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Aerodynamics > Definitions > Flashcards

Definitions Flashcards

1
Q

Circulation

A

Line integral of velocity tangent to line around a closes curve

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2
Q

Kutta condition

A

Airflow leaving trailing edge must leave smoothly

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3
Q

Why does the kutta condition exist

A

Exists to ensure a unique value of circulation around the aerofoil, hence a unique value of lift/ if not satisfied, velocity at trailing edge may have an infinite value

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4
Q

Inviscid

A

Zero Viscosity

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5
Q

What 2 conditions are true in an ideal flow

A

Inviscid and constant density

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6
Q

Streamline

A

Curve who’s tangent is everywhere in the same direction as a velocity vector field

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7
Q

Vorticity

A

The curl of a velocity field. A measure of a local rotation of a fluid

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8
Q

Irrotational flow

A

Vorticity is 0

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9
Q

What conditions are true for potential flow

A

Irrotational and Inviscid

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10
Q

What is the relationship between stream function and potential flow

A

Orthogonal

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11
Q

How can fluids for a complex flow be modelled

A

External flows around a body can be approximated as Irrotational and Inviscid. External potential flows can then be solved for, with viscosity effects applied within boundary layers

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12
Q

How can you approximate complex potential flows

A

By superposition and and adding simple potential flows around

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13
Q

Do potential flows actually exist

A

No; as flow with 0 viscosity is impossible

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14
Q

What force will not be seen when looking at the behaviour of an object in a potential flow

A

Drag

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15
Q

Circulation

A

Line integral of velocity around a closed curve (C)

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16
Q

What are the four elementary flows

A

Uniform flow, source flow,sink flow and free vortex flow

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17
Q

What blocks are involved in doublet flow

A

A source and a sink

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18
Q

Half body flow

A

A source in a free stream

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19
Q

What flow features are found on Circular cylinder with no lift

A

Uniform flow and doublet flow (source and sink)

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20
Q

What is d’alemberts paradox

A

Result of 0 drag when cylinder is modelled in a uniform potential flow

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21
Q

Building blocks of a lifting flow over a circular cylinder

A

Uniform flow + doublet + vortex flow

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22
Q

Kutta condition

A

A body with a sharp trailing edge will generate enough circulation to hold its stagnation point at the trailing edge

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23
Q

What is the thin aerofoil theory

A

For a given shape aerofoil, there exists a continuous distribution of vortices along the mean chord line with a unique vortex strength, so that a combined velocity field of upstream flow and vortex sheets satisfies boundary conditions

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24
Q

What is a trailing vortex

A

Two curling vortices that form at a wingtip and trail downstream.

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25
Q

What is downwash

A

Downward velocity induced by tip vortices along the span, directly behind the wing

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26
Q

What is induced drag

A

Drag penalty caused by generation of lift. Energy lost in wingtip vortices

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27
Q

Prandtls lifting line theory

A

One bound vortex along the wingspan, with 2 tip vortices extended to infinity. Circulation is constant along whole vortex model

28
Q

What are some limitations of the lifting line theory

A

Lift per unit span is constant (inaccurate)
Induced drag wildly inaccurate
Downwash infinite at tips

29
Q

What is the lifting line model

A

Uses multiple distributed horseshoe vortices
Different circulations at different spans
Good approximation of straight slender wings

30
Q

What happens to the compressibility of flows of M>1

A

They become compressible

31
Q

What is a shock wave

A

An extremely thin region (10^-5) or smaller across which flow properties change drastically and suddenly

32
Q

What happens to flow speed across a shockwave

A

It decelerates

33
Q

What are the 3 types of shocks

A

Normal shock
Oblique shock
Bow shock

34
Q

What is a normal shock

A

Adiabatic- but non isentropic

35
Q

What force is caused by a boundary layer

A

Skin friction drag

36
Q

What is a boundary layer

A

A thin layer of flow adjacent to a surface where flow is decelerated due to friction between a solid surface and a fluid

37
Q

What is the no slip condition

A

At the surface fluid particles in contact with a surface have zero relative velocity

38
Q

What is it called when a boundary layer separates from a surface

A

Stall

39
Q

How do you calculate total drag

A

Pressure drag + skin friction drag

40
Q

What sort of flow is friction drag higher in

A

Turbulent

41
Q

What sort of flow is more prone to separation

A

Flow separation is much more likely to occur in a laminar flow

42
Q

What factors can effect BL transition

A

Surface roughness
Surface temperature
Pressure gradients
Free stream turbulence
Reynolds number
Surface curvature

43
Q

What are the characteristics of laminar flow

A

Low skin friction
Smooth sheet of flow
Thickens slowly
Prone to transition at high RE
readily seperates

44
Q

What are the characteristics of a turbulent BL

A

High skin friction
Random flow with lots of mixing
Thickens rapidly
Reluctant to form at low RE
Can penetrate an adverse pressure gradients Free

45
Q

What is the displacement thickness of a BL

A

The distance by which streamlines are displaced due to the presence of the BL

46
Q

Momentum thickness

A

Proportional to the reduction in momentum flow due to the presence of a boundary layer
Proportional to total skin friction drag coefficient. Used to predict separation point

47
Q

What shape is used to approximate an aerofoil at low angles of attack

A

A flat plate

48
Q

What is the shape factor of a BL

A

Non dimensional number used to determine the nature of the BL flow. By definition H=disp/momentum thickness

49
Q

What is the H value of a laminar flow

A

2.6

50
Q

What is the h value of the turbulent flow

A

H=1.4

51
Q

Why do we need high lift devices

A

High loft devices increase the Cl, which decreases the Vstall, which allows the aircraft to takeoff at a lower speed/a speed it can achieve on the ground

52
Q

What is wing loading

A

The ratio of the aircraft weight to its wing area, which tells us how much weight can be supported by the wing

53
Q

What are the drawbacks of HLDs

A

Expensive
Complex
Heavy
Noisy

54
Q

What are the 2 main types of HLDs

A

TE devices - increase the effective angle that flow is turned through, increasing lift
LE devices - assists flow in negotiating the sharp turn from lower surface around the LE and back for a short distance on upper surface

55
Q

Types of LE devices

A

Leading edge Slats
Leading edge Droop nose
Leading edge flaps

56
Q

What is the purpose of the slat

A

Increase stall AoA which leads to a higher clmax

57
Q

What is the physical effect of a slat

A

Flow on the lower surface flows onto the upper surface, increasing the KE and reenergising the boundary layer Proportional

58
Q

What is a TE flap

A

A TE flap is a portion of the trailing edge that is hinged and can be deflected upwards or downwards to affect flow

59
Q

What effect will TE flaps have on performance

A

Lift curve moves more to the left, and zero lift angle becomes negative

60
Q

What are the types of TE flaps

A

Plain
Split
Slotted
Fowler

61
Q

What is a multi element device

A

An HLD that uses multiple moving parts I.e. double slotted flap

62
Q

What is a spoiler

A

An upper surface flap that causes separation to destroy lift. They increase the rate of descent. Increased drag but primary mechanism is to reduce lift

63
Q

Air brakes

A

Increase drag but don’t reduce lift
Normally found on the fuselage

64
Q

Suction and blowing

A

Used to manipulate a boundary layer to reduce separation.
Suction has holes to remove BL layers
Blowing adds air tangent to the surface to energise BL

65
Q

What is a Blown flap

A

Use of engine exhaust to introduce high are over the wing

66
Q

Vortex generators

A

Small vanes attached to wing, that energise BL and allow it to stay attached. This also assists effectiveness of flaps

Aerodynamics (1 decks)

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