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Aeronautical Laboratory > Airflow Visualization > Flashcards

Airflow Visualization Flashcards

1
Q

assumed to neglect the effects of friction in tackling situations involving fluid dynamics

A

inviscid

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2
Q

can either be compressible or incompressible

A

inviscid

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3
Q

the property of a fluid to resist shearing stresses and a tendency to adhere to solid surfaces

A

viscous

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4
Q

a measure of a fluid’s resistance to flow

A

viscosity

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5
Q

viscosity of water

A

1.0 centipoise

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6
Q

viscosity of honey

A

12,200.0 centipoise

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7
Q

the lower the centipoise, the _______ the viscosity

A

lower

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8
Q

is one way to measure a fluid’s resistance to flow when an external force is applied

A

dynamic viscosity

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9
Q

is another way to measure a fluid’s resistance to flow when no external force is applied except for gravity

A

kinematic viscosity

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10
Q

D’Alembert’s paradox in 1752 states that

A

both drag and lift are zero in potential flow with the following characteristics: incompressible, inviscid, irrotational, and stationary

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11
Q

properties involved in D’Alembert’s paradox of 1752

A

incompressible, inviscid, irrotational, stationary

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12
Q

it is the air flow that is closest to and in contact with the aircraft surface

A

boundary layer

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13
Q

it is a very thin layer of air lying over the surface of the wing and all other surfaces of the airplane

A

boundary layer

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14
Q

Why does the boundary layer tend to stick to the wing?

A

because air has viscosity

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15
Q

grows as the flow moves over the aerofoil

A

boundary layer thickness

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16
Q

The boundary layer thickens as more and more friction affect the flow as the distance along the surface increases. The surface of the aerofoil also contributes to the _________ at the surface.

A

shear stress

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17
Q

______ has dimensions of F/A and acts in a direction tangential to the surface.

A

Shear stress

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18
Q

Shear stress gives rise to a drag force called _________.

A

skin friction drag

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19
Q

What do you call when the fluid moves in parallel layers?

A

Laminar Flow

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20
Q

What do you call when the fluid moves chaotically with irregular fluctuations?

A

Turbulent Flow

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21
Q

Factors that affect the type of flow in the Boundary Layer

A
  • flow quality
  • shape
  • texture
  • pressure gradient
  • Reynolds Number
  • heating of the fluid
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22
Q

Reynolds Number was named after?

A

Osborne Reynolds

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23
Q

If the Reynolds number is small then the flow is __________.

A

laminar

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24
Q

If the Reynolds Number is high, then the flow is _________.

A

turbulent

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25
Q

Osborne Reynolds noticed that if the fluid velocity or diameter of the stream are _____ and the viscosity of the fluid is _____, the Reynolds number will be small.

A

small, large

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26
Q

Symbol for Reynolds Number

A

Re

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27
Q

formula of Reynolds Number

A

Re = ρVl / μₒ = Vl / v

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28
Q

On the boundary layer, the fluid’s interaction with the wall induces a ________ boundary condition and increases continuously to its value of V2 on the _____ of the boundary layer.

A

no-slip, outer edge

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29
Q

zero velocity at the wall condition in a boundary layer

A

no-slip

30
Q

Because high skin friction drag results to a turbulent flow, designers who want to reduce skin friction drag always try to ___________.

A

extend the point at the aerofoil’s surface with which laminar flows exist

31
Q

Formula of Laminar Boundary Layer Thickness

A

δ = 5.2x / sqr of Reₓ

32
Q

symbol for laminar local skin friction

A

Cfₓ

33
Q

Formula of Turbulent Boundary Layer Thickness

A

δ = 0.37x / 5th root of Reₓ

34
Q

a point in the flow where it changes or “transitions” from laminar flow to turbulent flow

A

transition point

35
Q

The transition point’s location in the chord is indicated by the ________ derived from the critical Reynolds Number.

A

critical chord xcr

36
Q

Friction also causes this phenomenon which in turn creates another source of aerodynamic drag due to separation.

