November 4, 2024-Ken Kiger, University of Maryland

November 4,  2024
4:00 p.m.
Room:  Torgersen Hall 2150
Speaker:  Ken Kiger, University of Maryland
Faculty Host: Dr. Christine Gilbert

"From ocean waves to metal casting: the varied forms of air entrainment by plunging jets"

Abstract:  Air entrainment is the phenomena by which a finite volume of gas is submerged into a liquid at its free surface and rendered discontinuous with the surrounding atmosphere. It is a ubiquitous occurrence wherever moving liquids and gases are in contact with each other, whether it is as mundane as filling a glass with water or at a larger scale via a turbulent hydraulic jump in a spillway, or aeration of a bioreactor. One canonical form of air entrainment occurs when a liquid jet plunges into a pool of the same liquid, where the process of air entrainment is determined by the momentum of the jet counteracted by the restoring forces of gravity and surface tension. In high viscosity liquids, this process is well understood to be controlled by momentum transfer via viscous shearing stress, leading to a scaling in the form of a critical Capillary number. For low viscosity liquids, however, much less is understood, both due to the typically unsteadiness that is commonplace in high Reynolds number conditions, in addition to the varied mechanisms they create to destabilize the gas/liquid interface. In this presentation, I will review two cases we have examined: 1) air entrainment by a translating plunging jet, representative of the conditions that occur with breaking waves, and 2) air entrainment by a stationary but oscillating forced jet, intended to elucidate the irregular features that commonly occur in naturally occurring plunging jets. The results of the first case show that the air entrainment can be well-captured by unsteady potential flow informed by a gravitational cavity collapse and pinch off borrowed from water entry problems when one appropriately accounts for the translational momentum component. The second problem is more complicated and depends on the details of the subsurface flow triggering instabilities in the vicinity of the meniscus where the impacting jet separates from the free surface.  Our contribution to understanding this behavior has been to show pathways to three-dimensional effects in the form of Faraday like waves that lower the threshold for air entrainment. 

Bio:  Dr. Kiger is a Keystone Professor in the Department of Mechanical Engineering and serves as the Associate Dean of Undergraduate Programs at the University of Maryland where he has been a faculty member since 1995.    Dr. Kiger’s research interests are in experimental fluid mechanics and turbulence, with an emphasis on two-phase flows and biological fluid mechanics. His work in two-phase flows have focused on particle-turbulence interaction, which has required the development of novel instrumentation approaches to resolve the momentum coupling between the phases, primarily in solid-liquid flows. His research interests in biological applications relate to hemodynamics, cardiovascular mechanics pertaining to cardiogenesis, disease, and artificial pumping devices, as well as the study of bio-locomotion and respiration.