## 4 Results and Discussion
### Simulation Results and Comparisons
#### 4.2.3 Gas Velocity Analysis and Comparisons  
The present study investigates the critical velocity of slug flow in a pipe, examining the effects of liquid accumulation volumes and pipe angles. Experimental results are presented in Figure 29, which displays the slugging speeds at different angles when the liquid accumulation volume is 50 mL, 80 mL, 100 mL, 200 mL, and 300 mL, represented by the blue, red, yellow, purple, and green dotted lines, respectively. It is revealed that an increase in the angle of the pipe leads to a higher slugging speed, as the inclined surface generates greater frictional forces that require an increased amount of kinetic energy from the slug. Additionally, a decrease in the critical velocity of slug flow is observed as the volume of liquid accumulation increases. This result can be attributed to the proximity of larger liquid volumes to the top of the pipe, increasing the likelihood of interface fluctuation touching the top of the pipe. Consequently, the accumulation of liquid at the elbow occurs when gas enters the pipe, and the fluctuation amplitude reaches the top of the pipe, triggering slug flow.  
The critical slug velocity of 200 mL of liquid accumulation at different angles using OLGA simulation, as depicted in Figure 30. The blue, red, yellow, and purple lines in the figure represent slugging speeds corresponding to pipe inclination angles of 5\({}^{\circ}\), 10\({}^{\circ}\), 15\({}^{\circ}\), and 20\({}^{\circ}\). The results indicate that the critical slug velocity increases as the pipe angle increases, owing to the increased friction in the inclined pipe, requiring greater kinetic energy from the slug to overcome it. The experimental slugging speeds of 200 mL of accumulated liquid at 5\({}^{\circ}\), 10\({}^{\circ}\), 15\({}^{\circ}\), and 20\({}^{\circ}\) are 3.71 m/s, 4.26 m/s, 5.79 m/s, and 7.65 m/s, respectively. In contrast, the OLGA simulation slugging speeds for the same volume of accumulated liquid at 5\({}^{\circ}\), 10\({}^{\circ}\), 15\({}^{\circ}\), and 20\({}^{\circ}\) are calculated as 3.423 m/s, 4.138 m/s, 6.85 m/s, and 9.404 m/s, respectively. The comparison of experimental and simulation  
Figure 28: The pressure signal at P3.  
Overall, the comparison between the experimental and OLGA simulation results shows good agreement regarding slug flow formation and propagation. The simulation results also provide a more detailed understanding of the fluid dynamics and flow behavior during slug flow formation, which can be useful for optimizing production processes and designing pipelines.  
OLGA's precision in forecasting intricate slug flow dynamics can be compromised, particularly within scenarios characterized by severe slugging or abrupt alterations in flow regimes. In the context of lengthy subaqueous compressed gas energy storage pipelines, this can impede accurate slug flow prediction. Enhancing the OLGA model's efficacy in underwater pipelines necessitates the incorporation of pragmatic instances of extensive gas transportation, thereby refining the model's capacity to anticipate fluid behavior in this unique context.