## 1 Introduction  
Underwater compressed gas energy storage (UW-CGES) is a novel technology that compresses and stores gases, such as air, natural gas, hydrogen, etc., underwater for energy storage. This technology involves delivering pressurized gas into underwater storage tanks where it is stored. Upon demand, the compressed gas can be conveyed through pipelines to power plants or other equipment for electricity production or other applications [1; 2; 3]. UW-CGES is accomplished through two distinct methods, cavern-style and tank-style, as displayed in Figure 1. Cavern-style involves utilizing natural underwater caverns or mines to store and manage energy. However, this approach requires specific geological conditions, reducing its applicability. Conversely, tank-style employs submerged tanks for storing compressed gas [4; 5; 6; 7]. The process involves compressing the gas, treating the tank for anti-corrosion and insulation, and transporting the compressed gas via pipelines to power plants or other facilities for energy release. Tank-style storage has advantages such as affordable construction, widespread commercialization, and mature technological development. In underwater compressed gas energy storage pipelines, a liquid accumulation may form due to gas supersaturation, temperature changes, water presence, and high gas flow rates [8; 9; 10; 11]. The specific reasons can be elucidated as follows:  
* Supersaturation of the gas inside the pipeline causes liquefaction and the formation of a liquid layer at the bottom. * Temperature changes lead to the cooling of gas inside the pipeline, causing partial liquefaction and accumulation. * Water is transported gas may react with cooled gas at the bottom of the pipeline, causing a liquid layer to form. * High gas flow rates induce unstable gas movement, promoting localized pressure changes and the consequent liquefaction and accumulation of some gas at the pipeline bottom. The occurrence of liquid accumulation in pipelines may generate local blockage, which elevates pressure and friction within the pipeline, ultimately leading to a more profound issue of slug flow [12; 13; 14]. The motion of slug flow in pipelines presents complex regimes that can engender erosion and wall damage, contributing to further corrosion and wear. Additionally, slug flow may give rise to acutely sharp pressure waves within pipelines that lead to violent gas-liquid media oscillations and consequential noise and vibration. Failure to promptly maintain and clean pipelines can lead to grave incidents such as blockages and explosions caused by slug flow [15; 16; 17; 18]. Currently, significant progress has been made in investigating the flow characteristics, motion laws, influencing factors, and other aspects of slug flow through numerical simulation [19; 20; 21], laboratory testing [22; 23; 24], and field observation [25; 26; 27]. Furthermore, some studies are dedicated to developing innovative technologies and devices for preventing and mitigating slug flow, which can enhance the effectiveness and safety of transport pipelines. Laboratory testing is a valuable tool for investigating the flow characteristics and motion laws of slug flow. However, it is essential to concede its potential limitations. For instance, reproducing the complex and dynamic environmental factors in field settings is challenging. Additionally, the experimental apparatus used can impact the fluid, introducing errors in the experimental results. Given that liquid slug flow's formation, evolution, and motion processes are highly intricate, precise experimental parameter design and measurement method adjustment are necessary to obtain accurate and reliable data. [PERSON] et al. [28] performed within a small-scale air-water test circuit, observing three distinct stages: slug formation, gas blowout, and transition. The research revealed a strong correlation between the spatiotemporal dynamics of flexible catenary risers and the severity of slug flow stages, liquid slug length, and liquid accumulation along the riser.