M2 Flashcards
Assumed top neglect the effects of friction in tackling situations involving fluid dynamics. It can either be incompressible or incompressible.
Inviscid
is the property of a fluid to resist shearing stresses and a tendency to adhere to solid surfaces.
Viscosity
It is deemed to be an important fluid property when analyzing fluid behavior and motion near solid boundaries
Viscosity
Viscosity of water
1.0centipoise
Viscosity of honey
12,200 centipoise
It is caused by intermolecular forces or the friction that is exerted when layers of fluid try to slip by one another.
Shear resistance
Two related measures of fluid viscosity
- Dynamic μ0
- Kinematic v
is one way to measure a fluid’s resistance to flow when an external
force is applied.
Dynamic Viscosity
Constant value of Dynamic Viscosity ( SI and Imperial)
μ0air= 1.789 x10^5 kg/ms
= 3.7272 X 10^4 ft^2/s
It is another way to measure this resistive flow of a fluid
when no external force is applied except the influence of weight.
Kinematic Viscosity
Constant value of Kinematic Viscosity ( SI and Imperial)
vair= 1.460 x 10^ -5 m^2/s
= 1.5723 x 10^-4 ft^2/s
——————— formulated a paradox about an ideal frictionless flow where it states that both drag and lift are zero in potential flow with the following characteristics:
- d’Alembert (1752)
characteristics:
-Incompressible
-Inviscid
-Irrotational; and
-Stationary
He suggested that drag could somehow result from a very thin region close to a solid boundary. Although his resolution to the paradox was incorrect, his insight to the concept of boundary layer was a huge contribution when it came to solving problems regarding viscous flows.
Ludwig Prandtl (1904)
a layer adheres to the surface because of friction between the gas and the surface of the solid material
Boundary Layer
It is airflow that is closest to and in contact with the aircraft, in other words, tight at the surface, the flow is zero,
Boundary Layer
There is a thin region of retarded flow in the vicinity of the surface. A flow field can be split into two regions:
-Boundary Layer
-Potential or Frictionless Flow ( outside the
boundary layer)
Inner edge of the boundary layer (a) (V=?)
Velocity= 0
the flow velocity is essentially the same to the flow velocity for the frictionless flow at V2, meaning that the properties on the outer edge of the boundary layer can be calculated from a
“frictionless flow analysis”
Outer edge of the boundary layer (b)
grows as the flow moves over the aerofoil.This happens as
more and more friction affects the flow as the distance along the surface increases.
Boundary Layer Thickness δ
The friction on the surface of the aerofoil also contributes to the __________at the surface.
shear stress τω
The shear stress has
dimensions of force/area and acts in a direction “tangential” to the surface τ w gives rise to a drag force called _________.
Skin Friction Drag
Flows in which the streamlines of a flow are smooth and regular and the fluid element moves smoothly along the streamline.
Laminar Flow
These are flows in which the streamlines break up and a fluid element moves in a random, irregular, and tortuous fashion.
Turbulent Flow
Factors that affect the type of flow in the Boundary Layer: (6)
-Quality (smoothness) of the flow approaching the body
-The shape of the body
-Surface roughness
-Pressure gradient
-Reynolds Number
-Heating of the fluid by the surface
This parabolic profile that illustrates the velocity of the flow as it goes through the boundary layer.
Velocity Profile
The velocity at the surface of the object where V(y = 0) is______.
zero
The velocity at the outer boundary where V(y = H) is ________.
V2
Named after Osborne Reynolds
Reynold’s Number
if the fluid velocity or diameter of the stream are ______ and the viscosity of the fluid is ________, the Reynolds number will be small, and with these cases, the flows tend to be laminar
small, large
is a dimensionless number linking viscosity, density, and a reference length.
Reynold’s Number
number is expressed as the ratio of inertial forces (resistance to change or
motion) to viscous forces.
Reynold’s Number
It grows as the flow progresses from laminar to turbulent
Velocity Profile
The ________resulting from turbulent flows are thicker than those in laminar flows
velocity profile
The _____________ for turbulent flows remains its velocity almost the same as the ones in the free stream and then rapidly decreases when it’s near the surface while the velocity profile for laminar flows gradually decreases from its free stream
velocity.
velocity profile
TRUE OR FALSE: Wall shear stress of laminar flow is MORE than wall shear stress of turbulent flow
FALSE, Wall shear stress of laminar flow is LESS than wall shear stress of turbulent flow
TRUE OR FALSE: Designers who want to reduce skin friction drag always try to extend the point at the aerofoil’s
surface with which laminar flows exist.
TRUE
suggests, is a point in the flow where it changes or “transitions” from laminar flow to turbulent flow.
Transition Flow
____________ is caused by friction; which in turn, creates another source of aerodynamic drag, called _________ due to separation or _______.
Flow Separation, pressure drag, Form drag
The two major consequences of the flow separating over an aerofoil are:
a. A drastic loss of lift. (stalling)
b. A major increase in drag caused by pressure drag due to separation.
Summary of viscous effects on drag:
1. ___________ due to shear stress at the wall.
2. ___________ due to flow separation Dp, sometimes identified as form drag
Skin friction drag Df, Pressure drag
The flow of fluids can be analyzed by:
-Theory
-Numerical computation
-Experiment.
is an important tool in experimental fluid mechanics, which can provide an overall picture of the flow field.
