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Unraveling the multi-stage evolution of thin gas films during liquid-liquid impact events via high-fidelity numerical simulations and theoretical analyses


Dr. Seyedshahabaddin Mirjalili


Stanford, CA


Thursday, February 23, 2023 at 11:00:00 AM   


MRDC Building, Room 4211


Dr. Minami Yoda


Owing to their long residence time under the free surface, micro-bubbles with sizes ranging from 10-100 microns impact interfacial heat and mass transfer, acoustic wave propagation, and traceability of ship vessels in oceanic environments. As such, the formation mechanism of such bubbles is a subject of great interest. Based on evidence from drop-pool impact experiments, the leading hypothesis for micro-bubble formation is a process known as Mesler entrainment, in which liquid interfaces collide, entrapping a thin gas film that sheds hundreds of micro-bubbles after its rupture. However, before my work, due to the microscopic scales involved, there was a limited understanding of Mesler entrainment and a lack of quantitative data related to the film and micro-bubbles. This understanding has significant practical value in modeling micro-bubble generation in large-scale multiphase flow simulations. In this talk, after briefly introducing a novel diffuse interface method for simulation of complex multiphase flows, I examine the drop-pool impact as a model problem to reveal how collisions between two arbitrarily-curved interfaces can lead to microbubble generation. First, I use boundary integral method (BIM) simulations and theoretical analyses to discover a capillary transition that prevents early contact, allowing the drop to penetrate further into the pool and providing a pathway for the formation of elongated films. Since Mesler entrainment can only happen if early contact is prevented, I use the occurrence of transition as a criterion to provide an upper boundary for the Mesler entrainment regime. I describe the spreading kinematics of the drop after transition and present asymptotic scaling laws governing the film thickness. Using a linear stability analysis, I explain how intermolecular Van der Waals forces can trigger the rupture of the thin film. Next, by employing my novel diffuse interface method, implemented in my high-performance in-house software, I delve into the post-rupture dynamics of the film to numerically simulate thin retracting gas films. These simulations reveal a new scaling law for gas film retraction velocity. Moreover, using high-fidelity 3D simulations, a transverse instability on the edge of the film is shown to be responsible for micro-bubble generation. I conclude the talk by outlining similarities between this multi-stage, multi-scale multiphase flow problem and the phenomenon responsible for the generation of aerosols in human lungs during breathing, known as the Bronchiole Fluid Film Burst (BFFB) mechanism.


Shahab Mirjalili is a research associate at the Center for Turbulence Research at Stanford University. He received his Ph.D. in Mechanical Engineering under the supervision of Professor Ali Mani at Stanford University in 2019. His dissertation focused on developing numerical methods for simulating multiphase flows with application to studying micro-bubble generation. Before that, Shahab earned his MS from Stanford University and a BS from Sharif University of Technology, both in Mechanical Engineering. He is the recipient of the Gallery of Fluid Motion Award, American Physical Society Division of Fluid Dynamics (2018). Shahab’s research focuses on improving the predictive understanding of multiphase flow problems involving multi-physics interactions across a wide range of scales and Reynolds numbers. His applications of particular interest include additive manufacturing processes, biophysical systems, energy and propulsion, and environmental flows.


Refreshments will be served.