Wind flow modelling

The speed and direction of the wind varies as it moves across a wind farm site due to changes in the terrain height, roughness and the presence of obstacles. The process of calculating these changes is known as ‘wind flow modelling’. There is a multitude of flow models that range from very simple models that are based on terrain height only to sophisticated CFD models. WindFarmer can use results from any wind flow model as long as they are available in WAsP RSF or WRG file format. WindFarmer also includes a simple flow model that combines customisable vertical shear models and a simple horizontal model that is based on the terrain height. The WAsP flow model can be run from inside WindFarmer if there is a WAsP licence available on the computer.

Flow model initiation

One frequency distribution at a measurement site is used as input for each WAsP from frequency distribution or simple flow model prediction. When there are frequency distributions at multiple heights, the frequency distribution closest to the target height is selected automatically. Within one flow calculation there will be many flow model predictions, 1 for each target height, measurement site with appropriate input data and target area.

Simple Flow Model for Feasibility Studies

The ‘Simple’ wind flow model has been included to enable wind flow model calculations to be performed quickly and cheaply, without the need for external software. As the name suggests, it uses a simple algorithm which can produce results with a level of accuracy appropriate for use in early feasibility studies. It is also useful for offshore or flat sites where it is an acceptable approximation to have no ambient wind speed variation across the site.

The wind speed is assumed to change linearly with terrain altitude relative to a given point, as suggested in [29] and [30]. When the terrain slopes are less than 3 degrees, the relationship between wind speed-up factor and the terrain height is

$$s = 1 + 0.001*\mathrm{\Delta}z$$

where ∆z is difference in terrain altitude in m. The topographic sensitivity is expressed in a speed-up change per m, here 0.001.

The following inputs are required to estimate the flow with the Simple Flow Model:

  • Topographic information: digital maps in a raster format (gridded elevations).

  • Wind data: a frequency distribution at a mast.

  • Definition of the area to be considered

Shear model

The Simple Flow Model will calculate the change in wind speed between the height above ground level at which the wind was measured, and the hub height. This is performed using the wind shear model.

The following shear models can be used:

Log law – the surface roughness length z0 is the input parameter. The speed-up factor between mast height and hub height is calculated as follows,

$$s = \frac{\ln\left( \frac{z_{\textrm{hub}}}{z_{0}} \right)}{\ln\left( \frac{z_{\textrm{mast}}}{z_{0}} \right)}$$

Power law – the power exponent, alpha, is the input parameter. The speed-up factor between mast height and hub height is calculated as follows,

$$s = \left( \frac{z_{\textrm{hub}}}{z_{\textrm{mast}}} \right)^{\alpha}$$

Displacement height at hub height and mast heights are considered automatically.

WAsP

The following inputs are required to estimate the flow with the WAsP flow model:

  • Topographic information: elevation and roughness maps in contour format.

  • Wind data: a frequency distribution at a mast.

  • Definition of the area to be considered

The required geographic location of the mast (in latitude/longitude) is based on the mast metric coordinates and the workbook projection.

The standard heights used in the WAsP simulations are automatically set to the required height of the mast and the turbine hub heights, considering displacement heights. The WAsP standard roughness lengths are used.

Possible differences from other WAsP calculations:

  • Wind resource data that are created with WAsP in WindFarmer do not use the WAsP WRG/RSF file rounding so there are small differences in the wind speed and direction distributions when comparing results with pre-calculated WRG/RSF files from WAsP and those created with WindFarmer.
  • Mean wind speeds at turbines are calculated differently when the association method is on. For each flow case (typically 12 x 50) in the frequency distribution that initiates the turbine, the bin center wind speed (0 m/s on the first bin) is multiplied by the speedup and normalized probability. The mean wind speed is the sum of all flow cases.

Wind speed adjustments (all flow models)

Wind speed adjustments are a applied as a speedup factor correction to the speedup between an initiation mast and a target location.

Flow calibration

A wind flow calibration factor can be entered in the Flow Model, Manual Adjustments task. The mean wind speed of all wind turbines and wind resources that have been calculated with the flow model are then multiplied with the calibration factor. Wind speed maps are also scaled by the calibration factor.