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Part 3 Aerodynamics > Module 2 Concepts > Flashcards

Module 2 Concepts Flashcards

1
Q

What is the definition of viscosity?

A

The measure of resistance to the relative movement of two neighbouring particles of a fluid

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2
Q

How does the viscosity of a liquid change with increased temperature?

A

Viscosity decreases. Eg. honey & Engine oil

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3
Q

How does viscosity of a gas change with increased temperature?

A

Viscosity increases

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4
Q

Why does viscosity fo illiquid decrease with increased temperature?

A

As temp increases the bond between particles becomes more relaxed as they have more energy to move about more freely.

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5
Q

What does viscosity of a gas increase with increased temperature?

A

The kinetic energy of each particle sin the gas is increases, so they move around faster so there are more frequent collisions between particles with creates more resistance to the change in relative velocity between them.

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6
Q

How does the viscosity of a gas change with increased pressure?

A

If pressure increases, and temp remains constant, the viscosity will increase.

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7
Q

Does an incompressible, viscous fluid flowing through a pipe satisfy Bernoulli’s equation? What will happen?

A

No. there will be loss in pressure at the downstream location as some energy is consumed to overcome the friction.

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8
Q

What is hf? in relation to viscous fluids in Bernoulli’s equation

A

loss of pressure head

-The difference in pressure head caused by the viscous friction.

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9
Q

What is the definition of Reynolds number?

A

A dimensionless number used to characteristics the regime of a flow

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10
Q

What ratio does Reynolds number show?

A

Ratio of dynamic forces to viscous force

Dynamic: ρvl
Viscous: µ

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11
Q

What is the relative speed of boundary layer particles at the surface? Why?

A

0

because they are ‘stuck’ there due to the effects of viscosity.

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12
Q

What is the boundary layer?

A

layer of fluid near the surface of an object in a fluid flow, where the speed of the particles is slower than that of the mainstream flow. It reaches from the surface where the speed is 0, the the point where speed is 0.99 of the mainstream speed

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13
Q

Describe the general characteristics of the boundary layer?

A
  • Speed of particles on the surface is 0
  • Thickness at the leading edge is 0
  • Speed increases as the perpendicular distance from the surface increases
  • Thickness is the distance from the surface to the point where speed is 99% of the free stream speed
  • Pressure of the fluid in the direction perpendicular to the surface is constant
  • Normally laminar closer to LE, then transition, then turbulent further aft
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14
Q

What is the structure and speed profile of the laminar boundary layer?

A
  • Relatively thin

- Speed increases approximately linearly

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15
Q

What is the structure and speed profile of the turbulent boundary layer?

A
  • Thicker than the laminar layer
  • Speed increases steeply near the surface
  • Speed becomes quite uniform near the edge of boundary layer
  • Has a viscous laminar sublayer underneath
  • Buffer zone between laminar sublayer and turbulent BL
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16
Q

When is a boundary layer considered laminar, transition or turbulent?

A

If Re is:

laminar < 510^5 < transition < 510^6 < Turbulent

17
Q

What is the effect of viscosity in the the boundary layer?

A
  • Creates a shear stressing a direction parallel to the direction of fluid flow
  • This shear stress is skin drag
18
Q

How does average speed and kinetic energy in laminar and turbulent BL compare?

A

Turbulent has a higher average speed/ kinetic energy than laminar due to sharp speed increase near the surface.

19
Q

How does the coefficient of skin drag change as Reynolds number increases?

A

Laminar: Initially it will decrease linearly with increasing Re
Transition: Cd increases to the max at the start of the transition region. From there it decreases linearly, at a low rate so that is its always higher than the max Cd in the laminar layer.

20
Q

How does the higher speed in the turbulent BL happen?

A

The mainstream flow contributes more kinetic energy to overcome the increased viscous friction. This caused the Cf / Cd to be higher.

21
Q

How does pressure change in the u direction over an aerofoil?

A

Decreased from LE to max thickness where it is minimum
beyond max thickness the pressure will increase again

LE to max thickness:n ∂p/∂u < 0
Max Thickness: ∂p/∂u = 0
max thickness to TE: ∂p/∂u > 0

22
Q

How does pressure change in the y direction over an aerofoil?

A

Constant

∂p/∂y = 0

23
Q

What is the adverse pressure gradient?

A

when the pressure in the u direction is increasing. It acts in the direction opposite to the direction of the fluid flow over the aerofoil

24
Q

What causes airflow to reach stagnation?

A
  • Kinetic energy is exhausted from the BL due it being used to overcome the viscous friction
  • Adverse pressure gradient
25
Q

What is the stagnation point? What is BL separation?

A

Stagnation point is the point over the aerofoil where particles velocity is 0 and there is not enough energy to support the increase in speed
BL separation is simply the boundary layer fluid particles leaving the surface of the object.

26
Q

What happens as a result of Boundary layer separation?

A
  • Changes pressure distribution
  • Cl decreases as separated section of wing does not produce lift
  • Centre of pressure moves forward
27
Q

Why does the boundary layer separate?

A

The idle/ stagnation particles accumulate and are pushed away from the surface by more particles arriving at the stagnation point.

28
Q

What can happen if airflow separates over an aileron?

A

Aileron reversal - deflecting the aileron will increase the AoA and cause the separation point to move forward, stalling it and causing the lift over it to reduce and have to opposite effect to what is intended

29
Q

What is form drag?

A

The drag created by the fore-aft pressure difference caused by airflow separation.

30
Q

How does Reynolds number affect form drag?

A

As Re increases, Form drag increases

31
Q

What are the three types of stall?

A
  • Accelerated/ dynamic stall
  • Low speed - High AoA stall
  • Aileron reversal/ tip-stall
32
Q

What happens when vortices are shed from behind the object?

A
  • They will shed in an alternating fashion (at a certain frequency) causing a Karman vortex street.
  • This can become audible if the frequency is right
33
Q

What can cause early airflow separation?

A
  • Sudden change in shape
  • Bumpy surfaces
  • Ice formation
  • Adverse pressure gradient
  • High AoA
34
Q

How can airflow separation be reduced to reduce form drag?

A

Streamlining and filleting

Part 3 Aerodynamics (6 decks)

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