## 4 Results
### Analyses of Cavitation Dynamics
#### 4.3.1 Comparisons with Hydrophone Measurements  
Different hydrophone configurations were tested in the cavitation tunnel of SVA Potsdam, depicted in Figure 13 and summarized in Table 3. The hydrophones were installed on the starboard side of the cavitation tunnel window.  
Figure 14 shows the results of the conducted hydrophone measurements. In the figures, we show six different hydrophone arrangements. HF1 and HF2 denote the hydrophones in an acoustic chamber, with HF1 mounted in the wall of the cavitation tunnel at a distance of 0.500 m from the origin of the cavitator (see Figure 13), and HF2 mounted a distance of 0.414 m from the cavitator. HF3 and HF4 are located longitudinally to the flow, and the distances to the cavitator are 0.543 m for HF3 m and 0.352 m for HF4. HF5 and HF6 are located transversally to the flow, and the distances to the cavitator are 0.522 m for HF5 m and 0.420 m for HF6. The sensors HF1, HF3 and HF5 are located upstream of the cavitator, and the sensors HF2, HF4 and HF6 are located downstream of the cavitator.  
Figure 14a shows the case with \(\sigma=0.53\), and Figure 14b shows the case with \(\sigma=1.27\). In both instances, the predicted sheet-cavitation shedding frequencies and their three harmonics are shown as well, cf. Section 4.2. We observe that in the case of \(\sigma=0.53\), the shedding frequency is picked up by hydrophones HF1, HF2, HF3 and HF4. Moreover, the measurements conducted with HF2, HF3 and HF4 show a smaller peak at the second harmonic of the sheet-cavitation shedding frequency. In the case of \(\sigma=1.27\), we see that the CFD predicted cavitation shedding frequency peak is visible only in the HF1 measurements. Other frequencies are also seen in the hydrophone measurements that are of greater magnitude than the ones induced by cavitation dynamics in the CFD simulations.  
Figure 13: Hydrophone arrangement in the starboard side of the cavitation tunnel. Figure adapted from [32], and reproduced with permission from [PERSON] (SVA).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Hydrophone** & **Installation** \\ \hline HF1 & Upstream, acoustic chamber \\ HF2 & Downstream, acoustic chamber \\ HF3 & Upstream, longitudinally to the flow \\ HF4 & Downstream, longitudinally to the flow \\ HF5 & Upstream, transversal to the flow \\ HF6 & Downstream, transversal to the flow \\ \hline \hline \end{tabular}
\end{table}
Table 3: Hydrophone arrangements in the cavitation tunnel.  
For instance, the shedding cavities formed into cloud-like cavitation structures both in the experiments as well as in the simulations, in both the investigated operation points. This should manifest itself as a high-frequency source of noise, the type of which is seen in the hydrophone signals in Figure 14: at frequencies between approximately 100 Hz and 1 kHz for the case with \(\sigma=0.53\) and at frequencies around 1-2 kHz for \(\sigma=1.27\), visible especially in the hydrophones HF1 and HF2. The high-frequency broadband noise is likely due in part to very rapid collapse events of the cavity clouds once they reach a region of sufficiently high pressure in the slipstream of the cavitator. A cavitation collapse event is assessed in Section 4.4. The best arrangement of hydrophones in the tests for the investigations with the standard cavitator turned out to be the one in the acoustic chamber, i.e., hydrophones HF1 and HF2.