I How does friction in a pipe occur, with the "no slip condition" ?

[lost captain]

TL;DR Summary

Since there is no sliding between the surface of the pipe and the surface of the water in direct contact with the pipe, how does friction happen?

Hello everyone
Let's say, we are having laminar flow in a cylindrical pipe. The fluid in direct contact with the pipe doesn't move (no slip condition), so there is no sliding between the surface of the pipe and the surface of the water. The friction that occurs is actually between this stationary layer of fluid and the rest moving fluid, so it's actually viscosity.

Does friction causes the viscosity to appear? I understand that viscosity is a property of a fluid that exists either way, but maybe without the friction of the pipe we whould not observe the property of viscosity.
So when a fluid moves in a pipe (laminar flow) do we have both friction and viscosity or just viscocity due to friction?

 

[A.T.]

Since there is no sliding between the surface of the pipe and the surface of the water in direct contact with the pipe, how does friction happen?

You don't need sliding to have friction. For example, have you heard of static friction? This is a similar concept.

The friction that occurs is actually between this stationary layer of fluid and the rest moving fluid, so it's actually viscosity.

How would that layer stay stationary, if the friction on it with rest of fluid wasn't balanced by friction with the wall?

I understand that viscosity is a property of a fluid that exists either way, but maybe without the friction of the pipe we whould not observe the property of viscosity.

To observe the velocity gradient you need both: friction with the wall and viscosity.

 

[anuttarasammyak]

Does friction causes the viscosity to appear?

In video https://www.google.com/search?q=wat...ate=ive&vld=cid:6518b6b3,vid:H_qPWZbxFl8,st:0 , I observe viscousity of water in surface free condition. So we know that friction with wall is not a cause of viscosity of water.

 

[lost captain]

You don't need sliding to have friction. For example, have you heard of static friction? This is a similar concept.

How would that layer stay stationary, if the friction on it with rest of fluid wasn't balanced by friction with the wall?

To observe the velocity gradient you need both: friction with the wall and viscosity.

First of all thank you for taking the time to reply
Okay i think i understand better now, it's like static friction in solids. Still what i am saying is that the friction with the walls in only applied to a thin layer of fluid, the rest of it, doesnt "feel" any friction from the wall. If it felt any, then viscosity whould also depend on wall friction of the pipe.
So when we talk about pressure loss due to friction on the pipe walls, do we mean that friction between the stationary fluid and the pipe? Essentially a static friction?
Or do we mean the viscosity between the stationary fluid and the moving fluid?

 

[DrClaude]

The no-slip condition is an idealization to allow for easier calculations. In reality, there can be friction between the fluid and the wall.

Also, viscosity is a property of the bulk fluid. It does not depend on the wall.

 

[lost captain]

In video https://www.google.com/search?q=wat...ate=ive&vld=cid:6518b6b3,vid:H_qPWZbxFl8,st:0 , I observe viscousity of water in surface free condition. So we know that friction with wall is not a cause of viscosity of water.

I'm sorry but the link doesn't work

 

[Arjan82]

Let's say, we are having laminar flow in a cylindrical pipe. The fluid in direct contact with the pipe doesn't move (no slip condition), so there is no sliding between the surface of the pipe and the surface of the water. The friction that occurs is actually between this stationary layer of fluid and the rest moving fluid, so it's actually viscosity.

Correct, although a little bit more accurate: viscosity is what causes friction in the fluid. Anywhere where there is a velocity gradient in the flow, viscosity is at work, i.e. that is what causes the gradient in laminar flow.

Does friction causes the viscosity to appear?

No, viscosity is caused by molecular interaction of the fluid parcels.

I understand that viscosity is a property of a fluid that exists either way, but maybe without the friction of the pipe we whould not observe the property of viscosity.

In some sense this is true, if there was no 'no-slip' condition there would not be a gradient in the flow necessary meaning friction does not do anything (look up 'plug flow'). However, this is a very rare situation, it does not happen with 'normal' fluids.

So when a fluid moves in a pipe (laminar flow) do we have both friction and viscosity or just viscocity due to friction?

[Edit]
The latter

Whoops, misread as 'friction due to viscosity'

 

[A.T.]

the friction with the walls in only applied to a thin layer of fluid,

Yes, an infinitesimally thin layer in the continuum model, which is of course an idealization.

the rest of it, doesnt "feel" any friction from the wall.

The rest of the fluid experiences friction from neighboring layers, or shear stress.

If it felt any, then viscosity whould also depend on wall friction of the pipe.

The shear stress depends on the friction of the pipe. Viscosity is the ability of the fluid to transmit shear stress, but it doesn't imply that you must have shear stress.

So when we talk about pressure loss due to friction on the pipe walls, do we mean that friction between the stationary fluid and the pipe? Essentially a static friction? Or do we mean the viscosity between the stationary fluid and the moving fluid?

The pressure loss along the pipe depends only on the external froces applied to the fluid, like the friction by the wall. The shear stresses due to viscosity are internal to the fluid and merely distribute momentum within it.

