Airflow Visualization Flashcards
assumed to neglect the effects of friction in tackling situations involving fluid dynamics
inviscid
can either be compressible or incompressible
inviscid
the property of a fluid to resist shearing stresses and a tendency to adhere to solid surfaces
viscous
a measure of a fluid’s resistance to flow
viscosity
viscosity of water
1.0 centipoise
viscosity of honey
12,200.0 centipoise
the lower the centipoise, the _______ the viscosity
lower
is one way to measure a fluid’s resistance to flow when an external force is applied
dynamic viscosity
is another way to measure a fluid’s resistance to flow when no external force is applied except for gravity
kinematic viscosity
D’Alembert’s paradox in 1752 states that
both drag and lift are zero in potential flow with the following characteristics: incompressible, inviscid, irrotational, and stationary
properties involved in D’Alembert’s paradox of 1752
incompressible, inviscid, irrotational, stationary
it is the air flow that is closest to and in contact with the aircraft surface
boundary layer
it is a very thin layer of air lying over the surface of the wing and all other surfaces of the airplane
boundary layer
Why does the boundary layer tend to stick to the wing?
because air has viscosity
grows as the flow moves over the aerofoil
boundary layer thickness
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.
shear stress
______ has dimensions of F/A and acts in a direction tangential to the surface.
Shear stress
Shear stress gives rise to a drag force called _________.
skin friction drag
What do you call when the fluid moves in parallel layers?
Laminar Flow
What do you call when the fluid moves chaotically with irregular fluctuations?
Turbulent Flow
Factors that affect the type of flow in the Boundary Layer
- flow quality
- shape
- texture
- pressure gradient
- Reynolds Number
- heating of the fluid
Reynolds Number was named after?
Osborne Reynolds
If the Reynolds number is small then the flow is __________.
laminar
If the Reynolds Number is high, then the flow is _________.
turbulent
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.
small, large
Symbol for Reynolds Number
Re
formula of Reynolds Number
Re = ρVl / μₒ = Vl / v
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.
no-slip, outer edge
zero velocity at the wall condition in a boundary layer
no-slip
Because high skin friction drag results to a turbulent flow, designers who want to reduce skin friction drag always try to ___________.
extend the point at the aerofoil’s surface with which laminar flows exist
Formula of Laminar Boundary Layer Thickness
δ = 5.2x / sqr of Reₓ
symbol for laminar local skin friction
Cfₓ
Formula of Turbulent Boundary Layer Thickness
δ = 0.37x / 5th root of Reₓ
a point in the flow where it changes or “transitions” from laminar flow to turbulent flow
transition point
The transition point’s location in the chord is indicated by the ________ derived from the critical Reynolds Number.
critical chord xcr
Friction also causes this phenomenon which in turn creates another source of aerodynamic drag due to separation.
flow separation
the drag caused by the flow separation is called _______
pressure drag or form drag
Two major consequences of the flow separating over an aerofoil
- drastic loss of lift
- major increase in drag caused by pressure drag due to separation
Viscous Effects on Drag
- skin friction drag
- pressure drag or form drag
symbol of skin friction drag
Df
symbol of pressure drag or form drag
Dp
Formula of total drag caused by viscous effects
D = Df + Dp
experimental flow visualization techniques are applied for several reasons, what are these?
- 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
visualization of light scattering from tracer particles
tracer particles
three conditions the tracers should meet
- neutral buoyancy
- high stability against mixing
- good visibility
It is used to visualize the flow that is away from the surface of the model.
smoke
It is an accurate way to visualize freestream flow.
Smoke Visualization
The smoke can be very useful in a wind tunnel with ___________.
low turbulence
Smoke can be used to detect ______ and ________ of separated flow.
vortices, regions
advantage of smoke visualization
inexpensive, easy to set up
cons of smoke visualization
- 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
_______ rely on the refractive index changes in the fluid.
optical methods
_________ is a phenomenon when a light passing through a certain medium bends toward or away its normal line according to the _____________ of the medium.
refraction, index of refraction
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.
Schlieren photography
Schlieren photography is similar to ________ and relies on the fact that light rays are bent whenever they encounter changes in density of fluid.
shadowgraph technique
Schlieren systems are used to visualize the flow away from the _________ of an object.
surface
Schlieren photography is used to locate _____________.
shockwaves
This method is performed by putting oil drops on the test model at its upstream location.
surface oil flow visualization
Oils used in the surface oil flow visualization can sometimes be dyed with ___________ or with ____________.
conspicuous pigments, fluorescent properties
In a surface oil flow visualization, surface oils must be applied _________ the experiment when the wind tunnel is still sopped.
before
It is an alternative approach as to the traditional techniques in measuring surface pressure distributions through the use of pressure taps.
pressure sensitive paints (PSP)
Pressure Sensitive Paint is essentially a luminescent dye dispersed in an oxygen permeable binder, also known as _______________.
oxygen quenching
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).
Pressure Sensitive Paint (PSP)
Pressure Sensitive Paint responds to __________. When excited, the intensity of the light it reflects will vary with _____________.
UV light, air pressure
________ are small lengths of string that are frayed on the ends.
Tufts
________ 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 _________.
Tufts, downstream
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.
size, weight
Popular material for thread tufts
- monofilament nylon
- polyester
- cotton number 60
_______ are thin, lightweight tufts with diameter of 0.01 mm <= 0.1 mm.
mini-tufts
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 ________________.
0.04 mm, boundary layer disturbances
Thread tufts used in experiments, are commonly _______ that are either directly attached to the surface or tied to a ______ called _____.
1 inch segments, wire mesh (screen), grid
It is placed behind the test model oriented perpendicularly to the air flow.
grid
