*4.2. DU06-W-200 Airfoil*

This airfoil was specifically designed for vertical axis wind turbine applications, with the objective of improving the self-starting abilities of this type of turbine. The available aerodynamic data of the DU06-W-200 airfoil in the literature, used to contrast the obtained results in the present database, are found in [24]. This source provides experimental work performed in the Low Turbulence Tunnel (LTT) at the Technical University of Delft. This wind tunnel has a 1.25 × 1.8 m test section and can achieve a maximum wind speed of 120 m/s. The tested model was made of solid aluminum with a 1.8 m span (the whole section width) and a chord of 0.25 m, and the aerodynamic coefficients were obtained with a six-component external balance. Two types of results are presented: "clean", corresponding to the bare airfoil in the wind tunnel with around 0.02% turbulence, and "dirty", which is the same testing conditions, but the airfoil features a zig-zag tape at 5% of the chord, simulating a much more turbulent test environment. From the available results, those corresponding to a Reynolds number of 300,000 have been chosen as a reference, as they are the closest to our experimental dataset.

### **5. Results**

In this section, the results of the different tests are presented. Particularly, the results have been divided depending on the type of prototype used in the test: flat plate or DU06-W-200 airfoil.

## *5.1. Flat Plate*

The aerodynamic coefficients (*CD*, *CL*) obtained with the flat plates are shown in Figure 5, compared with the data from the bibliography. Experimental curves are plotted with red discontinuous lines, using triangles for the 2D flat plate and squares for the 3D flat plate. On the other hand, 2D and 3D flat plate data from the bibliography are plotted with green and blue dotted lines, respectively.

**Figure 5.** Experimental drag and lift coefficients from 2D and 3D flat plates compared with data from the bibliography.

The drag coefficient (top plot) exhibits a remarkable agreement for both flat plates in the whole angular range, with a very slight drift at high pitching angles. Complementarily, the lift coefficients (bottom plot) also show a good overall agreement, although with some overestimation for angles smaller than 20 degrees. This can be attributed to the lack of a complete symmetry between both pressure and suction sides of the plates. In fact, only the pressure sides are completely flat, because of a slight engrossment of the suction side at the mid-chord to accommodate a sufficiently robust shaft. Thus, at low pitching angles, when the flow is still attached, this geometrical defect raises the pressure difference between both sides leading to an increase in the lift force. However, at higher pitching angles, the flow in the suction side is completely detached and the experimental data matches the reference

data with especially accurate results for the 3D flat plate. Despite the experimental lift coefficients for the 2D flat plate being slightly above the bibliography, the global trend is perfectly reproduced. The overall result suggests an accurate and precise performance of the balance, thus postulating it as a good candidate for airfoil testing.
