Induction-Model
The latest version of Openwind contains three different induction models show in figure 1 below.
Figure 1: Induction Model Settings
Forsting is the induction model by A.R.M Forsting, of which there is more discussion at the end of this section.
Rankine Half-Body (Gribben) is the model published by Gribben and available here. The implementation uses the reflected turbine images referenced in Gribben’s paper to strengthen the induction effect.
Vortex Cylinder (Branlard) is the model published by Branlard and referenced by Nygaard here
Consider Acceleration Effects - this option models speed-up zones on the shoulder of each turbine in addition to the slow-down in front of each turbine. See the Gribben paper for some nice illustrations of this.
Induction Deficit Combination Scheme - this is the way in which the induction deficit from one turbine is combined with all the others. These options are described in detail in the section on the N.O.Jensen wake model.
Induction Deficit + Wake Deficit Combination Scheme - this is the way in which the total induction deficit is combined with the total wake deficit at each point. Again the options are described in detail in the section on the N.O.Jensen wake model.
Remove turbine self induction effect up to a distance of X RD - this calculates the individual turbine induction effect at X rotor diameters upstream and subtracts this amount of deficit from the induction deficit experienced by that same turbine. This is in order to remove the upwind induction that is assumed and implicitly included in the turbine manufacturer’s power curve. To disable this, set X to something large like 10.
Individual turbine induction effects of multiple turbines can be combined using a variety of simple mechanisms in Openwind including:
RSS (root sum squared) – the overall induction deficit is the square root of the sum of the squares of the induction deficits from all the turbines in the workbook
Geometric – the overall induction is the product of the chaining of the induction efficiency from each individual turbine
Linear – the overall induction is the sum of the induction deficit over all turbines
Mixed – in which the overall induction is a weighted mix of RSS and Linear described above.
Openwind computes an induction effect by comparing the wind speed 2.5D (default) in front of a turbine, to the free speed without induction or wakes.
The induction effect can be combined with wake effects in a similar manner to how the induction effects are combined with each other. The current consensus is that the single turbine induction wind speed slow down should be the maximum induction effect across all turbines and should act as a global offset to current wake models. For this reason, the current default combination method for induction with wakes is linear, although geometric gives similar results.