Intro & Fluid Properties

Question 1

Give the definition of a fluid.

Answer 1

A substance in the liquid or gas phase.

Question 2

Distinguish between a solid and a fluid with their response to applied shear (or tangential) stress.

Answer 2

• A solid can resist applied shear stress by deforming.

- A fluid deforms continuously under applied shear stress no matter how small.

Question 3

What happens to the shear stress when the fluid is at rest? And if there is a pressure, state what type?

Answer 3

There is zero shear pressure, with a normal stress called ‘pressure’.

Question 4

In regards to fluids, we like to consider them as a continuum. Describe the term - ‘continuum’.

Answer 4

A continuous distribution of matter with no empty space.

i.e. Not the individual molecule but as a whole.

Question 5

Describe what an ‘inviscid/nonviscous’ fluid is?

Answer 5

A fluid that cannot sustain a shearing stress even when in motion. i.e. a substance which has zero viscosity.

In real world applications, water and air nearly have zero viscosity.

Question 6

What’s the difference between 1.) internal flow and 2.) external flow?

Note: Tie these different flows with viscous effects. ‘Viscous effects’ is the effect of what the thickness of a fluid has.

Answer 6

1.) The flow of fluid in a pipe or duct is internal flow if the fluid is completely bounded by solid surfaces.

Internal flows are dominated by viscous effects.

2.) The flow of fluid over a surface such as a wire, plate or pipe.

Viscous effects are significant near solid boundaries and in wake regions.

Question 7

What do you call a duct that is partially filled with liquid?

Answer 7

An open channel flow.

Question 8

What is the difference between 1.) ‘natural flow’ and 2.) ‘forced flow’?

Answer 8

1. ) The fluid motion due to natural means such as buoyancy.

2. The fluid motion is due to external means such as a fan or pump.

Question 9

What happens to the meaning of weight and gravity when you add the word ‘specific’ in front of it?

Answer 9

• The weight of fluid per unit volume

- Compares the ratio of a fluid specific weight to water’s specific weight.

Question 10

What stress does pressure experience?

Answer 10

Direct stress in general, but shear stress can be applied.

Question 11

Is pressure a scalar or a vector, and state why.

Answer 11

Pressure is a scalar because it is independent of direction i.e. In any direction, pressure is the same.

Question 12

It’s been consolidated (mentioned) that pressure in a fluid acts equally in all directions.

Using this knowledge, fill in the blank:

The fluid may be accelerating providing there is no ___ within the particles.

Answer 12

Relative motion.

Question 13

Give the definition of atmospheric pressure.

Answer 13

It’s the forced exerted on a surface by the air above it as gravity pulls it to Earth.

Question 14

What’s the relationship between atmospheric pressure and altitude?

Answer 14

As the altitude rises, the atmospheric pressure decreases. This is an inverse relationship.

Question 15

Why do your ears pop in airplanes?

Answer 15

As you go up, the atmospheric pressure becomes lower than the pressure of the air inside your ears.

Your ears pop because they are trying to equalise, or match, the pressure.

Question 16

What is the definition of shear force?

What’s a shearing motion?

Answer 16

A force parallel to the surface over which it acts.

When a real fluid causes one layer to move over another.

Note: Shear stresses develop within real fluids when they move

Question 17

What is the definition of viscosity and fluid viscosity?

Give an example of a high viscous fluid.

Answer 17

Viscosity is the resistance to flow. [High viscosity … honey, tar]

Fluid viscosity is the resistance to movement of one fluid layer over another.

Question 18

Describe what the velocity gradient is

Answer 18

The rate at which the velocity changes with distances across the flow.

Question 19

Describe what the ‘no-slip’ condition is.

Answer 19

The velocity of any fluid at the solid boundary will always be zero relative to the velocity of the surface.

Question 20

What is the relationship between the shear stress and the velocity gradient.

Answer 20

It’s direct proportionality and produces a constant of proportionality; dynamic viscosity.

Question 21

What behavior does a Newtonian fluid have and give some examples.

Answer 21

A Newtonian fluid has direct proportional relationship between shear stress and the velocity gradient.

Examples include water, diesel.

Question 22

Give two other types of fluid behaviors and their examples.

Answer 22

Pseudo plastic (shear thinning): gelatin, clay, milk.

Dilatant (Shear thickening): Concentrated solutions of sugar in water.

Question 23

Give the definition for the following term; Kinematic Viscosity

Answer 23

Used to indicate the ratio of viscous to inertial (mass) forces likely in motion.

I.e. How does the viscosity of a liquid change with temperature?

Question 24

Describe what the bulk modulus is?

Answer 24

The degree of compressibility of a substance.

Question 25

What relationship does the bulk modulus depend on?

Answer 25

The relationship between pressure and density.

Question 26

Explain the difference between 1.) isothermal and 2.) isentropic bulk modulus.

Answer 26

1. ) If compression occurs when the temperature is constant.
2. ) If there is no heat transfer during compression and if friction is absent.

Question 27

Explain why water might be compressible or incompressible.

Answer 27

Everything is incompressible, but water in particular has a high bulk modulus.

This means that the change in density is very small even for large pressure changes.

Question 28

When should compressibility be considered in relation to a gas?

Answer 28

When the relative velocity of the gas approaches the velocity at which sound is propagated in the gas.

Question 29

Describe what the Mach number is and its significance?

Answer 29

The Mach number is the ratio of actual fluid velocity, u, to sonic velocity.

If M«1, INCOMPRESSIBLE FLOW

if M»1, COMPRESSIBLE FLOW

Question 30

In relation to the ideal-gas law, what prerequisites does this gas have to obey?

Answer 30

The state of the gas is neither close to liquefaction (vapors) nor to dissociation (normally at very high T).

Question 31

What is surface tension?

Answer 31

Surface molecules are compressed more tightly at the surface, with less distance between them then the surface below.

Question 32

What does the value of the surface tension depend on?

Answer 32

• The nature of the liquid.
• Nature of substance in which the liquid acts with.
• Temperature and pressure.

Question 33

What’s the difference between a concave meniscus and a convex meniscus?

Answer 33

A concave meniscus is when the molecules have a stronger attraction to the container.

A convex meniscus has a stronger attraction to each other.

Question 34

What is capillary attraction?

Answer 34

The ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to external forces like gravity.