Confusion in uderstanding the concept of viscosity

[arpatil_31]

Hello friend...
I have confusion in understanding the concept of viscosity. Please help me.
Viscosity is the property of the fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid. Here both top and adjacent layer causes shear stress on each other.
Now, my confusion is that:
what is mean by less viscous and highly viscous fluid? and
what happens when viscosity decreases or increases?
As per definition viscosity offers resistance for movement of fluids. So I am thinking that viscosity increases means frictional resistance increases which causes increase in turbulence.
But in book I read that during turbulent flow viscosity decreases and fluid is less viscous. And for laminar flow, fluid is highly viscous.
Also, engine oils are highly viscous or less viscous?
Please help me. Thanks in advanceRegards
Atul Patil

 

[Simon Bridge]

what is mean by less viscous and highly viscous fluid?

You just described what viscosity is -

As per definition viscosity offers resistance for movement of fluids. So I am thinking that viscosity increases means frictional resistance increases which causes increase in turbulence.

Hmmm ...

Highly viscous: think a thick syrup or steak sauce.
Less viscous: think water in a bath or sink.
Turbulence will get you vortexes - whirlpools.
So do you associate whirlpools more with the flow of a syrup or of water?

What characterizes the motion of a syrup flowing compared with water?

 

[arpatil_31]

Thanks simon...
Now my doubt is clear.

 

[Simon Bridge]

Well done.
One of the neat things about physics is that you can always go back and look at some real-life case you are familiar with.

 

[Chestermiller]

Our old friend, Sir Isaac Newton, did experiments on fluids. He found that, if you have two parallel plates with a fluid in the gap between them, and you move one of the plates in the direction parallel to the other, the force per unit area on each of the plates will be proportional to the difference in their velocities, divided by the distance between the plates. We call the constant of proportionality the viscosity, and we call this relationship Newton's Law of Viscosity.

 

[Simon Bridge]

That's great as a description of what viscosity is, but we see further down in post #1, OP seems to be asking about how to reason out a particular problem using the description. i.e. how does this definition mean that you get more turbulence in low-viscosity fluids?

I kinda side-stepped that question by appealing to common experience ;)

 

[Chestermiller]

That's great as a description of what viscosity is, but we see further down in post #1, OP seems to be asking about how to reason out a particular problem using the description. i.e. how does this definition mean that you get more turbulence in low-viscosity fluids?

I kinda side-stepped that question by appealing to common experience ;)

In turbulent flow, the fluid is not less viscous, contrary to what the OP indicated. The viscosity is a physical property of the fluid, and not of the flow.

In turbulent flow, the fluid velocity fluctuates with time on the small scale, even though, on the large scale, the time averaged velocity may be constant. This situation occurs when the inertial contributions to the force balance overshadow the viscous contributions, so that the viscous damping is no longer effective in stabilizing the laminar regime, and there is a transition to turbulence. In the case of fluid flow in a pipe, for example, the transition from laminar to turbulent flow occurs at a Reynolds number of 2100; the Reynolds number is given by

$$Re=\frac{ρvd}{μ}=\frac{ρv^2}{\left(μ\frac{v}{d}\right)}$$

Notice that, in the final version of this expression, the numerator is the inertial stress and the denominator is the viscous stress.

When the flow is turbulent, the time averaged stresses are typically much higher than in laminar flow. This has a big effect on the mean velocity distribution and on the pressure variations in the system. For example, in flow in a tube, the pressure drop in turbulent flow is much higher than one would calculate using the Hagen Poiseuille equation applicable to laminar flow.

 

[Simon Bridge]

In turbulent flow, the fluid is not less viscous, contrary to what the OP indicated. The viscosity is a physical property of the fluid, and not of the flow.

Actually I think that's well spotted - I just figured the phrasing was due to English being a second language.
I figured what was intended was that turbulence is more commonly encountered in less viscous fluids.
But it could be read either way...

 

[Chestermiller]

Actually I think that's well spotted - I just figured the phrasing was due to English being a second language.
I figured what was intended was that turbulence is more commonly encountered in less viscous fluids.
But it could be read either way...

Good point.

Chet

 

[arpatil_31]

It's good discussion

Thanks to Simon and Chestermiller

 

[Simon Bridge]

No worries - part of why more experienced people are prepared to tackle these questions is that it helps us sort out our own ideas. ;)