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Aerodynamics > Definitions > Flashcards

Definitions Flashcards

(66 cards)

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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
What is downwash
Downward velocity induced by tip vortices along the span, directly behind the wing
26
What is induced drag
Drag penalty caused by generation of lift. Energy lost in wingtip vortices
27
Prandtls lifting line theory
One bound vortex along the wingspan, with 2 tip vortices extended to infinity. Circulation is constant along whole vortex model
28
What are some limitations of the lifting line theory
Lift per unit span is constant (inaccurate) Induced drag wildly inaccurate Downwash infinite at tips
29
What is the lifting line model
Uses multiple distributed horseshoe vortices Different circulations at different spans Good approximation of straight slender wings
30
What happens to the compressibility of flows of M>1
They become compressible
31
What is a shock wave
An extremely thin region (10^-5) or smaller across which flow properties change drastically and suddenly
32
What happens to flow speed across a shockwave
It decelerates
33
What are the 3 types of shocks
Normal shock Oblique shock Bow shock
34
What is a normal shock
Adiabatic- but non isentropic
35
What force is caused by a boundary layer
Skin friction drag
36
What is a boundary layer
A thin layer of flow adjacent to a surface where flow is decelerated due to friction between a solid surface and a fluid
37
What is the no slip condition
At the surface fluid particles in contact with a surface have zero relative velocity
38
What is it called when a boundary layer separates from a surface
Stall
39
How do you calculate total drag
Pressure drag + skin friction drag
40
What sort of flow is friction drag higher in
Turbulent
41
What sort of flow is more prone to separation
Flow separation is much more likely to occur in a laminar flow
42
What factors can effect BL transition
Surface roughness Surface temperature Pressure gradients Free stream turbulence Reynolds number Surface curvature
43
What are the characteristics of laminar flow
Low skin friction Smooth sheet of flow Thickens slowly Prone to transition at high RE readily seperates
44
What are the characteristics of a turbulent BL
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
What is the displacement thickness of a BL
The distance by which streamlines are displaced due to the presence of the BL
46
Momentum thickness
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
What shape is used to approximate an aerofoil at low angles of attack
A flat plate
48
What is the shape factor of a BL
Non dimensional number used to determine the nature of the BL flow. By definition H=disp/momentum thickness
49
What is the H value of a laminar flow
2.6
50
What is the h value of the turbulent flow
H=1.4
51
Why do we need high lift devices
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
What is wing loading
The ratio of the aircraft weight to its wing area, which tells us how much weight can be supported by the wing
53
What are the drawbacks of HLDs
Expensive Complex Heavy Noisy
54
What are the 2 main types of HLDs
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
Types of LE devices
Leading edge Slats Leading edge Droop nose Leading edge flaps
56
What is the purpose of the slat
Increase stall AoA which leads to a higher clmax
57
What is the physical effect of a slat
Flow on the lower surface flows onto the upper surface, increasing the KE and reenergising the boundary layer Proportional
58
What is a TE flap
A TE flap is a portion of the trailing edge that is hinged and can be deflected upwards or downwards to affect flow
59
What effect will TE flaps have on performance
Lift curve moves more to the left, and zero lift angle becomes negative
60
What are the types of TE flaps
Plain Split Slotted Fowler
61
What is a multi element device
An HLD that uses multiple moving parts I.e. double slotted flap
62
What is a spoiler
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
Air brakes
Increase drag but don’t reduce lift Normally found on the fuselage
64
Suction and blowing
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
What is a Blown flap
Use of engine exhaust to introduce high are over the wing
66
Vortex generators
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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