A numerical study on clustering characteristics of rising bubbles

Horizontal clustering is observed in bubbly flows under specific flow conditions. Although previous studies have explored the flow conditions and mechanisms of clustering, the characteristics of bubble clustering are still not fully understood. We conduct numerical simulation of rising air bubbles in stagnant water to investigate their cluster characteristics. Monodispersed rising bubbles with different equivalent bubble diameters (d_eq = 0.5, 1, and 1.5 mm) are simulated in a periodic domain. The bubbles of d_eq = 0.5 mm tend to maintain their initial distribution, while larger ones (d_eq = 1 and 1.5 mm) interact among themselves, leading to clustering. As bubble clusters grow, their rising velocity decreases. The bubble clusters are sheet-like and are aligned nearly parallel to the horizontal direction. Also, bubbles within a short distance (r_ij < 2d_eq) tend to align horizontally. The bubble-to-bubble collision during clustering causes a counter-rotating vortex pair between bubbles. These vortices induce a downward velocity, keeping the bubbles horizontally aligned.