Free Vortex Wake Settings

Wake Modelling

  • Wake Integration Type: This sets the velocity integration method for the wake nodes during the free wake convection step. EF: A simple 1st Order Euler Forward integration. PC: A 2nd Order Predictor Corrector integration method. PC2B: A second Order Predictor Corrector Backwards integration scheme.

  • Wake Rollup: This actives or deactivates the wake self-induction.

  • Include Trailing/Shed Vortices: This sets if trailing (streamwise) or shed (spanwise) vortices are generated at the blades trailing edge during every timepstep.

  • Wake Convection: The user can choose here which free-stream velocity contributes to the total convection velocitzy of the wake nodes. BL: The convection velocity is the mean boundary layer velocity (as a function of height). HH: The convection velocity is the constant hub-height velocity. LOC: The convection velocity is evaluated locally at each wake node position.

  • Wake Relaxation Factor: This factor can be used to relax the wake by blending out the starting vortex. The factor controls how long the wake is allowed to be after a given number of rotor revolutions or timesteps (depending on the Count Wake Length In setting). Such as a value of 0.5 allows for a wake length of 5 revolutions after the rotor has undergone 10 revolutions. A factor of 1 deactivaes the blending.

  • First Wake Row Length Factor: This factor can be used to assign a shortened length to the newly created wake elements at the trailing edge so that the newly created shed vorticity is in closer proximity to the blade. A factor of 1 deactivates the shortening.

  • Max Num. Elements / Norm. Distance: These two values are used to cut-off the wake after a fixed numbner of vortex elements has been created (Max. Num. Wake Elements) or after a vortex element has reached a distance (normalized by rotor diameter) from the hub that is larger than Norm. Distance.

  • Wake Reduction Factor: This factor filters out wake elements that have a circulation smaller than the maximum circulation in the wake multiplied by this factor. In most cases this effectively removes shed vorticity that does not significantly affect the wake induction (see Fig. 67).

Visualization of the wake reduction approach.

Fig. 67 Visualization of the wake reduction approach.

  • Count Wake Length In: This setting controls how the age of a vortex element is counted. Either as a number of rotor revolutions, or as a number of timesteps that have passed since the elemnt was created.

  • Particle Conversion after [Revolutions/Timesteps]*: (Only QBlade-EE) This setting controls when a vortex filament is converted into a vortex particle. If the vortex elemnt has reached an age (in timesteps or revolutions) equal to this value it is converted into a particle.

  • Wake Zones N/1/2/3 in [Revolutions/Timesteps]: This setting controls the length of the different wake zones. The length is either counted in rotor revolutions or in timesteps, depending on the setting (Count Wake Length In). Each wake zone has a successively coarser discretization (depending on the Wake Zones Factor settings) to reduce the total number of free wake elements and thereby to speed up the simulation.

  • Wake Zones 1/2/3 factor: These (integer) factors control by how much the wake is coarsenend in between the different wake zones. A factor of 4 means that when transitioning from one zone to the next 4 wake elements are replaced by a single wake element to coarsen the wake resolution (see :numref:fig-wakezones`.png`).

Visualization of the wake zoning approach.

Fig. 68 Visualization of the wake zoning approach.

Vortex Modelling

  • Fixed Bound Core Radius (% Chord): This sets the fixed core radius of the bound blade vortices. Defined as a fraction of the local blade chord.

  • Initial Wake Core Radius (% Chord): This sets the intial core radius of the free vortices that are created at the blades trailing edge. Defined as a fraction of the local blade chord.

  • Turbulent Vortex Viscosity: This value is used in the vortex core growth model, see Vortex Core Desingularization.

  • Include Vortex Stretching: This option activates vortex stretching, see Vortex Core Desingularization.

  • Maximum Vortex Stretching Factor: After the cummulative vortex strain rate has reached a value larger than this factor it is automatically removed from the wake.

Turbine Gamma Iteration Parameters

  • Relaxation Factor: This relaxation factor is used when the blade circulation is updated during the circulation iteration.

  • Max. Epsilon for Convergence: The convergence criteria for the blade circulation.

  • Max. Number of Iterations: The maximum number of blade circulation iterations that will be carried out.