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Fluids > Chapter 1 > Flashcards

Chapter 1 Flashcards

1
Q

What is density?

A

Mass (m) per unit Volume (V), that is, the amount of mass per unit volume. ρ=m/V. Units = kg/m^(3)

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2
Q

2 different types of force in fluids?

A
  1. BODY FORCE (NO MOTION)

2. SURFACE FORCE (MOTION)

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3
Q

What are body forces?

A

Body forces are associated with the action of gravity. This is simply the weight of the fluid particles calculated as W=mg. Although fluid is at rest, it can still gain a force.

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4
Q

What are surface forces?

A

Surface forces can be written in terms of the normal (Fn) and Tangential (Ft) components to the surface.

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5
Q

Eqn for normal force?

A

Fn = pA => p=Fn/A (p=pressure)

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6
Q

Eqn for tangential force?

A

Ft=τA => τ=Ft/A (τ (tau) =shear stress)

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7
Q

What is pressure (in fluid mechanics)?

A

Pressure is defined as the normal force per unit area, it is also known as the normal stress.

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8
Q

What is stress(in fluid mechanics)?

A

Stress is defined as the tangential force per unit area.

It is also known as the tangential stress/

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9
Q

What is viscosity?

A

When a fluid is in motion, the resistance to the movement of one layer of fluid over and adjoining one is referred to as the viscosity of the fluid. That is, the tangential or shear force gives the viscosity. Viscosity is a property of a fluid which determines the amount of resistance to a shear force. Shear forces of a fluid are parallel to the surfaces over which they act, the resisting forces must be in the exactly opposite direction and so they too are parallel to the surfaces.

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10
Q

What happens if there is no shear force?

A

(No motion), no shear force, no viscous effect.

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11
Q

Can you derive an expression (the Newtonian relation) for viscosity of a fluid between 2 paralled plates, one moving and one fixed?

A

YES OR NO

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12
Q

What does the Newtonian relation satisfy?

A

The fact that the shear stress of a fluid is proportional to the rate of deformation (or the strain rate).

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13
Q

What is kinematic viscosity equation?

A

ν(nu)=μ(mu)/ρ

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14
Q

What can the velocity gradient at the moving wall be approximated as?

A

y=h, so du/dy=U/h

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15
Q

From this what does the shear stress become?

A

τ=F/A=μU/h

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16
Q

How to work out the units of μ?

A

F/A=μU/h => μ=Fh/AU => units of μ = Nm/m^(2)m/s=Ns/m^(2)

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17
Q

What are the two physical causes of viscosity?

A
  1. Resistance between the fluid layers. 2. Momentum exchange or transfer while fluid is in motion
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18
Q

What can viscosity be effected by?

A

Viscosity of a fluid may be affected by the temperature, it is usually a function of fluid temperature T, μ=μ(T).

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19
Q

What is a Newtonian fluid?

A

If the viscosity of a fluid is always constant, it is a Newtonian fluid

20
Q

What is a Non-Newtonian fluid?

A

When the viscosity of a fluid changes (with temp).

21
Q

Can you draw the graph comparing Newtonian and Non-Newtonian fluids?

A

YES OR NO

22
Q

What is an incompressible fluid?

A

If the density of a fluid is constant, the fluid is said to be incompressible

23
Q

What is a compressible fluid?

A

If the density of a fluid is variable

24
Q

When can a fluid flow be treated as incompressible?

A

If the Mach number (M) is less than around 0.3

25
Q

What is a steady flow?

A

We mean that the velocity components (u,v,w) and the thermodynamic components (T,P,ρ) etc, at every given location in the flow do not change with time:
d(u,v,w,P,T,ρ,etc)/dt = 0.

26
Q

What is an unsteady flow?

A

d(u,v,w,P,T,ρ,etc)/dt is not equal to zero.

27
Q

Can you draw the diagram of uniform vs non uniform flows? What are the related equations?

A

YES OR NO and:
Uniform: dU/dx = 0
Non uniform: dU/dx NOT=0

28
Q

What is the Mach Number?

A

M is defined as the ratio of fluid speed to the local speed of sound: M=U/a (U is fluid speed and a is local sound/sonic speed).

29
Q

What are the limits of M for different types of flow?

A 
If:
M<a> SUBSONIC FLOW
M>1, U>a => SUPERSONIC FLOW
M=1, U=a => SONIC FLOW
M>3, U>3a => HYPERSONIC FLOW
30
Q

What is internal flow?

A

When the fluid flow is confined by walls, it is usually referred to as an internal flow. E.g. inside pipes, channels, nozzles, pumps, engine combustors, etc… even blood flow.

31
Q

What is external flow?

A

The flow of a fluid over an object. E.g. in aerodynamics (flows over wings, airplanes, etc)., flow over a car.

32
Q

Describe laminar and turbulent flow in layers?

A

The fluid flows in layers or laminae in laminar flow, as opposed to turbulent flow in which the velocity components have random fluctuations imposed by their mean values.

33
Q

Descirbe laminar and turbulent flow in paths?

A

In laminar flow, the paths of individual particles of fluids are straight everywhere, while for turbulent flow the paths for individual particles are no longer straight but are chaotic.

34
Q

What is Reynolds Number?

A

A non-dimensional constant, Re=ρud/μ (d for circle cross section, x for a flat surface distance)

35
Q

For a pipe (internal), what are the limits on Re to describe flow type?

A

If:
Re4000, Turbulent flow
2000<4000, transition

36
Q

For flow plate (external), what are the limits on Re to describe flow type and can you draw a diagram of this?

A

ReRe_c, Turbulent

37
Q

What is Re_c?

A

the critical Reynolds Number.

38
Q

Difference in attractive forces between the molecules of a solid and a fluid?

A

Solid: so strong that it can retain its shape.
Fluid: small so fluid will flow under a small force/stress and the flow will continue as long as the stress is present.

39
Q

Difference in deformation between the molecules of a solid and a fluid?

A

Solid: an ideal elastic solid will deform when a load is applied and return to its original state when the load is removed.
Fluid: a fluid continuous to flow for as long as the force is applied and will not recover to its original form when the force is removed.

40
Q

Difference in shear stress between the molecules of a solid and a fluid?

A

Solid: shear stress proportional to deformation (strain).
Fluid: shear stress proportional to rate of deformation (strain rate).

41
Q

What is the difference between a liquid and a gas?

A

A liquid is less compressive so density variation is little with temp and pressure and liquids have a definite volume.

42
Q

What is the standard value for a? (Local speed of sound)§

A

343 m/s

43
Q

What is the equation for a? (local speed of sound)

A

a=sqrt(KRT) [K=polytropic constant, R=Gas constant, T=Temp]

44
Q

In calculating v, how is it measured/what kind of velocity is it?

A

v = BULK velocity (average velocity, uniform velocity). This means we can just rely on one velocity measurement from a single point in the flow. v=Q/A (Q=flow rate, A=cross-sectional area). Using this equation we don’t need to worry about where v is measured. It is therefore easy to get Re for internal flow because A can easily be calc.

45
Q

What is fully developed flow?

A

When fluid enters a pipe at uniform velocity, after a certain length the shape of the flow will change (after a length L) to be a parabolic shape of non uniform velocity due to the no slip condition and viscous effect. Once this parabolic shape is reached, the flow is fully developed.

46
Q

Eqn to calculate L till fully developed flow?

A

L = 0.06Red

Fluids (6 decks)

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