Airflow Visualization (new)

Question 1

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

Answer 1

inviscid

Question 2

can either be compressible or incompressible

Answer 2

inviscid

Question 3

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

Answer 3

viscous

Question 4

a measure of a fluid’s resistance to flow

Answer 4

viscosity

Question 5

viscosity of water

Answer 5

1.0 centipoise

Question 6

viscosity of honey

Answer 6

12,200.0 centipoise

Question 7

the lower the centipoise, the _______ the viscosity

Answer 7

lower

Question 8

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

Answer 8

dynamic viscosity

Question 9

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

Answer 9

kinematic viscosity

Question 10

D’Alembert’s paradox in 1752 states that

Answer 10

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

Question 11

properties involved in D’Alembert’s paradox of 1752

Answer 11

incompressible, inviscid, irrotational, stationary

Question 12

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

Answer 12

boundary layer

Question 13

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

Answer 13

boundary layer

Question 14

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

Answer 14

because air has viscosity

Question 15

grows as the flow moves over the aerofoil

Answer 15

boundary layer thickness

Question 16

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.

Answer 16

shear stress

Question 17

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

Answer 17

Shear stress

Question 18

Shear stress gives rise to a drag force called _________.

Answer 18

skin friction drag

Question 19

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

Answer 19

Laminar Flow

Question 20

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

Answer 20

Turbulent Flow

Question 21

Factors that affect the type of flow in the Boundary Layer

Answer 21

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

Question 22

Reynolds Number was named after?

Answer 22

Osborne Reynolds

Question 23

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

Answer 23

laminar

Question 24

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

Answer 24

turbulent

Question 25

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.

Answer 25

small, large

Question 26

Symbol for Reynolds Number

Answer 26

Re

Question 27

formula of Reynolds Number

Answer 27

Re = ρVl / μₒ = Vl / v

Question 28

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.

Answer 28

no-slip, outer edge

Question 29

zero velocity at the wall condition in a boundary layer

Answer 29

no-slip

Question 30

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

Answer 30

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

Question 31

Formula of Laminar Boundary Layer Thickness

Answer 31

δ = 5.2x / sqr of Reₓ

Question 32

symbol for laminar local skin friction

Answer 32

Cfₓ

Question 33

Formula of Turbulent Boundary Layer Thickness

Answer 33

δ = 0.37x / 5th root of Reₓ

Question 34

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

Answer 34

transition point

Question 35

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

Answer 35

critical chord xcr

Question 36

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

Answer 36

flow separation

Question 37

the drag caused by the flow separation is called _______

Answer 37

pressure drag or form drag

Question 38

Two major consequences of the flow separating over an aerofoil

Answer 38

- drastic loss of lift - major increase in drag caused by pressure drag due to separation

Question 39

Viscous Effects on Drag

Answer 39

- skin friction drag - pressure drag or form drag

Question 40

symbol of skin friction drag

Answer 40

Df

Question 41

symbol of pressure drag or form drag

Answer 41

Dp

Question 42

Formula of total drag caused by viscous effects

Answer 42

D = Df + Dp

Question 43

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

Answer 43

- 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

Question 44

visualization of light scattering from tracer particles

Answer 44

tracer particles

Question 45

three conditions the tracers should meet

Answer 45

- neutral buoyancy - high stability against mixing - good visibility

Question 46

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

Answer 46

smoke

Question 47

It is an accurate way to visualize freestream flow.

Answer 47

Smoke Visualization

Question 48

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

Answer 48

low turbulence

Question 49

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

Answer 49

vortices, regions

Question 50

advantage of smoke visualization

Answer 50

inexpensive, easy to set up

Question 51

cons of smoke visualization

Answer 51

- 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

Question 52

_______ rely on the refractive index changes in the fluid.

Answer 52

optical methods

Question 53

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

Answer 53

refraction, index of refraction

Question 54

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.

Answer 54

Schlieren photography

Question 55

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

Answer 55

shadowgraph technique

Question 56

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

Answer 56

surface

Question 57

Schlieren photography is used to locate _____________.

Answer 57

shockwaves

Question 58

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

Answer 58

surface oil flow visualization

Question 59

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

Answer 59

conspicuous pigments, fluorescent properties

Question 60

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

Answer 60

before

Question 61

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

Answer 61

pressure sensitive paints (PSP)

Question 62

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

Answer 62

oxygen quenching

Question 63

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).

Answer 63

Pressure Sensitive Paint (PSP)

Question 64

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

Answer 64

UV light, air pressure

Question 65

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

Answer 65

Tufts

Question 66

________ 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 _________.

Answer 66

Tufts, downstream

Question 67

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.

Answer 67

size, weight

Question 68

Popular material for thread tufts

Answer 68

- monofilament nylon - polyester - cotton number 60

Question 69

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

Answer 69

mini-tufts

Question 70

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 ________________.

Answer 70

0.04 mm, boundary layer disturbances

Question 71

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

Answer 71

1 inch segments, wire mesh (screen), grid

Question 72

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

Answer 72

grid