 

[lost captain]

The shear stress depends on the friction of the pipe. Viscosity is the ability of the fluid to transmit shear stress, but it doesn't imply that you must have shear stress.

Doesn't sheer stress depend only on viscosity and the velocity gradient? I think im missing something major here. Isn't sheer stress equal to the viscosity times the velocity gradient? So from where do we know that sheer stress depends on friction of the pipe wall?
If shear stress depends of friction of the pipe then shouldn't it be different for different pipe roughness?

 

[A.T.]

Isn't sheer stress equal to the viscosity times the velocity gradient? So from where do we know that sheer stress depends on friction of the pipe wall?

If there was no friction with the wall, would there be a velocity gradient and thus sheer stress?

 

[lost captain]

If there was no friction with the wall, would there be a velocity gradient and thus sheer stress?

As long as some fluid was stationary at the wall yes there whould be sheer stress. The pipe could be very rough or very smooth but that whould not mater because the fluid flos in contact with the stationary- non moving fluid, not in contact with the pipe

 

[lost captain]

There is shear stress between the moving layers of the fluid, this internal friction is caused due to viscosity of the fluid.

Then there is shear stress between the moving fluid and the stationary fluid(no slip condition) this is again internal friction, since its between layers of fluid, and again this is caused by viscosity.

And then there is shear stress between the walls of the pipe and the stationary, fluid. This is not between the layers of the fluid, this is between the fluid and the pipe. Does viscosity play any role here? I think not. So this friction is not internal right? So shouldn't this friction depend on the roughness of the pipe?

 

[A.T.]

If there was no friction with the wall, would there be a velocity gradient and thus sheer stress?

As long as some fluid was stationary at the wall ...

Why would some fluid be stationary at the wall if there was no friction with the wall? I'm talking about about steady state solutions for water flowing through the the pipe here.

 

[A.T.]

So shouldn't this friction depend on the roughness of the pipe?

The friction with the wall depends on the roughness of the wall.

Does viscosity play any role here?

I guess it depends on how you define it exactly. Here is a paper arguing wall friction should be decoupled from fluid viscosity for some applications:
https://pubs.aip.org/aip/jcp/articl...-decoupled-from-fluid?redirectedFrom=fulltext

 

[lost captain]

The friction with the wall depends on the roughness of the wall.

In laminar flow it doesn't

 

[lost captain]

In laminar flow it doesn't

Yes that's why it makes even more sense, that the friction that we call "friction between the fluid and the pipe walls" is actually between the moving fluid and the non moving stationary fluid that is in direct contact with the pipe walls. Thats what im actually asking

 

[lost captain]

Im sorry if i wasn't clear enough from the beginning, and thank you for trying to explain to me what's going on

 

[anuttarasammyak]

I'm sorry but the link doesn't work

Sorry for that. Watch .

 

[anuttarasammyak]

Yes that's why it makes even more sense, that the friction that we call "friction between the fluid and the pipe walls" is actually between the moving fluid and the non moving stationary fluid that is in direct contact with the pipe walls. Thats what im actually asking

Imagine lumps of clay is running through pipe instead of liquid water. clay surface sticks on the pipe wall (friction) clay lumps defrm ( viscosity ) dragged from stick surface part and pushed force (pressure difference). Thus clay limps could go through the pipe any way. Though clay is more sticky and viscous than water, the same things take place.

Flow separation of boudary layer separation would be a topic of your interest. https://en.wikipedia.org/wiki/Flow_separation

I draw my comments on the figure in Wiki.

 

[lost captain]

Imagine lumps of clay is running through pipe instead of liquid water. clay surface sticks on the pipe wall (friction) clay lumps defrm ( viscosity ) dragged from stick surface part and pushed force (pressure difference). Thus clay limps could go through the pipe any way. Though clay is more sticky and viscous than water, the same things take place.

Flow separation of boudary layer separation would be a topic of your interest. https://en.wikipedia.org/wiki/Flow_separation

The clay that is going to flow in the end is the clay that had not stick to the wall, hence friction is between this moving clay and the other caly that had been stuck to the wall surface of the pipe

 

[A.T.]

Yes that's why it makes even more sense, that the friction that we call "friction between the fluid and the pipe walls" is actually between the moving fluid and the non moving stationary fluid that is in direct contact with the pipe walls.

But those frictions have the same magnitude, otherwise the fluid in direct wall contact would not remain stationary. So I don't see what difference it makes, and how it makes "more sense".

 

[Arjan82]

Sorry for that. Watch .

This video, although nice, mostly shows surface tension effects. It does not show viscosity effects.

 

[Arjan82]

The reason that a 'normal' fluid has zero velocity at the wall is because at molecular scale a wall is not flat at all. The bumpiness of the solid at micro (or even lower?) scale is causing the fluid to stagnate.

