Underwater compressed air energy storage was developed from its terrestrial counterpart. It has also evolved to underwater compressed natural gas and hydrogen energy storage in recent years. UWCGES is a promising energy storage technology for the marine environment and subsequently of recent significant interest attention. However, it is still immature. In this study, the latest progress in both academic and industrial fields is summarized. Additionally, challenges facing this emerging technology are analyzed. The pros and cons of UWCGES are provided and are differentiated from the terrestrial variant. Technical, economic, environmental, and policy challenges are examined. In particular, the critical issues for developing artificial large and ultra-large underwater gas storage accumulators and effective underwater gas transportation are comprehensively analyzed. Finally, the demand for marine energy storage technology is briefly summarized, and the potential application scenarios and application modes of underwater compressed gas energy storage technology are prospected. This study aims to highlight the current state of the UWCGES sector and provide some guidance and reference for theoretical research and industrial development.

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An experiment study on the cross flow-induced vibration of a flexible cylinder with two degrees of freedom had been conducted in a towing tank. The test cylinder was a 45 cm long Tygon tubing with outer and inner diameter of 7.9 mm (5/16 in) and 4.8 mm (3/16 in), giving a mass ratio of 0.77 and an aspect ratio of 56. It was towed from rest up to 1.6 m/s before slowing down to rest again over a distance of 1.6 m in still water, covering the range of Reynolds number from 1500 to 13000 and reduced velocity from 4 to 35. Multi-mode vibration and sudden shift between different modes were observed. The vibration amplitude, frequency and mode were quantified. The results obtained during the brief constant towing speed were expressed in term of the corresponding Reynolds number or reduced velocity. These findings were cast with respect to the existing knowledge in the literature.

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Numerical modeling of stratified boundary layer over complex terrain has been an ongoing challenge in the field of environmental fluid dynamics. In this work, we present a computational framework aiming to tackle that challenge. The key components of the framework are residual-based variational multiscale method, isogeometric analysis, and weak imposition of Dirichlet boundary condition. The framework is validated against a laboratory experiment on strongly stratified flow past a three-dimensional bell-shaped hill. Good agreement is observed for qualitative flow physics, with the predicted occurrences of flow separation, recirculation, and hydraulic jump closely matching those in the experiment. In addition, the dividing-streamline height and wavelength of lee wave computed from the present framework compare well to the theoretical predictions. We show that the present framework is able to tackle various degrees of stratifications. The effect of weak imposition of Dirichlet boundary condition on the performance of the framework is also examined. This paper is concluded with an outlook toward applying the present framework to modeling microscale stratified flow past real-world terrains by simulating stratified flow past a two-dimensional environmental terrain.