Assignment – 2

Task

one

Explain

the term viscosity making reference to the following

·

Attraction – 1

·

Layers within the fluid – layers when moving

-1

·

The nature of shear stress and strain –

·

Boundary layer

Viscosity is the term used to describe the thickness of a liquid, i.e. a

fluid that flows easily has what’s known as a low resistance to deformation and

is therefore a low viscosity fluid, if a fluid has a high resistance than it

would be a viscous fluid. This is a very loose description of what is a

complicated term. In technical terms viscosity is the measurement of a fluids

internal flow resistance and the best way to describe this is by looking at the

layers within the fluid.

Santner Foundation. 2017.

Fluids as opposed to gases or solids have a degree of attraction between

molecules that is strong enough to keep them together but not strong enough to

keep them rigid hence why it can ‘flow’. If you imagine a fluid travelling over

flat ground and the fluid to be made up into tiny layers than it would flow as

the diagram shows above. This is known as laminar flow and when testing a

fluids viscosity it is crucial that the conditions are suitable to create

laminar flow. It allows the flow to be in a orderly structure where the

molecules of the fluid do not change between layers, if the fluid is turbulent

it will not create laminar flow and will normally mean the viscosity reading

will be higher than reality. These two

types of flow can be see below;

Santner Foundation. 2017.

When measuring Viscosity one of the factors is the shear rate this

relates well to the layer diagram, when the layer breaks away from below it is

‘shearing’ the behaviour of the fluid when it does this defines whether it is a

Newtonian or Non-Newtonian fluid.

As with any movement there are forces acting upon fluids when in motion,

the shear stress is the measure of the force of friction which is acting upon a

body in the path of it. i.e. if you take a river for an example the force would

be acting upon the bed of the river. When the shear stress occurs on a body it

then causes deformation this is measured by the size of the angle it is

deformed by (?) this is known as the shear strain.

When a fluid is in motion as shown above in the diagrams with the

laminar flow, the outer or bottom are subject to a large amount of shear stress

this greatly reduces the velocity at this point of the fluid as you move

further away from this point the shear stress reduces due to the lack of

deformation and thus the velocity increases. The point between the outer layer

as discussed and the point of 99% of maximum velocity in the fluid is classed

as the boundary layer simply because it occurs on the boundary of the fluid.

·

B . Explain the effect of shear

rate, temperature and pressure on viscose fluids.

With

fluctuations in the environment the properties of fluids change. For example

with increase in temperature the fluid may become more viscous or in turn its

fluidity increase.

There

are various models for the relationship between the shear viscosity and

temperature in fluids. One of the more commonly used is the Exponential model

created by Reynolds in 1886 the equation he derived is

?

(T) = ?0 exp (-?T)

T

= Temperature

?0

=

The same can be said for pressure whether this

is the pressure in the atmosphere the fluid is being used or whether the fluid

itself is ‘under’ pressure. As with the temperature this can result in the

fluids viscosity changing this is normally due to an increase in pressure. In

most liquids the viscosity changes because the molecular structure of the liquid

compresses this cause a reduction in free volume which causes a resistance to

the flow.

·

C. Explain the difference between dynamic and kinematic viscosity and

state the relationship between them. You should include the SI units in which

each are measured. Explain equation

There is one big difference between dynamic and kinematic viscosity and

this is whether an external force is acting upon the fluid or not. Dynamic

viscosity measures a fluids resistance to flow when an external force is

applied whereas kinematic viscosity measures the resistance to flow when no

external force is applied apart from gravity. Density plays a key part in

kinematic viscosity whereas with dynamic viscosity it is not a part of it this

means you can have a fluid with the same dynamic but different kinematic viscosity.

The equations is ;

Kinematic Viscosity = Dynamic viscosity/density

Dynamic viscosity is measured in Centipoise (cP)

Kinematic viscosity

is measured in Centistokes (cSt)

2 – Describe the falling sphere method for

measuring viscosity.

You need to include the equations used for the

calculations.

Mohsin

Siddique, Assistant Professor 2017

This method for measuring viscosity is called the falling sphere method.

Utilising a spherical ball, a tube and the sample liquid. This type of viscosity

measuring is normally used for Newtonian fluids. essentially in order to

measure the viscosity you place a sphere of a known size at the top of the tube

and time it falling through the liquid using known markings on the cylinder to

measure distance and this allows us to calculate the viscosity, this is

repeated three times to prove. This type of test can be controlled such as with

cooling or heating jackets to maintain set temperatures, one of the obvious

limitations of this type of test is it is only viable on transparent liquids so

you can see the sphere moving. This test can also be called the rolling ball if

the cylinder is angled at less than 80 degrees.

Using stokes law we measure the time taken for the sphere to fall from

one defined mark to the next. The ball will fall at a constant velocity balanced

by the resistance of the viscous fluid. The equations are;

– we then re-arrange this equation to make the

dynamic viscosity the subject

– this is to calculate dynamic viscosity.

– This is the equation for all falling sphere

viscometers

– This is the equation used to calculate the

drag force on the sphere