## 2 Experiments  
### Underwater Assisted Gas Flow Visualization  
To observe and analyze the assist gas flow, a dummy specimen made of transparent acrylic was prepared. A 100 mm-thick acrylic plate was placed underwater and formed a gap of 1 mm and 2 mm; the nozzle was placed on top of the acrylic plate. Experimental conditions are shown in Table 1. The assist gas flow pattern, depending on the gas pressure (P\({}_{\text{g}}\)), the gap between acrylic plates (G\({}_{\text{p}}\)), SOD (D\({}_{\text{n}}\)), and nozzle types, was captured by the CCD camera as shown in Figures 2 and 3. It was assumed that, when the penetration depth was deep, the assist gas flow rate between the cut surfaces was high, and the molten metal could be easily removed in the actual underwater laser-cutting process.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test No.**} & \multirow{2}{*}{**Nozzle Type**} & \begin{tabular}{c} **Gap between the** \\ **Plates (G\({}_{\text{p}}\), mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Stand-Off Distance** \\ **(D\({}_{\text{n}}\), mm)** \\ \end{tabular} &
\begin{tabular}{c} **Assist Gas** \\ **Pressure (P\({}_{\text{g}}\), bar)** \\ \end{tabular} \\ \hline
1-1 & Conical & 1 & 1 & \\
1-2 & Conical & 2 & 1 & \\
2-1 & Supersonic & 1 & 1 & \\
2-2 & Supersonic & 2 & 1 & 4, 6, 8, 10 \\
3-1 & Conical–flat & 2 & 0 & \\
3-2 & Conical–flat & 2 & 1 & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Underwater assist gas flow visualization test parameters.  
Figure 1: Nozzle types and their cross-sectional views: (**a**) conical, (**b**) conical–flat, and (**c**) supersonic.  
Assist gas penetrated deeply in the experiments of test No. 1-1 and No. 1-2, as well as in the experiments of test No. 2-1 and No. 2-2, in the case of \(\rm G_{p}=2\) mm. The penetration depth of the assist gas when \(\rm P_{g}=10\) bar and \(\rm G_{p}=1\) mm was almost the same as that of \(\rm P_{g}=6\) bar and \(\rm G_{p}=2\) mm. When comparing the experiments of tests No. 1-1 and No. 2-1, the penetration depth of the assist gas was less than expected when using a supersonic nozzle versus a conical nozzle. From the results of tests No. 1-2, No. 2-2, and No. 3-2, in the case of \(\rm G_{p}=2\) mm, there was no significant difference in the penetration depth of assist gas depending on the nozzle types. From the experimental results of tests No. 3-1 and No. 3-2, when the \(\rm D_{n}\) was set to \(0\) mm, the assist gas penetrated slightly deeper than \(\rm D_{n}=1\) mm, but its effect was insignificant compared to effect of the differences in the gap between plates. Therefore, it can be assumed that increasing the kerf width in underwater laser cutting is the most dominant parameter for assist gas flow and aerosols generation reduction.  
Figure 3: Underwater assist gas flow visualization depending on the nozzle types, the gap between the plates (\(\rm G_{p}\)), and the SOD (\(\rm D_{n}\)). (**a**) Test No. 1-1: conical; \(\rm G_{p}=1\) mm; \(\rm D_{n}=1\) mm. (**b**) Test No. 1-2: conical; \(\rm G_{p}=2\) mm; \(\rm D_{n}=1\) mm. (**c**) Test No. 2-1: supersonic; \(\rm G_{p}=1\) mm; \(\rm D_{n}=1\) mm. (**d**) Test No. 2-2: supersonic; \(\rm G_{p}=2\) mm; \(\rm D_{n}=1\) mm. (**e**) Test No. 3-1: conical–flat; \(\rm G_{p}=2\) mm; \(\rm D_{n}=1\) mm. (**f**) Test No. 3-2: conical–flat; \(\rm G_{p}=2\) mm; \(\rm D_{n}=0\) mm.  
Figure 2: Underwater assist gas flow visualization experiment setup.