Aerodynamics

Question 1

Definition Aerodynamics

Answer 1

• 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

Question 2

Classification of types of flow

Answer 2

• density
• friction
• turbulence
• steadiness
• Newton’s definition
• velocity

Question 3

Classification of types of flow according to density

Answer 3

• compressible = variable density

or incrompressible = constant density (wind turbine technology)

Question 4

Classification of types of flow according to friction

Answer 4

• viscous flow = high influence of friction or

inviscid flow = no friction involved (wind turbine technology)

Question 5

Classification of types of flow according to turbulence

Answer 5

• laminar flow = regular

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

Question 6

Classification of types of flow according to steadiness

Answer 6

• steady state flow = timeindependent (WET 1)

or unsteady state flow = timedependent (WET 2)

Question 7

Classification of types of flow according to Newton’s definition

Answer 7

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

Question 8

Classification of types of flow according to velocity

Answer 8

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

Question 9

Mach number:

Answer 9

ratio between current flow velocity and sonic speed

Question 10

Pressure

Answer 10

normal force per unit area

Question 11

Density

Answer 11

mass per unit volume

Question 12

Streamline

Answer 12

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

Question 13

Streamtube

Answer 13

• bunch of streamlines in 3D

- impermeable

Question 14

Control volume

Answer 14

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

Question 15

Shear stress

Answer 15

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

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

Question 16

Air foil

Answer 16

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

Question 17

Chord length

Answer 17

length between leading and trailing edge alonge the chord line

Question 18

Relative thickness

Answer 18

Ratio between maximum thickness and chord length

Question 19

Camber

Answer 19

distance between mean camber line and chord line

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

Question 20

Reynolds number

Answer 20

• 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)

Question 21

Momentum equation

Answer 21

• 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

Question 22

Bernoulli equation

Answer 22

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

Question 23

Origin of aerodynamic forces

Answer 23

• pressure and shear stress distribution over the body surface

Question 24

Reference point for resulting aerodynamic force R

Answer 24

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

Question 25

Angle of attack

Answer 25

- 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

Question 26

Lift force

Answer 26

- component of the resultant force R perpendicular to the relative inflow velocity urel

Question 27

Drag force

Answer 27

- component of the resultant force R parallel to the relative inflow velocity urel

Question 28

Boundary layer concept

Answer 28

devide flow around a body into two areas: - undisturbed flow - boundary layer

Question 29

Undisturbed flow

Answer 29

- relative far away from the body - small to zero velocity gradients - fricition is not important - thick layer

Question 30

Boundary layer

Answer 30

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

Question 31

Favourable pressure gradient

Answer 31

- velocity increasing - pressure decreasing - unproblematic

Question 32

Unfavourable pressure gradient

Answer 32

- velocity decreasing - pressure increasing - stall can appear

Question 33

Stall

Answer 33

- 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

Question 34

Strategy to retard stall

Answer 34

- 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

Question 35

Stall link to angle of attack

Answer 35

- 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