**8. Conclusions**

The present study scrutinized the impacts of terrain-induced turbulence on wind turbine blades of wind turbine #10 in the Kushikino Reimei Wind Farm (established in November of 2012) in Hashima Ichikikushikino City, Kagoshima Prefecture, Japan, measuring actual data of wind conditions and strains of wind turbine blades. High-resolution numerical simulations were performed on the basis of large-eddy simulation (LES). The research findings were as follows:

On the basis of measurement data obtained by the above auto-measuring system from 3 November 2015, 0:00 a.m. JST to 17 March 2016, 7:00 a.m. JST, the output results of sensors were extracted using wind speed and direction sensors for the control of the wind turbine, which were installed on the wind turbine nacelle. These data results were analyzed to examine the behavior of airflows that were generated surrounding wind turbine #10. Consequently, it was confirmed that the turbulence intensity of the easterly wind with a speed of 10 m/s or lower frequently exceeded the turbulence intensity (Class A) that was defined in the International Electrotechnical Commission (IEC). Furthermore, it was inferred that the influences of Mt. Benzaiten (elevation 519 m) were a cause of the increased turbulence intensity when easterly wind occurred.

During the measurement period of this investigation (3 November 2015, 0:00 a.m. JST–17 March 2016, 7:00 a.m. JST), the maximum value of 0.23 for DEL on the wind turbine blades under easterly wind was identified for the time period 9:40–9:50 a.m. JST of November 13, 2015. It was revealed that if airflow with a property of DEL 2.03 continued for 5.88 years, the total load on wind turbine blades would reach the design load for the designed service life.

The three-dimensional airflow structures, which were considered to be terrain-induced turbulence formed under the easterly wind from Mt. Benzaiten (elevation 519 m), were closely examined using high-resolution numerical simulations on the basis of large-eddy simulation (LES). The present study defined the U-K scale\_1 as the value obtained by calculation where the standard deviation assessed at the wind turbine hub height was normalized by the wind speed obtained at the maximum height above the ground point of the flow boundary layer. Therefore, the existence of the terrain-induced turbulence was quantitatively identified. The threshold value of the U-K scale\_1, the wind condition index, was determined as 0.2, and this index was confirmed to not be dependent on the height direction distributions of inflow wind speed, the influences of horizontal grid resolution, and the influences of the computed azimuth. Under these circumstances, the U-K scale\_1 is a unique indicator that distinguishes it from the IEC turbulence category. Moreover, this scale is based on the premise of comparison with actual measurement data and was found as a research result of about 15 years, including this research result. The U-K scale\_1 is extremely e ffective as a guideline for solving the problem of generation loss and failure caused by topographical turbulence.

A linear trend was recognized in wind turbine operations at 4 m/s or higher between the nacelle wind speed (and its standard deviation) and damage equivalent load (DEL), and it was possible to approximate to the regression line. For an index regarding load, the U-K scale\_2 was defined as the fatigue damage evaluation index, which was obtained by the calculation using two types of regression lines calculated based on actual measurement values and the design value on the basis of Bladed: one regression line was the northerly wind as the low-turbulence flow case, and the other was that of the easterly wind as the high-turbulence flow case. The U-K scale\_2 was equal to the ratio of the integrated value for the measured DEL to the integrated value for the design DEL (Bladed).

Data on both northerly and easterly winds were extracted from measurement data for the year April 2015–March 2016, corresponding to wind turbine operations with a speed of 4 m/s or higher. Using these extracted data and the U-K scale\_2, the influence of terrain-induced turbulence on the age-related degradation of wind turbine blades was evaluated quantitatively. Consequently, it was revealed that the results of northerly wind were within the design value, with the U-K scale\_2 being 0.86 < 1.0. In contrast, for easterly wind, the U-K scale\_2 was 1.60 > 1.0 and exceeded the design value. Furthermore, the integrated value of the fatigue damage was approximately 1.9 times greater in the case of easterly wind compared to northerly wind. Based on this result, it was notably indicated that the blades of wind turbine #10 were directly and strongly a ffected by terrain-induced turbulence when easterly winds occurred.

**Author Contributions:** The study idea, plan and design were conceived by T.U. And, all authors prepared the manuscript.

**Funding:** This work was supported by JSPS KAKENHI Grant Number 17H02053. **Acknowledgments:** We owe our deepest gratitude to Kyudenko New Energy Co., Ltd., whose encouragemen<sup>t</sup> and support in providing the in situ data enabled us to develop our research, and to E-Wind Co., Ltd., whose commitment to data analysis supported us in completing our investigation. This work was supported by both the collaborative research of Kyushu University and West Japan Engineering Consultants, Inc. and the collaborative research of Kyushu University and Hitachi, Ltd. We pay our heartfelt respect and gratitude to all the organizations and individuals for their particular technique and wisdom.

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