## 5 Conclusions  
In this paper, three-dimensional vortical flow phenomena in the wind turbine were investigated by the unsteady Reynolds averaged Navier-Stokes (RANS) flow simulation with SST turbulence model. Based on the investigation, the studies for enhancing aerodynamic performance were carried out including the optimization of the innovative blade tip design and the optimal layout of the nacelle anemometer. The results are summarized as follows:  
1. The three-dimensional vortical flow field in the wind turbine was dominated by the tip vortex and hub separation vortices. The tip vortex starts to be formed near the rotor tip leading edge. The hub separation vortices were generated near the rotor hub leading edge and develop nearly in the spanwise direction.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Nacelle Anemometer Positions** & **Flow Angle [deg]** & **Turbulence Intensity** \\ \hline Point 1 & \(-\)10.90 & 0.0096 \\ \hline Point 2 & \(-\)11.10 & 0.0041 \\ \hline Point 3 & \(-\)11.15 & 0.0035 \\ \hline Point 4 & \(-\)11.78 & 0.0092 \\ \hline Point 5 & \(-\)11.98 & 0.0037 \\ \hline Point 6 & \(-\)11.67 & 0.0030 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Effect of anemometer positions on FA and \(T_{i}\).  
Figure 16: Flow angle time history on each nacelle anemometer position with varying azimuth angle.  
2. The winglet and ogee rotor tip shape, which is remarkably sensitive to the aerodynamic characteristics, were optimized to achieve higher aerodynamic performance while lowering aerodynamic thrust force and tip vorticity.
3. The interaction between the rotating blade and the stationary tower brings about the high pressure fluctuation on the tower surface with 3P frequency, which makes the aerodynamic performance and thrust force remarkably fluctuate with 3P frequency. The vortices around the rotor hub are also shed in synchronization with 3P frequency. The vortex shedding is not the prominent effect on the aerodynamic performance and thrust force in the wind turbine.
4. The unsteady nature of the vortex shedding from the rotor hub led to the generation of the complicated and vortical flow field around the nacelle anemometer and wind vane. The flow angle (FA) time history is dominated by the fluctuation with the 3P frequency. As the distance between the rotor hub and the nacelle anemometer becomes closer, the higher the FA fluctuation and turbulence intensity occur. Regardless of the anemometer position, the time averaged FA value is \(11.43^{\circ}\) in average.  
[PERSON]-[PERSON] compiled the literature review, conceptualized the ideas, performed the numerical simulation, and wrote the bulk of the paper; [PERSON] organized the concept idea and paper structure, compiled the literature review, contributed in structuring and review of the paper, and partially wrote the paper. Both authors have read and agreed to the published version of the manuscript. This study has been supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2020R1G1A1099560). Additionally, this work was supported by Brain Pool Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (grant number: 2019H1D3A2A02102093). I acknowledge the administrative and technical support of Floating Offshore Wind Energy System Engineering Department of University of Ulsan. The authors declare no conflict of interest.