Now we're going to learn a little bit about fluid viscosity.

At the end of this video, you should be able to

explain the concept of viscosity using some of the fundamental equations, and

you'll be able to see how viscosity is measured and you'll be

able to understand how viscosity affects the efficiency of a hydraulic system.

Viscosity is the resistance of fluid to flow.

And high viscosity means high resistance.

So I've got a syringe of water, which has a medium viscosity and

it's very easy for me to push the water out though the syringe.

In the other syringe I've got a high viscosity, fluid, which is honey.

And here it's really hard for me to push that out

through the syringe because its offering a lot resistance to the flow.

Formally, viscosity is the ratio of sheer stress to the velocity gradient.

So we're going to do a quick review of some

of the fluid mechanics you learned as an undergraduate engineering student.

So the image you see here is a ideal situation where at the bottom is a

fixed plate and at the top is a moving plate and a fluid is in between the two.

And you can see how the, you can see how this line.

Represents the the velocity of the fluid going from zero to the

velocity of the plate at the top u, and varying along the way.

And along the top there's a force that's

pushing the plate to move against the fluid.

Now in a pipe.

You don't have that top plate moving that way.

So on the right is more the situation you have

in the pipe where again the velocity is zero down

on the pipe wall and then goes up to roughly

a uniform velocity down in the middle of the pipe.

So the fundamental equation for viscosity.

Is that it's the ratio of the shear stress to the velocity gradient.

So here's the shear stress tao, which has to do with

the force that's pushing the plate at the top or the force

pushing against the fluid, and then here's the velocity gradient and then

the ratio between the two, is that viscosity that we're talking about.

Now a Newtonian fluid, that viscosity is independent of the sheer stress.

So you can use a fixed value for the viscosity

but a non-Newtonian fluid, then it varies with the sheer stress.

Fortunately in a hydraulics, most oils and waters and fluids of that type

are Newtonian fluids so you can use a fixed value for the viscosity.

Doesn't change with the sheer stress.

The unit for viscosity is the Pascal second, but it's an

awkward unit to use, so the more common unit, is the centipoise.

Which is one milliPascal second, named after Poiseuille, who is a 19th century

French physicist who worked out a lot of the fundamental laws of fluid mechanics.

So some of the, more common fluids in these

units, so water has a viscosity of one centipoise.

And this is at 20 degrees C.

The motor oil that you use in your car has about, viscosity of about 200 centipoise

and that honey that I howed you on

the table, has a viscosity of about 10,000 centipoise.

We've been talking about viscosity and there's

actually two different ways to think about viscosity.

The one we've been talking about so far is

the absolute viscosity indicated by the Greek letter Mu.

But it's, common to think about this other constant of kinematic viscosity

of which really relates to how the fluid would flow under the influence of gravity.

So that kinematic velocity which is the Greek letter Mu

is the absolute viscosity divided by the density of the fluid.

So this, this is common because it's easier to measure and often

you'll see fluids in hydraulic fluids specified in terms of their

kinematic velocity compared to the absolute velocity.

It's SI units come out to be meter squared divided by seconds, but the more

common unit is the centistoke which is ten to the minus six meter square per second.

So for example water is about one centistoke and

common hydraulic oils are between 20 and 70 centistokes.

Here's one way of measuring the kinmatic,

viscosity, which is with a saybolt viscosimeter.

And on this device, you put the fluid you want to measure in the middle and because

the viscosity is temperature dependent you surround it

with an oil bath at a very controlled temperature.

And then, you let the fluid run through a precision orifice, into

a cup and time how long it takes to fill up 60 millileters.

And the amount of time is the viscosity of that fluid

in sable universal seconds, so sometimes you'll see the viscosity in SUS units.

Typically the tests are done at, for example, 40 degrees C and 100 degrees C.

So hydrologic oils tend to be about around 150 SUS units.

Or the same at 32 centistokes at 30, 40 degrees C.

Now, the ideal viscosity turns out to be a tradeoff.

In other words, there is no ideal viscosity for a

fluid that you want to use in a fluid power system.

So this plot shows the viscosity.

On the x-axis compared to the efficiency on the y-axis.

And the red line plots the mechanical efficiency.

So what's this saying is as the fluid gets thicker and thicker, it becomes

harder to push it through small orifices, just like I showed you with the honey.

Which means that the overall efficiency of

the system goes down because you're losing a

lot of your mechanical energy to friction

as this fluid is moving through the orifices.

So this would say that you want a low viscosity fluid.

But on the other hand, the volumetric efficiency,

which is the blue line, that efficiency goes up

as the fluid gets thicker and the volumetric

efficiency is very low as the fluid gets thinner.

And what's going on there is that you have a

very thin fluid, then it's going to leak by all your seals.

So for example if you have the piston that we

talked about with seals around the edge, and you have a

very thin fluid, all the fluid is not going to be pushed

by the piston, it's just going to leak by the piston.

So.

The overall efficiency which is in the green line is shown here.

And what you want is a fluid, that kind of, is in this region here.

That's the right, range and the right trade

off between the volumetric efficiency and the mechanical efficiency.

Now the other thing to think about, which is

really important, is that the viscosity changes with temperature.

And certainly in fluid power systems, the temperature

can change a lot, both with the environment

that the system is in, but also, as it, over time, it can, it can heat up.

and, and you you know that for the honey

I showed you earlier, when it gets hard to, squeeze

out of the tube, you stick it in the

microwave to heat it up and viscosity goes way down.

The viscosity index is a dimensionless number that

indicates how much that fluid viscosity changes with temperature.

So a, high viscosity, index as indicated by, the red line

in this chart, is a fluid that doesn't change much with temperature.

And a low viscosity, fluid is one that changes a lot with temperature.

So what you're looking for, is a, high viscosity fluid.

Particularly if your equipment's running over a wide temperature range.

Engine motor oil is an example of a high viscosity fluid because

its multi-grade, and it can handle winter starts and summer traffic jams.

So for example, the SAE 10W30 oil that you might likely have in your car.

Is one viscosity at a low temperature.

Because so temperature goes down.

You don't want it to get to thick.

And then it's a little bit thicker at a high temperature so

it doesn't get too thin and leak by all of your seals.

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Fluid Viscosity

流体动力学基本原理

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