Visualization
TRUE OR FALSE: By applying these some methods of visualization, a flow pattern can be observed directly or recorded using a camera.
TRUE
Experimental flow visualization techniques are applied for several reasons:
- 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 or model.
Flow visualization may be classified into three basic principles:
-Tracer Particles
-Optical Methods
-Surface Flows
Visualization of light scattering from tracer particles.
Tracer Particles
Since the ___________ from the fluid
molecules are extremely weak, tracer particles such as dust, smoke, and dye are seeded in the flow, making the
seemingly transparent nature of the fluids observable.
Rayleigh scattering
There are mainly three conditions the tracers should meet:
-Neutral buoyancy
-High stability against mixing,
-Good visibility.
___________ can be improved if the tracer particles can not only re-scatter the light incident to the particles but rather emit their own.
Signal-to-noise ratio
one of the oldest flow visualization methods to exist, still continues to be an important experimental tool in the study of flow dynamic phenomena.
Smoke Visualization
used to visualize the flow that is away from the surface of the model.
Smoke
Accurate way to visualize freestream flow
can be very useful in a wind tunnel with low turbulence.
Smoke
can be used to detect vortices and regions of separated flow.
Smoke
_________ can be generated in a wind tunnel by introducing smoke through small pipes in front of the test model.
Smoke Lines
Advantages of Smoke Visualization
- Inexpensive
- Easy to set up
Disadvantages of Smoke Visualization
- Extended use of this method fills the wind tunnel with smoke.
-Residue of the smoke can also stick to the walls of the wind tunnel skin friction drag on the walls and disturb air flow going through the test section.- may produce unnecessary skin friction drag on the wall.
These are another old visualization technique that is used in both flight test and wind tunnel testing.
Tufts
The visualization of water flows by means of dye is as popular as the use of smoke in air flows. Unlike smoke, appropriate dyes can fulfill almost any requirements of an ideal tracer.
Visualization by dye in water
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 downstream.
Tufts
Flow visualization by Tufts
Factors that need to consider when attaching stuffs:
- size
-weight
monofilament nylon, polyester or cotton number 60 sewing threads.
Thread Tufts
________ used in experiments, are commonly 1 inch segments that are either directly attached to the surface or tied to a wire mesh (screen) called grid, which is placed behind the test model oriented perpendicularly to the air flow
Thread Tufts
thin, lightweight tufts with diameter [0.01 mm ≤ 0.1mm].
Mini Tufts
Due to the small size of these ________, the proportional amount of glue quantities required to attach them to the test model’s surface becomes small in quantity as well, about 0.04 mm, thus avoiding too much boundary layer disturbances.
Mini Tufts
The issue in the ________ occurs in the method on how to record such thin tufts due to the small amount of reflected light. However, this conundrum was solved through the use of ___________.
Mini Tufts, fluorescent dyes
Used to cause the mini-tufts to fluoresce, enhancing their size for better visibility
Ultraviolet flash
relying on the refractive index changes in the fluid.
Optical methods
a phenomenon when a light passing through a certain medium bends toward or
away its normal line according to the index of refraction of the medium.
Refraction
an imaginary line perpendicular to the surface of the medium
Normal Line
Optical Methods:
As a light ray passes through a ______ medium, it tends to ________ the normal line. On the other hand, as light passes through a ______ medium it tends to _______ from the normal.
denser, bend towards, less dense,bend away
It is similar to the shadowgraph technique and relies on the fact that light rays are bent whenever they encounter changes in density of a fluid
Schlieren Photography
- are used to visualize the flow away from the surface of an object
- It is used to locate shockwaves.
Schlieren Photography
A type of flow visualization for observations of flow characteristics close to the surface of the test model
Surface Flow
The surface will then be coated with a certain material which will have an interaction with the fluid flow and develop a visible pattern, which is assume to have been taken by the fluid.
Surface Flow
Three different interaction processes can be used for generating different kinds of information. These interactions can be:
-Mechanical
-Chemical
-Thermal in nature
This method is performed by putting oil drops on the test model at its upstream location.
Surface Oil Flow Visualization
________ used in this method can sometimes be dyed with conspicuous pigments or with fluorescent properties.
Oils
_________ must be applied before the experiment when the wind tunnel is still sopped.
Surface oils
The use of _______________ to measure surface pressure was an alternative approach to the traditional techniques in measuring surface pressure distributions through the use of pressure taps.
Pressure Sensitive Paints
____________is essentially a luminescent dye dispersed in an oxygen permeable binder, also known as oxygen quenching.
Pressure Sensitive Paints
Pressure Sensitive Paints:
TRUE OR FALSE: The paint which contains these luminophore molecules are then illuminated with a light source emitting light of short wavelengths, commonly performed by passing a light through blue or Ultraviolet filters, in order to excite the paint. Returning to its ground state, the collision of the luminophores with the oxygen molecules causes the paint to emit light with less intensity.
TRUE
Paints use the same approach as PSP’s, only without oxygen quenching
Temperature Sensitive Paints
A type of fluid flow visualization where it would be possible to observe flow phenomena which are essentially inviscid.
Off-the-surface flow visualization
A type of fluid flow visualization where it would be possible to observe flow phenomena which are dominated by the effects of viscosity.
Surface flow visualization