A

flow separation

37
Q

the drag caused by the flow separation is called _______

A

pressure drag or form drag

38
Q

Two major consequences of the flow separating over an aerofoil

A
  • drastic loss of lift
  • major increase in drag caused by pressure drag due to separation
39
Q

Viscous Effects on Drag

A
  • skin friction drag
  • pressure drag or form drag
40
Q

symbol of skin friction drag

A

Df

41
Q

symbol of pressure drag or form drag

A

Dp

42
Q

Formula of total drag caused by viscous effects

A

D = Df + Dp

43
Q

experimental flow visualization techniques are applied for several reasons, what are these?

A
  • to get an impression of fluid flow around a scale model of a real object, either in qualitative or quantitative form
  • as a source of inspiration for the development of new and better theories of fluid flow
  • to verify a new theory of model
44
Q

visualization of light scattering from tracer particles

A

tracer particles

45
Q

three conditions the tracers should meet

A
  • neutral buoyancy
  • high stability against mixing
  • good visibility
46
Q

It is used to visualize the flow that is away from the surface of the model.

A

smoke

47
Q

It is an accurate way to visualize freestream flow.

A

Smoke Visualization

48
Q

The smoke can be very useful in a wind tunnel with ___________.

A

low turbulence

49
Q

Smoke can be used to detect ______ and ________ of separated flow.

A

vortices, regions

50
Q

advantage of smoke visualization

A

inexpensive, easy to set up

51
Q

cons of smoke visualization

A
  • extended use fills wind tunnel with smoke
  • residue of smoke can stick to walls of wind tunnel skin friction drag on the walls and disturb air flow going through the test section
52
Q

_______ rely on the refractive index changes in the fluid.

A

optical methods

53
Q

_________ is a phenomenon when a light passing through a certain medium bends toward or away its normal line according to the _____________ of the medium.

A

refraction, index of refraction

53
Q

It is similar to the shadowgraph technique and relies on the fact that light rays are bent whenever they encounter changes in density of fluid.

A

Schlieren photography

54
Q

Schlieren photography is similar to ________ and relies on the fact that light rays are bent whenever they encounter changes in density of fluid.

A

shadowgraph technique

55
Q

Schlieren systems are used to visualize the flow away from the _________ of an object.

A

surface

56
Q

Schlieren photography is used to locate _____________.

A

shockwaves

57
Q

This method is performed by putting oil drops on the test model at its upstream location.

A

surface oil flow visualization

58
Q

Oils used in the surface oil flow visualization can sometimes be dyed with ___________ or with ____________.

A

conspicuous pigments, fluorescent properties

59
Q

In a surface oil flow visualization, surface oils must be applied _________ the experiment when the wind tunnel is still sopped.

A

before

60
Q

It is an alternative approach as to the traditional techniques in measuring surface pressure distributions through the use of pressure taps.

A

pressure sensitive paints (PSP)

61
Q

Pressure Sensitive Paint is essentially a luminescent dye dispersed in an oxygen permeable binder, also known as _______________.

A

oxygen quenching

62
Q

It is a method for measuring air pressure on a test article. It requires less preparation compared to installing a pressure tap system, and produces a large number of data points (one every pixel).

A

Pressure Sensitive Paint (PSP)

63
Q

Pressure Sensitive Paint responds to __________. When excited, the intensity of the light it reflects will vary with _____________.

A

UV light, air pressure

64
Q

________ are small lengths of string that are frayed on the ends.

A

Tufts

65
Q

________ are attached to the surface of the model using some strong adhesive such as tape or glue, and as the air flows over the test model, the tuffs are blown and point _________.

A

Tufts, downstream

66
Q

Some factors that you need to consider when attaching tufts are its _____ and _______ just t make sure that the tufts move together with the flow and not alter the flow itself.

A

size, weight

67
Q

Popular material for thread tufts

A
  • monofilament nylon
  • polyester
  • cotton number 60
68
Q

_______ are thin, lightweight tufts with diameter of 0.01 mm <= 0.1 mm.

A

mini-tufts

69
Q

Due to the small size of mini-tufts, the proportional amount of glue quantities required to attach them to the test model’s surface is small, about _____, thus avoiding too much ________________.

A

0.04 mm, boundary layer disturbances

70
Q

Thread tufts used in experiments, are commonly _______ that are either directly attached to the surface or tied to a ______ called _____.

A

1 inch segments, wire mesh (screen), grid

71
Q

It is placed behind the test model oriented perpendicularly to the air flow.

A

grid

Aeronautical Laboratory (3 decks)

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