## 3 Results
### IWT Tests
#### 3.2.3. IWT Test Results
Rather, the Lei for the uncoated surface seems to be slightly shorter than that measured on the coated surface. Here, the values of Lei measured for the coated surface in Section 8 and uncoated surface in Section 11 are reported as isolated dots and are not included in the trend due to a substantial deviating rate. This deviation could be due to some imperfections in the nacelle surface, highlighted in these tests more than in the others. Finally, by comparing the rivulets' lengths in Figure 10c,f, it is notable that these are much longer and that the rivulets' distribution is denser on the uncoated surface than on the coated one. Figure 9: Results of test 1 performed on the uncoated test article (**a–d**) and of test 2 carried out on the coated test article (**e–h**). Figure 11: Impingement length measured after tests 3 and 4. Figure 10: Results of test 3 (**a–c**) on uncoated surface of test article and test 4 (**d–f**) on coated surface of test article. Test 5-6: T = \(-\)12 \({}^{\circ}\)C; v = 70 m/s; H = 3000 m; MVD = 20 \(\mu\)m; LWC = 0.3 g/m\({}^{3}\); \(t\) = 140 s--T Effect (Baseline for Tests 7 and 8)  
The effect of the decreased temperature was assessed with tests performed at \(-\)12 \({}^{\circ}\)C. Pictures of tests performed on uncoated and coated surfaces are displayed in Figure 12a-j, respectively. As for baseline tests 1 and 2, sparse ice droplets having spherical shapes appear on both coated and uncoated surfaces, displaying a typical dry rime ice accretion [27]. However, the coated surface has more isolated ice droplets, which result in a less dense transition area, and the ice droplets have larger dimensions than those accreted on the uncoated surface. Average thicknesses of accreted ice on uncoated and coated surfaces were 1.51 mm and 0.77 mm, respectively. No significant difference can be observed in the impingement lengths (see Figure 13). Figure 12: Results of test 5 (**a**–**e**), on uncoated test article and test 6 (**f**–j) on coated test article. Test 7-8: T = \(-\)12 \({}^{\circ}\)C; v = 70 m/s; H = 3000 m; MVD = 20 \(\mu\)m; LWC = 0.3 g/m\({}^{3}\); \(t\) = 420 s--Exposure Time and Temperature Effect  
Starting from tests 5 and 6 performed at \(-\)12 \({}^{\circ}\)C, the exposure time was increased to 420 s. Pictures of tests 7 and 8, performed on uncoated and coated surfaces, were shown in Figure 14a-j, respectively. By comparing pictures in Figure 12 at a shorter exposure time with those of Figure 14, it can be observed that the ice accumulated on both uncoated and coated surfaces increases as the exposure time increases, as expected. The accreted ice assumes two different shapes: it is milky with crystalline structures on the upstream area, typical of prime ice, whereas it appears to be less dense with columnar structures at the stagnation line, typical of glaze ice [27; 29]. Quite no differences can be observed in the shape and appearance of the ice grown on coated and uncoated surfaces. However, it was found that the impingement length of ice accreted on the coated surface is on average 3 mm shorter than that measured on the uncoated surface (Figure 15). Finally, measured thickness was 3.46 mm and 2.97 mm for uncoated and coated surfaces, respectively. Test 12-13-14: T = \(-\)5 \({}^{\circ}\)C; v = 95 m/s; H = 3000 m; MVD = 20 \(\mu\)m; LWC = 0.3 g/m\({}^{3}\); \(t\) = 140 s--V Effect  
Velocity was increased to 95 m/s in tests 9 and 10; pictures of results are displayed in Figure 16a-d for uncoated and Figure 16e-h for coated test article. A test of repeatability was performed on the coated surface (test 11), and results are shown in Figure 17. Compared with the baseline tests (Figure 9), it was found that as the incoming flow velocity increased, the water droplets on the uncoated surface would move more rapidly downstream and then freeze as thin and narrow runback rivulets due to the increased aerodynamic stress acting on the rivulet flows, as observed also by [PERSON] et al. [27]. A striking contrast is the dynamic of icing on the coated surface.