Flow characteristics and drag modulation in bubbly flow of a finite hull under varying flow confinements

The bubbly flow in microbubble drag reduction of a finite hull is a typical near-wall three-dimensional turbulent two-phase problem, which is influenced by varying flow confinements. Using the trim angle, heel angle, draft, and water depth as different flow confinements in this study, the Euler–Euler two-phase flow model is employed to investigate the flow characteristics, drag modulation, and underlying mechanisms. The results indicate that the streamwise wall inclination angle can affect the gas distribution and modulate the structure of the flow separation by altering the gas–liquid interphase forces. It may have a decoupling effect on drag reduction. The positive streamwise wall inclination angle helps expand the gas coverage area and reduce frictional drag. For the considered ship, it enhances flow separation, thereby increasing viscous pressure drag. The spanwise wall inclination angle induces lateral bubble migration and localized gas leakage, which reduces gas coverage and weakens the modulation of flow separation, thereby diminishing the drag reduction of frictional drag and viscous pressure drag. The immersion depth of the object affects gas distribution by affecting the degree of blocking in the flow field. The smaller immersion degree results in an overall increase in flow field velocity, thereby affecting the gas diffusion and drag reduction effects. The spatial constraints of the flow field increase the overall flow velocity and promotes uniform downstream diffusion of gas. However, its impact on the modulation effect of the flow field near the tail by bubbles is relatively weak.