Key role of void fraction in bubble clouds created by plunging liquid jet impacts

A jet plunging in a pool of same, or different liquid, entrains air and often forms a bubble cloud. This ubiquitous phenomenon is widely encountered in nature, like breaking waves in water bodies, and industrial applications like reducing foam formation in chemical processes. Through a simple force balance model, Guyot et al. (PRL 124, 194503 (2020)) recently elucidated the key role of void fraction on the bubble cloud depth and thereby, introduced two general scenarios, namely, inertial and buoyancy-dominated bubble clouds. Here, we further explore the influence of void fraction using novel lab–scale experiments with injectors ranging from 1.2 to 10 mm in diameter, wherein the jet fall height is varied up to one meter in order to control the air entrainment process via the impact velocity and jet morphology. In particular, we carefully measure void fraction in bubble clouds using optical probes. We first show that, for a given impact velocity, bubble cloud depth decreases with an increase in jet fall height, due to air-intake augmentation during free-fall and impact. Furthermore, we demonstrate a transition from inertial to buoyant clouds at a critical impact velocity, based on jet diameter and void fraction. New scaling laws on the cloud void fraction are also proposed.