Direct Numerical Simulation of Bubble Formation Through a Submerged “Flute” with Experimental Validation

Direct Numerical Simulation (DNS) is often used to uncover and highlight physical phenomena that are not properly resolved using other Computational Fluid Dynamics (CFD) methods due to shortcuts taken in the latter to cheapen computational cost. In this work we use DNS along with interface tracking to take an in-depth look at bubble formation, departure, and ascent through water. To form the bubbles air is injected through a novel orifice geometry not unlike that of a flute submerged underwater, which introduces phenomena that are not typically brought to light in conventional orifice studies. For example, our single-phase simulations show a significant leaning effect wherein pressure accumulating at the trailing nozzle edges leads to asymmetric discharge through the nozzle hole, and an upward bias in the flow in the rest of the pipe. In our two-phase simulations, this effect is masked by the surface tension of the bubble sitting on the nozzle, but it can still be seen following departure events. After bubble departure, we observe the bubbles converge towards an ellipsoidal shape, which has been validated by experiments. As the bubbles rise, we note that local variations in the vertical velocity cause the bubble edges to flap slightly, oscillating between relatively low and high velocities at the edges. Thus, causing the bubble edges to periodically lag and lead the bulk bubble mass.