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Windenergietechnik > Aerodynamics > Flashcards

Aerodynamics Flashcards

1
Q

Definition Aerodynamics

A
  • a branch of fluid dynamics
  • consider gas such as air as a fluid
  • studies the motion o gases and the forces acting on a body (assumed rigid) exposed to gas flow
  • main parameter: flow velocity and pressure distribution
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2
Q

Classification of types of flow

A
  • density
  • friction
  • turbulence
  • steadiness
  • Newton’s definition
  • velocity
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3
Q

Classification of types of flow according to density

A
  • compressible = variable density

or incrompressible = constant density (wind turbine technology)

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4
Q

Classification of types of flow according to friction

A
  • viscous flow = high influence of friction or

inviscid flow = no friction involved (wind turbine technology)

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5
Q

Classification of types of flow according to turbulence

A
  • laminar flow = regular

or turbulent flow = random, irregular (Wind flow in PBL)

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6
Q

Classification of types of flow according to steadiness

A
  • steady state flow = timeindependent (WET 1)

or unsteady state flow = timedependent (WET 2)

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7
Q

Classification of types of flow according to Newton’s definition

A
  • Newtonian flow = sher stress proportional to strain rate = air, water (Wind energie technologie)
    or non-newtonian flow = viscoelastic material
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8
Q

Classification of types of flow according to velocity

A
  • Subsonic flow Ma < 0,8 (WET)
  • Transonic flow
  • Supersonic flow
  • Hypersonic flow Ma > 5
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9
Q

Mach number:

A

ratio between current flow velocity and sonic speed

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10
Q

Pressure

A

normal force per unit area

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11
Q

Density

A

mass per unit volume

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12
Q

Streamline

A
  • curve tangential to the velocity vectors of particles of a fluid at a given instance of time
  • streamlines can not intersect
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13
Q

Streamtube

A
  • bunch of streamlines in 3D

- impermeable

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14
Q

Control volume

A
  • reasonably large, finite region of the flow
  • fixed in space
  • arbitrary volume
  • remains constant (conservation of mass: mass in = mass out)
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15
Q

Shear stress

A

product of the viscosity coefficient (material parameter) and the velocity gradient

  • high coefficient = high influence of friction
  • high gradient = high influence of friction
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16
Q

Air foil

A
  • aerodynamic section
  • low drag
  • rounded shape at front (nose) = leading edge
  • ## sharp edge at back = trailing edge
17
Q

Chord length

A

length between leading and trailing edge alonge the chord line

18
Q

Relative thickness

A

Ratio between maximum thickness and chord length

19
Q

Camber

A

distance between mean camber line and chord line

  • 0 at leading and trailing edge
  • symmetric airfoils do not have any camber
20
Q

Reynolds number

A
  • dimensionless numer
  • ratio between inertia and viscous forces
  • material depended
  • characteristic length is chord length
  • low Re (< 2000) = high viscosity, laminar flow
  • high Re (>3500)= low viscosity , turbulent flow (WET 100.000 - 10.000.000)
21
Q

Momentum equation

A
  • Principle of conservation of linear momentum:
  • time rate of change of linear momentum of a body (produkt of mass and accleration) is equal to the sum of the forces acting on the body
22
Q

Bernoulli equation

A
  • Principle of conservation of energy
  • sum of energy stays constant
  • for incompressible, inviscid, steady state flows
23
Q

Origin of aerodynamic forces

A
  • pressure and shear stress distribution over the body surface
24
Q

Reference point for resulting aerodynamic force R

A
  • not point of attack
  • quarter chord point
  • point on chord line at 25% of the chord length measured from the leading edge
25
Q

Angle of attack

A
  • the angle between the direction of the relative inflow velocity and the chord line
  • alpha
  • has a major influence on the pressure and shear stress distribution and thus on the lift and drag forces
26
Q

Lift force

A
  • component of the resultant force R perpendicular to the relative inflow velocity urel
27
Q

Drag force

A
  • component of the resultant force R parallel to the relative inflow velocity urel
28
Q

Boundary layer concept

A

devide flow around a body into two areas:

  • undisturbed flow
  • boundary layer
29
Q

Undisturbed flow

A
  • relative far away from the body
  • small to zero velocity gradients
  • fricition is not important
  • thick layer
30
Q

Boundary layer

A
  • near the body
  • large velocity gradients
  • viscous forces predominate
  • friction is important
  • thin layer
  • flow can be laminar or turbulent
31
Q

Favourable pressure gradient

A
  • velocity increasing
  • pressure decreasing
  • unproblematic
32
Q

Unfavourable pressure gradient

A
  • velocity decreasing
  • pressure increasing
  • stall can appear
33
Q

Stall

A
  • boundary layer separates from the body
  • drastic change in pressure distribution
  • large increase of drag (pressure drag)
  • is linked with high loads and a drop in power production
34
Q

Strategy to retard stall

A
  • energizing the boundary layer flow (e.g. golf balls)
  • late stall point
  • WT: vortex generators, strips to glue to the blade, trigger turbulence in boundary layer
35
Q

Stall link to angle of attack

A
  • increase angle of attack (move nose upwards) = suction at top (velocity increase), pressure at bottom surface
  • flow seperates on the suction side fom trailing edge to leading edge

Windenergietechnik (6 decks)

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