*3.6. Influence of Droplet Impact Velocity Variations*

In this section, the strain-stress analysis intends the consequence of a difference on the droplet impact velocity over the reference case for the testing coupon. Three cases of study are related with variations of the given parameter multiplying its value by 1, 0.6 and 1.4 for Cases 1, 2, and 3, respectively as shown in Table 6. It is important to note that a maximum impact velocity in operational conditions of wind turbine blades (only in offshore fields) should be defined around 170 m/s.

**Table 6.** Modelling input data for variation cases in droplet impact velocity and considering semi-infinite substrates, hs > 2d Cs CL .


Figure 34 clarifies the direct related variation in the strain-rate values with the impact velocity for the three different cases. The corresponding influence on the strain frequency spectrum is depicted in Figures 35 and 36. The principal working strain frequency range is evenly distributed in the periods of time closer to the impact with values from 1 to 7 MHz for Cases 2 and 3, pointing out an important influence of the impact velocity with the frequency range during impact event.

**Figure 34.** Strain-Time evolution for Cases 1–3 at middle coating layer, 50% thickness, considering the substrate-filler as semi-infinite with variation in droplet velocity.

**Figure 35.** Spectrogram for strain evolution at the middle coating layer, 50% thickness, considering substrate-filler as semi-infinite with variation in droplet velocity. Case 2.

**Figure 36.** Spectrogram for strain evolution at the middle coating layer, 50% thickness, considering substrate-filler as semi-infinite with variation in droplet velocity. Case 3.

#### **4. Conclusions**

Numerical and analytical models have been used in this work as a tool to analyze coating LEP wear surface erosion performance. The modelling description offers a guidance in the analysis based on the material fundamental properties. It is required for a complete analysis to define criteria for identifying suitable acoustical matching of LEP coating and composite substrate interfaces.

Complex material models are considered to observe the highly transient material behavior during waterdrop collisions that require to define the range of frequency of its data set to account for strain rate dependence. The simplified single droplet impact modelling developed in this work has been implemented and its capabilities assessed. The simulated analysis pondering different operational and configuration cases used in industry has been discussed in detail and limits the working frequency in a range of 0.5–7 MHz. The analysis has been developed assuming constant values of mechanical properties during the impact event in order to imitate the Springer modelling assumptions. The upper limit of 5 MHz allows one to consider a conservative constant value for the appropriate measurement of the material impedance providing a limit on the stiffness variation of the viscoelastic response of the selected material. Then, a procedure for the measurement of acoustic impedance with a time-of-flight technique of a thin viscoelastic layer using a planar ultrasonic transducer for the frequency regime of interest can be developed. Details of such developments are reported by the authors in linked research [20].

The material impedance characterization may be obtained at the appropriate Ultrasound frequency testing for the erosion performance modelling input data to avoid lack of accuracy. The computational tool presented would allow one to define erosion performance estimators depending on the relative acoustic impedance of liquid, coating and substrate materials definition reducing the Rain Erosion Testing campaigns to evaluate the rain erosion resistance of selected top-coating systems.

**Author Contributions:** Designed and developed the computational tool, L.D. and J.R.; Conceived and implemented the material testing specimens, A.Š.; Determined the research program, defined the computational framework scope, its use and the interpretation of the results, supervised the work and wrote the paper, F.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has been partially funded by the DEMOWIND-2 Project "Offshore Demonstration Blade (ODB)" funded by MINECO with reference PCIN-069-2017, by the ESI-Group Chair at CEU-UCH and from the European Union's Horizon 2020 research and innovation program under grant agreement No 811473. Project "LEP4BLADES".

**Conflicts of Interest:** The authors declare no conflict of interest.
