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fluid mechanics and energy transfer > fluid mechanics > Flashcards

fluid mechanics Flashcards

1
Q

What is one bar?

A

105 N/m2

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2
Q

What do shear stresses result from?

A

Forces applied at a tangent.

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3
Q

What is pascal’s law?

A

Pascal’s law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.

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4
Q

what is the pressure gradient?

A

dp/dz = -ρg

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5
Q

what is the head?

A

for pressure use P=ρgh to find it

Represents energy per unti weight

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6
Q

What is gauge pressure?

A

Pressure relative to the atmosphere.

Pa= 0

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7
Q

How does a baramometer work?

A

[insert photo]

atmosphereic pressure= ρgh + vapour pressure

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8
Q

What is the function of a manometer?

A

Compare pressure in a vessel to atmospheric pressure and thus find gauge pressure.

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9
Q

What are the limitations of the piezometer?

A

Can’t measure pressure less than atmospheric

Can’t be used for gases

If it is of high pressure, it would be too tall.

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10
Q

How does a U-tube manometer work?

A

Glass tube curved to give two vertical columns.

It is filled with a liquid of different density

Used to find difference in pressures.

P-Patm=ρ2h2g-ρ1h1g

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11
Q

What equation would you use to find pressure difference with a manometer with different diameter ends?

A

p1=p2 +ρgz(1+(d2/D2))

Where z is the difference in heights

where D is the larger diameter.

If D is a lot larger than d, then p1=p2 +ρgz

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12
Q

What is the force acting on a vertical dam?

Where does it act?

A

ρgBh2 / 2

Where B is length of dam

1/3 up the side of the dam

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13
Q

What is the no-slip condition?

A

When a fluid flows past a surface, the fluid adjacent to the solid moves at the same speed as the solid.

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14
Q

Under what conditions do you get a linear velocity gradient between a stationary and moving plate?

A

The gap between the plates and the velocity is relatively small.

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15
Q

What is the symbol and unit for shear rate?

A

ℽ and s-1

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16
Q

What is the symbol and unit for shear stress?

A

𝛕

N/m2

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17
Q

What is 𝛕 equal to?

A

μ*(dy/dy)

where μ is dynamic viscosity

(viscosity units = Ns/m2)

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18
Q

What is kinematic viscosity

A

Symbol: curley ν

dynamic viscosity / density

(m2/s)

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19
Q

How do you find shear rate?

A

=du/dy

Difference in speeds of plates divided by distance between them.

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20
Q

What is the equation for drag?

A

D=0.5*area*u2*density*drag coefficient

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21
Q

What is laminar flow?

A

Small deviations in fluid flow willbe damped by viscosity and it will be smooth

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22
Q

What is reynolds number?

A

interial forces / viscous forces

= ρ*u*d/ dynamic viscosity

= u*d/ kinematic viscousity

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23
Q

When does laminar and turbulant flow happen?

A

laminar when Re<2000

Turbulent when Re>4000

Though these only apply to pipes

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24
Q

What are the units for reynolds number?

A

It has none

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25
Q

What is Stokes law?

A

When flow is laminar, CD= 24/Re

For it to be laminar Re<0.1

Learn it

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26
Q

How do you find buoyancy?

A

density*volume*g

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27
Q

How can mass be stored in the controle volume?

A

A change in density

A change in controle volume size

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28
Q

What is the bernoulli equation?

A
29
Q

How do you find the head with velocity?

A

velocity2 / 2g

30
Q

What does the bernoulli equation show?

A

That the energy per unit weight at one point is the same at a different point.

31
Q

What is is a pitot tube?

A

Cylindrical, open ended tube, pointing into the flow of fluid.

32
Q

What is the total head to the entrance to a pitot tube?

A

As the velocity of fluid there is 0, it is p/ρg +z

33
Q

What is stagnation pressure?

A

The fluid stops at a point and v falls to 0

As P +0.5*ρ*v2 +zρg needs to be conserved the pressure has to increase to P +0.5*ρ*v2

This is the stagnation pressure.

34
Q

What is a different way to express the bernoulli equation?

A

P +0.5*ρ*v2 +zρg

35
Q

What is the discharge coefficient?

A

for when calculated discharge is less than expected

Cd=actual / ideal

36
Q

What is dynamic pressure?

