L4: Wind Turbine Technology I Flashcards
What assumptions are made for 1-D momentum theory? (6)
- Homogenous, incompressible, steady state flow
- Inviscid flow
- Infinite number of blades
- Non-rotating flow
- Pressure discontinuity across rotor disk
- Pressure equal to ambient pressure far from rotor disk
Define the Betz limit
The ideal maximum fraction of the power in the wind that can be extracted by a rotor
In BEM theory, what are a and a’?
a: axial induction factor
a’: tangential induction factor
In BEM theory, what do phi, alpha and beta represent? How are they related?
phi: inflow angle
alpha: angle of attack
beta: twist angle/ twist + pitch angle
phi = alpha + beta
Define the local angle of attack
The angle between the blade element chord and the total incident wind speed
In BEM theory, what is dT?
dT: Infinitesimal thrust acting on the blade element strip (change in axial momentum)
In BEM theory, what is dQ?
Infinitesimal torque acting on the blade element strip (change in angular momentum)
What are u1, u2, u3 and u4 in BEM theory?
u1 = Undisturbed wind speed, before interacting wind the rotor u2 = u3 = Wind speed at the rotor section u4 = Wind speed at a section far enough after the rotor, where the local pressure is equal to the undisturbed pressure
Give the main limits to actuator disc theory
- The flow is not rotating (in reality the flow rotates due to the action of the rotor)
- The rotor has an infinite number of blades (in reality a rotor has a finite number of blades)
- The flow is inviscid (in reality viscous drag forces are not zero)
What directions do lift, drag, thrust and torque act in?
Lift: Perpendicular to total velocity
Drag: Parallel to total velocity
Thrust: Perpendicular to rotor plane
Torque: Parallel to rotor plane
What is the largest source of error in BEM results?
Uncertainties in aerofoil data
What is dynamic stall? (Note - not the process)
A non-linear unsteady aerodynamic phenomenon that occurs when the angle of attack changes suddenly
What temporarily increases the lift in dynamic stall?
Change in angle of attack causes a strong vortex being shed from the leading edge which travels downstream. Its associated low pressure causes lift increase
What causes the stall in dynamic stall?
As soon as the vortex passes the trailing edge the lift reduces dramatically, causing stall
What causes oscillation in angle of attack (associated with dynamic stall)?
Variation of local wind velocity over rotor disc (mainly sheared wind profile and large-scale turbulence)
