Quantitative analysis of the upwelling behavior of methane bubbles in nature and numerical simulations

Abstract

In recent years, methane gas released from the seafloor has been considered a sub-oceanic resource, and comprehending the dynamics of methane bubbles in the ocean has garnered increasing attention. Therefore, in this study, we conducted a quantitative analysis of the behavior of ascending methane bubbles with and without a hydrate layer. The bubbles were filmed at two natural gas seep sites and reproduced by numerical simulations using two-dimensional motion analysis software. The simulations were performed with gas bubbles and methane hydrate (MH)¹ bubbles spouting from a nozzle in a computational domain filled with pure water to assess the validity of image analysis for in-situ data, whereas numerical models and physical properties were utilized for the current two-phase (gas-liquid) simulations. The rising velocity, size, circumference, circularity, and maximum diameter of the methane bubbles were examined to understand the effects of the MH layer on the statistical and stochastic features of ascending methane bubbles. Based on the statistics of the above variables, the gas bubbles had a higher rising velocity and smaller circularity than the MH bubbles when the bubble sizes were identical. In addition, stochastic analysis indicated that the circularity of the MH bubble was uniquely determined by their size, due to the more rigid skin of the MH bubbles compared to that of gas bubbles. Consequently, discrepancies in bubble dynamics between methane gas and MH bubbles were clarified in this study.