of it.