Shape Oscillation-Induced Early Detachment of Bubble from a Submerged Microcapillary Nozzle.

Bubble growth and detachment are basic phenomena in boiling and water splitting processes and are essential for their heat transfer performance and hydrogen production efficiency. In this study, we investigate the formation and detachment of bubbles from a submerged capillary nozzle. Here, we report a new bubbling regime and term it unstable bubbling, where early and normal bubble detachments coexist. The early detachment leads to the generation of many finer bubbles, ranging from one-fifth to one-half of the volume of the permanently detached bubbles driven by buoyancy. The occurrence conditions for unstable bubbling are identified by developing a regime map. The visualization of bubble-induced liquid flow and energy analysis suggest that the oscillating detached bubble provides excess surface and kinetic energies to the growing bubble, supporting early bubble detachment. Two dimensionless numbers are derived considering the effect of these energies, which can be applied to successfully predict the occurrence of early detachment. The present results not only provide physical insight into bubble interactions but also offer potential techniques for generating fine bubbles.

Computational fluid dynamics investigation on aortic hemodynamics in double aortic arch before and after ligation surgery.

Double aortic arch (DAA) malformation is one of the reasons for symptomatic vascular rings, the hemodynamics of which is still poorly understood. This study aims to investigate the blood flow characteristics in patient-specific double aortic arches using computational fluid dynamics (CFD). Seven cases of infantile patients with DAA were collected and their computed tomography images were used to reconstruct 3D computational models. A modified Carreau model was used to consider the non-Newtonian effect of blood and a three-element Windkessel model taking the effect of the age of patients into account was applied to reproduce physiological pressure waveforms. Numerical results show that blood flow distribution and energy loss of DAA depends on relative sizes of the two aortic arches and their angles with the ascending aorta. Ligation of either aortic arch increases the energy loss of blood in the DAA, leading to the increase in cardiac workload. Generally, the rising rate of energy loss before and after the surgery is almost linear with the area ratio between the aortic arch without ligation and the ascending aorta.