Shear Stress and Viscosity

Question 1

Define Shear Stress

Answer 1

Force tending to cause deformation of a material that acts parallel to area

Question 2

Define normal stress

Answer 2

The direction of the deforming force is perpendicular to the cross-sectional area it’s acting upon

Question 3

What is a Fluid

Answer 3

A substance that is in the liquid or gas phase

Question 4

How to distinguish between a solid and a fluid

Answer 4

The substance ability to resist shear stress. A solid will deforming uniformly whereas a fluid with deform continuously; in solid stress is proportional to strain, but in fluids stress is proportional to strain rate.

When a constant shear force is applied, a solid eventually stops deforming at some fixed strain angle, whereas a find never steps defining and approaches a constant rate of strain.

Question 5

What is a superfluid

Answer 5

A fluid with flows with no viscosity so flows with no loss in kinetic energy (no friction)

Question 6

Which two isotopes can form a superfluid

Answer 6

He-4
He-3

Question 7

what characteristics define a gas and liquid are

Answer 7

gas = are highly compressible with very little to no viscosity
Liquid = are incompressible fluids that have high viscosity

Question 8

Define Specific Gravity and what it tells you

Answer 8

It’s a unit-less ratio that describes the density of an object relative to the density of water. To calculate SG divide the density of an object by the density of water.

If:
SG > 1 = sink in water
SG < 1 = float in water

Question 9

Specific Weight

Answer 9

also known as unit weight

Is define as the weight per unit volume - N/m^3

W = ρ * g

Question 10

How do we know a fluid is a Newtonian Fluid

Answer 10

When the viscosity is independent of the shear rate

τ = μ * du/dy

Question 11

Newtons Law of Viscosity

Answer 11

Shear stress is proportional to velocity gradient
τ = μ*du/dy

τ = shear stress
μ = viscosity
u = velocity
y = perpendicular distance from fixed lower surface

Question 12

Describe the behaviour of a plastic fluid under stress

Answer 12

Shear stress (τ) must reach a minimum before flowing

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Question 13

Describe the behaviour of a pseudo-plastic fluid under stress

Answer 13

(Shear thinning)
Viscosity decreases as the rate of shear strain increases

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Question 14

Describe the behaviour of a dilatant fluid under stress

Answer 14

(Shear thickening)
Viscosity increases as the rate of shear strain increases

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Question 15

Describe the behaviour of a Newtonian fluid under stress

Answer 15

Shear stress is proportional to shear rate strain (linear)

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Question 16

Describe the behaviour of a viscoelastic fluid under stress

Answer 16

If shear stress changes suddenly the material behaves like plastic

Question 17

Describe the behaviour of a thixotrop fluid under stress

Answer 17

Viscosity decreases with time

Question 18

Describe the behaviour of a rheopectic fluid under stress

Answer 18

Viscosity increases with time

Question 19

What is the difference between dynamic viscosity and kinematic viscosity

Answer 19

Dynamic viscosity is the resistance to movement of one layer of a fluid over another (units = Pa*s). Kinematic viscosity a measure of a fluids internal resistance to flow under gravity, its expressed as ratio of fluid dynamic viscosity to its density (units = m^2/s)

Question 20

How do you workout kinematic viscosity

Answer 20

absolute viscosity (ρ)
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fluid density (μ)

units = m^2/s

Question 21

Describe the behaviour of a Bingham plastic fluid under stress

Answer 21

Shear stress (τ) must reach a minimum before flowing like a Newtonian fluid

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