A

The 0.5*ρ*v2 bit of the bernoulli equation

Also known as the total head

37
Q

How do you find the speed of fluid in a tube with a pitot meter?

A

Install a pitot tube and a standpipe (pipe perpendicular to main flow) and measure the difference in height.

difference = v2/2g

38
Q

What is momentum/s?

A

ρ*area*v2

39
Q

How does conservation of momenum apply to fluids?

A

Momentum per second out of controle volume = momentum per second into controle volume

40
Q

Express the friction factor in terms of shear stress

A

f=(8𝜏)/(v^2ρ)

41
Q

What is the formula for the head loss in a pipe due to friction?

A

h=f*(L/D)*(V2/2g)

42
Q

When flow is laminar, what is the expresstion for the friction factor?

A

64 / Re

43
Q

How are local head losses found?

A

Local head loss = Kl*(V2/2g)

where Kl is a constant for a particular fitting

44
Q

How do you find Kl when the dimeter changes alot?

A

Kl=(1-(A2/A1))

A1 is intial diameter and A2 final diameter

45
Q

What is Ks?

A

The average roughness level

46
Q

When is it worth considering local loses?

A

When the pipe is short in length, otherwise friction dwarfs it.

47
Q

What is relative roughness?

A

Ks / diameter

48
Q

How do you find power needed to maintain flow in a pipe?

A

Mass per second* height loss*g

49
Q

A differential manometer is used to determine the pressure difference between two cross-sections A and B in a horizontal pipe in which water is flowing. The deflection of the mercury in the gauge is 0.58m. Calculate the pressure difference between sections A and B

A
50
Q

How do you find the force due to a water jet?

A

Force is rate of change of momentum. The water will lose all it’s momentum, so just find the momentum of the water.

Force=density of water*area*velocity2

51
Q

How do you find the force exeted by a nozzle on a fluid?

A

Use Bernoulli’s equation to find P1 (P2=0 and Z1-Z2 =0)

Forces to right =pressure @1 -force-pressure@2=change in momentum

=Momentum @2 -momentum @1

52
Q

What is an equation for the darcy friction factor?

A

f=(8τ)/(ρ*v2)

53
Q

How can you find the friction factor when the flow is laminar?

A

f=64/Re

54
Q

How do you use the cole-brook white formula?

A

Assume f to be a certain number e.g. 0.02

Put it into LHS and RHS. If they are disimilar, use the RHS to reverse calculate and find f. Try in

55
Q

What do you use the moody diagram for?

A

Finding the friction factor for smooth pipes, laminar flow, finiding friction facotr when you have relative roughness and Re

56
Q

How would you find the pressure gradient along a pipe having found the head loss?

A

Use p=ρ*g*h to find pressure drop and divide by pipe length

57
Q

What are the fundamental units?

A

Length, Mass and Time.

There are others but don’t worry about them.

58
Q

What is buckingham pi theorem?

A

If an equation involving k variables is dimensionally homogeneous, it can be reduced to a relationship among k-r independent dimensionless groups, where r is the minimum number of basic dimensions required to describe the variables.

We will be usung L, M and T so r=3

59
Q

What is force in fundamental units?

A

MLT-2

60
Q

How many variables are in a pi term?

How do you chose them?

A

3 as R=3.

You can’t use the independent varaible e.g. what you are trying to find

You can’t use things with the same units e.g. length, area and volume.

61
Q

What are forces at terminal velocity?

A

weight-drag-bouancy=0

62
Q

What is a venturimeter?

A

A tube that narrows and then widens again.

63
Q

How would you find the shear force acting on a pipe from the loss in head due to friction?

A
64
Q

If there is a nozzle with a smaller diameter than the supply, how would you find total head?

A

Find the velocity at the exit (use smaller diameter) and use V2/2g.

65
Q

What are the processes that make up a carnot cycle?

A

Two isothermal and two adiabatic.

66
Q

What does a pitot static tube measure?

A
67
Q

What is the equation for the work done in an isothemal process?

A

w=R*T*ln(V1/V2)

68
Q

How do you find the log mean temperture difference?

A

Find the temperture differences between the hot pipe and the cold pipe.

fluid mechanics and energy transfer (4 decks)

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