Seminar: Thursday, March 9, 2023 10:30 AM, ME 2054

Q&A Immediately Following—11:30 AM

Flow mechanisms affecting cavitation Inception in boundary layers and in turbulent shear flows

Abstract: 

The presentation summarizes several experimental studies aimed at elucidating flow phenomena affecting cavitation inception. The first reveals the mechanisms sustaining attached cavitation inception on curved surfaces having pressure minima followed by regions of adverse pressure gradients, which either thicken the boundary layer or cause local flow separation. Microbubbles originated from the collapse of occasional travelling bubble cavitation are trapped in the thin low momentum regions close to the wall. These bubbles migrate slowly upstream either under the influence of the adverse pressure gradients when the flow remains attached or carried by the recirculating flow when the boundary layer is separated. Owing to the low local pressure, these bubbles grow by non-condensable gas diffusion until they reach the thickness of the low-momentum zone. At that time, they are either swept downstream by the external flow or they become nuclei for new attached cavitation events, which generate new microbubbles, allowing the process to sustain itself. These phenomena do not occur when the adverse pressure gradients are too mild to create low-momentum zones with sufficient thickness to facilitate the slow upstream migration and growth. The second study examines cavitation inception in a turbulent shear layer behind a backward facing step. Inception occurs in multiple points along 1 mm diameter and 2-5 mm long secondary quasi-streamwise vortices (QSVs) stretched between the primary spanwise vortices. The rate of cavitation events increases with the Reynolds number (Re). Time resolved volumetric velocity in the noncavitating flow is measured using tomographic particle tracking, and the 3D pressure distribution is determined by spatial integration of the material acceleration. Analyses in Eulerian and Lagrangian reference frames reveal that the pressure is lower, and its minima last longer within the QSVs compared to the surrounding flow. The intermittent low pressure regions, whose sizes and shapes are consistent with those of the cavities, are likely to be preceded by axial vortex stretching and followed by contraction. Such phenomena have been observed before in simulations of stretched vortex elements. The pressure minima last longer with increasing Reynolds number, a trend elucidated in terms of viscous diffusion of the stretched vortex core. The impact of cavitation nuclei on the scaling trends is studied under “natural” and controlled seeding. Results show that for both cases, the Reynolds number scaling is not caused by differences in concentration or entrainment rates of the micro-bubbles.

Research Interests:

Dr. Joseph Katz received his B.S. degree from Tel Aviv University, and his M.S. and Ph.D. from California Institute of Technology, all in mechanical engineering. He is the William F. Ward Sr. Distinguished Professor of Engineering, and the director and co-founder of the Center for Environmental and Applied Fluid Mechanics at Johns Hopkins University. He is a Member of the National Academy of Engineering, as well as a Fellow of the American Society of Mechanical Engineers (ASME), the American Physical Society, and American society of Thermal and Fluids Engineering. He has served as the Editor of the Journal of Fluids Engineering, and as the Chair of the board of journal Editors of ASME. He has co-authored more than 430 journal and conference papers. Dr. Katz research extends over several fields, with a common theme involving experimental fluid mechanics, and development of advanced optical diagnostics techniques for laboratory and field applications. His group has studied laboratory and oceanic boundary layers, turbomachinery flows, flow-structure interactions, cerebral and cardiac vascular flows, plankton swimming, as well as cavitation, bubble, and droplet dynamics, the latter focusing on interfacial phenomena associated with oil spills.