CHAM CS - WES - Part 2

Figure 12 shows a colour map of thecharacteristic turbulence intensitydefined by the IEC standard. As theIEC proposes values of 18% and16% for the A and B classes, themap provides clear hints foroptimisation of the wind farm’sconfiguration.It should be noted that not allparameters are verified bymeasurements, because some of theparameters are difficult or evenimpossible to measure.2019181716151413However, it is already apparent thatstate-of-the-art flow simulation is thebest possible way to estimate these.Figure 12:Spatial variation of characteristicturbulence intensity calculated by CFD1210Hyperlink to primary textNote on elevation data processingIt is often asked how digital terrain data can be imported into PHOENICS. Inpractice, terrain data often comes in as height contour lines (e.g. DXF maps orWAsP map files), digitised from 1:25000 or 1:5000 topographical maps. Inorder to use that to construct a BFC grid with horizontal and vertical gridzooming, the contours must first be carefully interpolated to the defined surfacegrid nodes. Application of standard 2D-interpolation can lead to defects liketerrace forming between lines or overshoots. Therefore the "ray angleinterpolation" or one of its refinements may be used. For each grid point anumber of equally distributed "rays", and their intersection with the nearbycontours, are considered. An interpolation weight is assigned to each ray andits crossing contour, depending on the local angle between ray and contour,thus assigning higher weights to rays crossing a contour at 90° while assigninglower weights for sharper angles. The 3D BFC grid may be constructed on thisbasis over the surface and exported in ASCII grid format to PHOENICS, where itmay be smoothed using the Laplace grid smoothing tool. In order to promotenumerical properties, the initial surface may be slightly smoothed only in thevicinity of the outer grid boundaries. The level of detail of the initial heightmodel should fit to your horizontal grid spacing. In any case the resultingsurface grid needs to be inspected carefully.