That said, laminar flow does not notice a change in wall roughness, i.e. the friction force is not influenced by how rough the surface is. Of course, you can make the roughness elements very big, but at a certain moment the laminar flow trips to turbulence (or, at extremely low velocities, the flow can not really be said to be along a 'flat' surface)

For turbulent boundary layer there is a roughness below which the surface is called hydrodynamically smooth (for which I could not find a wiki reference, it is only referenced as 'hydraulically smooth flow' in the article about the 'law of the wall', which is about a part of the turbulent boundary layer, but the wikipedia for that reference does not yet exist though...). This means roughness has no influence on how large the friction becomes. Or in other words: polishing the surface to a roughness below the roughness at which the boundary layer flow becomes hydrodynamically smooth will not reduce the friction.

But if the roughness elements becomes higher than, roughly speaking, the 'laminar sublayer' (the inner most part of a turbulent boundary layer, see the 'law of the wall' wiki page I refered to earlier) then the roughness will have an influence and will make the friction increase.

Do notice however that I do not say roughness has no influence, it is what causes the no-slip boundary condition. What I say is that for a laminar boundary layer and for a turbulent boundary layer that is hydrodynamically smooth, a change in roughness will not cause a change in friction force.

Also, if you want to compute the actual friction between a wall and a fluid for a hydrodynamically smooth surface, all you need to know is the viscosity and velocity gradient at the wall (so you need to know the velocity profile of the fluid up onto the wall). So you don't need to know anything about the state of the surface of the solid.

To my best knowledge, even if the surface would be smooth up to the atomic scale, it will still behave as a no-slip surface for 'normal' fluids. This is also because there are attractive and repulsive forces between the fluid molecules and the solid molecules (Adhesive forces? But molecular dynamics is really outside of my area of knowledge).

I find it a bit unhelpful, maybe even confusing, to make a distinction between
1) The fluid friction between the stagnant part of the fluid at the wall and the fluid right next to it
and
2) The friction between that same stagnant fluid and the wall.
This is because in fluid dynamics (and actually also in solid mechanics) you make the assumption of a continnuum for a fluid. This model makes this stagnant fluid layer infinitesimally thin. So you can not really identify it as a 'layer' and distinguish it from the rest. So for me they are kind of the same thing. For me there is just friction between the fluid and the wall, and from that point on there is only friction that is internal to the fluid which changes continuously through the fluid domain (if it changes, which it usually does).

ps: by 'normal' fluids I mean the things that we normally associate with a fluid, water, air, alcohol, oil, etc. Not things like clay, starch, emulsions etc.

 

[Arjan82]

And then there is shear stress between the walls of the pipe and the stationary, fluid. This is not between the layers of the fluid, this is between the fluid and the pipe. Does viscosity play any role here? I think not. So this friction is not internal right? So shouldn't this friction depend on the roughness of the pipe?

So, to be clear here: viscosity has no influence on the no-slip condition (AFAIK, i.e. in 'normal' conditions), but it does have an influence on how large the friction becomes, i.e. it changes the gradient of the fluid at the surface. This friction is indeed not internal and roughness is what causes the no-slip condition (i.e. the friction between the fluid and the wall, notice that 'friction between fluid and wall' and 'no-slip condition' are the same thing). However, a change in roughness may or may not change the friction force, see my previous post.

 

[lost captain]

Also, if you want to compute the actual friction between a wall and a fluid for a hydrodynamically smooth surface, all you need to know is the viscosity and velocity gradient at the wall (so you need to know the velocity profile of the fluid up onto the wall). So you don't need to know anything about the state of the surface of the solid.

And the same thing holds true for any point inside the moving fluid.
If we want to compute the shear stress and so the friction between 2 fluid layers
(for any distance from the pipe walls) all we need is the viscosity and the velocity gradient, right?
I want to point out that the method we used to calculate the friction between the fluid and the walls of the pipe is the same method for calculating the internal friction between 2 layers of the fluid.
This way of thinking actually made me separate 2 friction( one between stationary fluid and wall and the other between moving flhid and stationary)

But after reading your last reply, i see that this distinction is wrong, as the layer of moving fluid is way to thin to be considered a layer. Okey, yes i understand that.
So in fluid dynamics wall and stationary fluid are treated as one thing and not separately, and we kinda think that the non moving fluid is part of the wall, that's why call it friction with the wall, even though the moving fluid is actually in direct contact with the non moving fluid and not the wall... right?


Thank you once again

 

[Arjan82]

And the same thing holds true for any point inside the moving fluid.
If we want to compute the shear stress and so the friction between 2 fluid layers
(for any distance from the pipe walls) all we need is the viscosity and the velocity gradient, right?

Yes

I want to point out that the method we used to calculate the friction between the fluid and the walls of the pipe is the same method for calculating the internal friction between 2 layers of the fluid.

For hydrodynamically smooth surfaces this is indeed true. You just have to be considered with the fluid domain and apply the no-slip condition (i.e. put zero velocity) at the boundaries of the fluid domain that are in contact with the wall. Then you can compute the friction force using viscosity and the velocity gradient.

So in fluid dynamics wall and stationary fluid are treated as one thing and not separately, and we kinda think that the non moving fluid is part of the wall, that's why call it friction with the wall, even though the moving fluid is actually in direct contact with the non moving fluid and not the wall... right?

Yes, that's pretty much correct.