Aerodynamics Flashcards
Definition Aerodynamics
- 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
Classification of types of flow
- density
- friction
- turbulence
- steadiness
- Newton’s definition
- velocity
Classification of types of flow according to density
- compressible = variable density
or incrompressible = constant density (wind turbine technology)
Classification of types of flow according to friction
- viscous flow = high influence of friction or
inviscid flow = no friction involved (wind turbine technology)
Classification of types of flow according to turbulence
- laminar flow = regular
or turbulent flow = random, irregular (Wind flow in PBL)
Classification of types of flow according to steadiness
- steady state flow = timeindependent (WET 1)
or unsteady state flow = timedependent (WET 2)
Classification of types of flow according to Newton’s definition
- Newtonian flow = sher stress proportional to strain rate = air, water (Wind energie technologie)
or non-newtonian flow = viscoelastic material
Classification of types of flow according to velocity
- Subsonic flow Ma < 0,8 (WET)
- Transonic flow
- Supersonic flow
- Hypersonic flow Ma > 5
Mach number:
ratio between current flow velocity and sonic speed
Pressure
normal force per unit area
Density
mass per unit volume
Streamline
- curve tangential to the velocity vectors of particles of a fluid at a given instance of time
- streamlines can not intersect
Streamtube
- bunch of streamlines in 3D
- impermeable
Control volume
- reasonably large, finite region of the flow
- fixed in space
- arbitrary volume
- remains constant (conservation of mass: mass in = mass out)
Shear stress
product of the viscosity coefficient (material parameter) and the velocity gradient
- high coefficient = high influence of friction
- high gradient = high influence of friction
Air foil
- aerodynamic section
- low drag
- rounded shape at front (nose) = leading edge
- ## sharp edge at back = trailing edge
Chord length
length between leading and trailing edge alonge the chord line
Relative thickness
Ratio between maximum thickness and chord length
Camber
distance between mean camber line and chord line
- 0 at leading and trailing edge
- symmetric airfoils do not have any camber
Reynolds number
- 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)
Momentum equation
- 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
Bernoulli equation
- Principle of conservation of energy
- sum of energy stays constant
- for incompressible, inviscid, steady state flows
Origin of aerodynamic forces
- pressure and shear stress distribution over the body surface
Reference point for resulting aerodynamic force R
- not point of attack
- quarter chord point
- point on chord line at 25% of the chord length measured from the leading edge
Angle of attack
- 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
Lift force
- component of the resultant force R perpendicular to the relative inflow velocity urel
Drag force
- component of the resultant force R parallel to the relative inflow velocity urel
Boundary layer concept
devide flow around a body into two areas:
- undisturbed flow
- boundary layer
Undisturbed flow
- relative far away from the body
- small to zero velocity gradients
- fricition is not important
- thick layer
Boundary layer
- near the body
- large velocity gradients
- viscous forces predominate
- friction is important
- thin layer
- flow can be laminar or turbulent
Favourable pressure gradient
- velocity increasing
- pressure decreasing
- unproblematic
Unfavourable pressure gradient
- velocity decreasing
- pressure increasing
- stall can appear
Stall
- 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
Strategy to retard stall
- 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
Stall link to angle of attack
- 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
