## 4 Experimental Setup
To measure the out-of-plane surface velocity of the WTS test specimen, the LSV scan head is positioned horizontally to the test specimen. A linear shaped fine grid consisting of 171 scan points is defined on the top of the stator arm, see Figure 5. The choice of number of grid points is based on the measurement accuracy and on the time duration of measurement. The number of scan points and the measurement area is kept unchanged for each set of experiment, see Figure 5. Furthermore, to find a correlation between several parameters to carry out all the experiments a parameter study on the reference WTS test specimen has been performed and their values can be seen in Table 3. Additionally, to prevent spectral leakage in the measured signals, the Hann-window function is adopted for the entire experiment. The experiment is first carried out on the reference specimen, i.e., WTS test specimen without the HCDP (see Figure 6a) to examine the reproducibility of the experimental results. It has been found that reassembling the stator arm of the reference specimen twice while keeping the stator ring fixed, shows a good agreement between the system response, see Figure 10 in Appendix A. Furthermore, to control the stability of the reproducibility of the experimental setup, an empty HCDP is mounted between the stator ring and stator arm. To assemble the HCDP, four identical steel blocks along with bolts, nuts, and the washer has been used. The obtained results confirm the robustness of the experimental setup in term of reproducing the system response after reassembling process except for a marginal shift in frequency around 350 Hz, see Figure 10 in Appendix A. The same amount of frequency shift around 350 Hz has been also observed for the reference test specimen, see Figure 10 in Appendix A. However, this higher frequency shift will not affect the present study, as the peak values are the major parameter for studying the effect of particle damper on vibration attenuation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & Complex average & cut-off frequency & Sampling frequency & Frequency resolution \\ \hline
**Value** & 6 & 1000 Hz & 12.8 kHz & 781.25 mHz \\ \hline \hline \end{tabular}
\end{table} 
Table 3: List of parameters for data acquisition. Figure 6 shows all the possible configuration of HCDP on the WTS test specimen that has been investigated in this contribution. The partially filled HCDPs and empty HCDP are investigated at each possible position on the test specimen. The WTS test specimen without HCDP, i.e., reference test specimen is given in Figure 6a. Firstly, the single-unit (SU) HCDP is mounted between the stator ring and stator arm, and its effect on test specimen vibration attenuation is studied, see Figure 6b. The best performing partially filled HCDP at this location is combined with its corresponding HCDP with different FR to investigate the multi-unit (MU) HCDP influence on transmission path damping, see Figure 6c. Afterwards, the SU HCDPs are attached to the outer wall of the stator ring, which is opposite to the excitation point, see Figure 6d. The material performance ranking obtained through this test is used for constructing a MU HCDP set-up at the same location, see Figure 6e. Similarly, MU HCDP is assembled to the inner wall of the stator ring to investigate their effect on the WTS test specimen vibration attenuation, see Figure 6f. Likewise, SU HCDP is  
Figure 4: Experimental setup of the wind turbine stator test specimen for evaluating the transmission path damping: (**a**) Test specimen, (**b**) Shaker power amplifier, (**c**) Force sensor amplifier, (**d**) Control and post-processing unit, (**e**) Laser vibrometer scan head. Figure 5: Linear shape grid consisting of 171 scan points. attached to the inner wall of the stator ring for vibration attenuation analysis, see Figure 6g.