Bubble formed by underwater rapid high-pressure gas jet near the free surface and the induced water mound

This study numerically investigates how geometric parameters influence bubbles generated by underwater high-pressure gas jets and the resultant free-surface water mounds. Comparing rigid jetting devices with different aspect ratios and pre-cracked casings with varied initial geometries, we held initial bubble volume, internal pressure, free-surface distance, and buoyancy parameter constant. Using the Finite Volume Method, we simulated the full bubble pulsation cycle (i.e., expansion, collapse, and secondary expansion), analyzed the evolution of phase fraction and flow-field pressure, and quantitatively characterized bubble morphology and mound dynamics. Results show that a higher aspect ratio shortens the pulsation period, significantly increases mound height, narrows mound width, and reduces mound mass. Among geometries, conical bubbles most significantly enhance mound height due to efficient axial energy-focusing; cylindrical ones are intermediate; inverted cones are lower due to dispersed focusing; and spheres are nearly isotropic. For a given aspect ratio, conical bubbles are optimal for maximizing mound height, with a critical water skirt height occurring within a specific aspect ratio range. This work provides valuable references for designing and optimizing underwater high-pressure gas jetting devices.