# Simulative Investigation of the Radar Cross Section of Wind Turbines
## 3 Results
### RCS of the Rotor Blades  
The results of the parameter studies in the following serve to investigate the influence of construction parameters on their radar cross section. The blades were placed in the simulation domain with the leading edge parallel to the z axis, with the same orientation: with the leading edge facing 0\({}^{\circ}\), the top side 90\({}^{\circ}\), the trailing edge 180\({}^{\circ}\), and the profile's lower facing 270\({}^{\circ}\). The blade's tip points towards +z, and the root is placed at the origin. In Figure 2(a), the rotor blades in their default configuration, modelled with PEC material, are presented. As can be seen, common characteristics are hard to make out. There is usually a stronger signal return at the leading edge: upwards between 50-90\({}^{\circ}\) and downwards between 230-260\({}^{\circ}\). Blade v1 features strong peaks at 70\({}^{\circ}\) and 235\({}^{\circ}\), while blade v2 shows those at 80\({}^{\circ}\) and 350\({}^{\circ}\) and blade v3 shows those at 90\({}^{\circ}\) and 245\({}^{\circ}\). Polarization sensitivity is low overall. In order to quantify blade-size influence, blades v2 and v3 were simulated after being scaled to 53 m and 63 m. The results are displayed in Figure 2(b),c. As can be seen, the chance in size leads to a slight increase in RCS of about 3 dB and to small movement of the characteristic peaks. Figure 4 shows the different blade's RCS, with FRP, assumed to be lossless, in different thicknesses, and the PEC simulation (see Figure 2(a)) as reference. The thickness was varied from 5-20 mm in 5-mm steps [44]. Larger values of up to 10 cm are reported in References [45,46], which are considered for the multilayered panels presented below. A common feature is the increase of signal return with thickness increase. For blades v1 (Figure 3(a)) and v3 (Figure 3(c)), the signal return with FRP is only rarely greater than with PEC, although differences between the polarizations are clearly visible. However, blade v2 in Figure 3(b) shows significant differences depending on polarization: the H polarization shows an entirely new backside lobe at 135-200\({}^{\circ}\), which is just as strong as the main front lobe at 350\({}^{\circ}\). Introducing dielectric losses with a loss tangent \(\tan\delta=0.05\) introduces only minimal changes, as shown in Figure 5.  
The panel in Figure 6 shows a parameter study where a layer of foam (\(\varepsilon_{\mathrm{r}}=1.1\)) of varying thickness (5-20 mm in 5-mm steps) was placed below FRP of varying thickness (10-50 mm in 10-mm steps). From row to row downwards, the thickness of the FRP increases by 5 mm. The trend towards higher RCS with thicker FRP is also present here. As above, blade V2 shows significant differences depending on polarization. The RCS of the dual-layer model is only rarely larger than that of the single-layer FRP model, with the differences never larger than 3 dB. Figure 7 shows a parameter study with an FRP sandwich: a layer of foam encased between two layers of FRP, one on the bottom as well and one on the top side. As in Figure 6, the thickness of the FRP increases by 5 mm from row to row downwards. Here, the estimate of a single-layer FRP-only model as a worst-case assumption for the RCS does not hold anymore. This is caused by the fact that layered structures work as resonators, which, depending on their thickness and composition, add interference, either constructive or destructive in nature. Figure 2: Comparison of the radar cross section (RCS) (in dB m\({}^{2}\)) of the nonrotating components of WT v1 and WT v2. The different tower shapes cause significant changes in the RCS, especially in the monostatic case (at 0\({}^{\circ}\)) with more than 10 dB difference. This is also the case for the tower with nacelle and hub, where the flat shape of WT v1 causes very strong peaks into a single direction. (**a**) Multistatic RCS: tower only; (**b**) monostatic RCS: tower with nacelle and rotor hub.