## 5 Results and Discussions
Furthermore, this observation also leads to a conclusion that attaching the HCDP on the outside of the WTS wall is not only effective for broadband damping but also favorable for industrial implementation after proper optimization. To evaluate the effect of particle damper the SU HCDP is fixed on the outer wall of the WTS ring, which is opposite to the excitation point, see Figure 6d. The experimental results of this configuration are plotted in Figures 10-11. From the narrow band spectrum, it can be noted that the reference test specimen and test specimen with attached SU HCDP have several resonances peaks, see Figure 11. Near the first resonance peak (47.7 Hz), all SU HCDP can reduce the vibration amplitude significantly. It is observed that for the first natural frequency, the SU HCDP filled with stone powder and sand with a filling ratio of FR1 are most effective for vibration attenuation and reduce the vibration amplitude by 7.1 dB and 6.5 dB, respectively, see Figure 11. Furthermore, it has been observed that attaching SU HCDP to the outer wall of the WTS ring also reduce the vibration amplitude near the third and fourth resonance peaks. However, for the resonance near 150 Hz, the vibration amplitude of the WTS test specimen with attached SU HCDP is higher than the reference values of WTS test specimen. Additionally, a slight frequency shift has been also observed that is coming because of HCDP extra mass. Moreover, attaching the SU HCDP at this location is increasing the vibration amplitude for the resonance peak around 100 Hz. Nevertheless, the overall damping characteristic of stone powder and sand is very similar, which can also be observed in their SL value of surface velocity. An empty HCDP is also attached to the WTS ring to show that the vibration reduction is mainly coming due to the particle damping mechanism and not only because of mass, see Figures 11-12. The influence of empty HCDP for each SU HCDP configuration at a different location on the WTS test specimen is investigated because its response on FRF is dependent on mounting position. From the above discussion, it can be concluded that the presented configuration shows a significant reduction in vibration amplitude for the resonance peaks near 50 Hz, 150 Hz, and 355 Hz. It has been also observed that stone powder and sand show the maximum vibration reduction near first resonance peaks in comparison to other granular materials. Conclusively, attaching the SU HCDP to the outer wall of the generator ring is not effective for broadband damping, which can also be observed in the SL values of SU HCDP in comparison to the reference specimen SL value. Figure 8: One-third octave representation of FRF of WTS test specimen. MU HCDP mounted to the outer wall of the stator ring, opposite to the excitation point. SL value of surface velocity is given in dB. Figure 8.0 Narrow band spectrum representation of FRF of WTS test specimen. MU HCDP mounted to the outer wall of the stator ring, opposite to the excitation point. SL value of surface velocity is given in dB. Figure 8.1 Difference in one-third octave band: FRF of MU HCDP mounted to the outer wall of the stator ring, opposite to the excitation point. Sum level of surface velocity is given in dB. The experimental results for the HCDP position between stator arm and stator ring are plotted in Figures 14-15. Resonance frequencies of the WTS test specimen can be seen in Figure 8.2 Placing the SU HCDP between the WTS arm and ring can reduce the vibration amplitude significantly near 150 Hz. A very similar effect has been also observed for the resonance near 355 Hz, see Figure 8.3 However, for the first resonance peak near 50 Hz, only SU HCDP filled with tungsten powder (green dotted line) and MU HCDP filled with sand (violet dashed line) are effective for vibration attenuation and reduce the vibration amplitude up to 3.5 dB and 1.49 dB, respectively, see Figure 8.4 MU HCDP set up in this configuration is not very useful in comparison to SU HCDP setup.