## 2 Methods  
The proposed methodology relies on the efficient implementation of aeroelastic WT MBS simulations and the purposeful analysis of the resulting simulated data to reach realistic accelerated test series covering the experimental design space, while profiting from DOE principles. Figure 3 provides an overview of the simulation and data analysis steps leading to the targeted factor combinations that can be applied to the V52 drivetrain during testing. Figure 3 also indicates the orientations of the x, y, and z axes with respect to the WT drivetrain, which will be followed throughout this paper. Further, Table 1 details the factors as well as clarifies the factor IDs that are used in Figure 3 and will be used herein after to refer the respective factor.  
As shown in Figure 3, the proposed method to identify realistic factor combinations, in this case 6-DOF rotor loads and rotational speed combinations, begins with the implementations of simulated wind fields covering a variety of DLCs according to the IEC 61400-1 standard [43]. This step is followed by the application of the simulated wind fields to an MBS model of the WT under investigation, Vestas V52 in this case, in the virtual environment. To clarify the WT models illustrated in Figure 3, Table 2 lists the main specifications of the Vestas V52 WT on which the MBS WT models are based. The drivetrain of the WT model is simplified since the inertia and stiffness of the drivetrain represents its main impact on the rotor loads. This simplification allows for a significant decrease in computational cost and therefore speed, which is needed to cover a large variety of DLCs and provide a more complete picture of the factor combinations that are to be expected during the lifetime of the WT. Section 2.1 details the simulation approach followed in this investigation. The resulting simulated time-series of the factors are then processed to bin each factor into five levels as indicted in Figure 3 for Mz as an example. Section 2.2 provides a description of the simulated data. The five levels are selected based on the range of values of each respective factor while taking into account the control behavior of the specific combination of the test rig and WT drivetrain at hand. The resulting factor level combinations are then tallied and are used for generating the test series for the planned tests. At this stage, highly frequent load combinations can also be covered in higher level resolution by a process outlined in Section 2.3, which describes the data analysis performed in this investigation. Lastly, Section 2.4 provides an overview of the considerations and challenges to be considered for an effective practical implementation of the proposed method.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**ID** & **Subject(s)** & **Measurement** & **Axis of Measurement** \\ \hline Mx & & & x \\ My & Rotor input load & Moment & y \\ Mz & & & z \\ \hline Fx & & & x \\ Fry & Rotor input load & Force & y \\ Fz & & & z \\ \hline n & Rotor & Rotational speed & x \\ \hline \hline \end{tabular}
\end{table}
Table 1: Explanation of experimental factors under investigation.  
Figure 3: Overview of proposed methodology for generation of factor combinations for V52 drivetrain testing.