Gas layers on solid surfaces can greatly reduce the drag on objects moving within liquids, by bypassing the no-slip boundary condition. For free-falling Leidenfrost spheres these vapor layers move the separation points towards the back side thereby reducing the form-drag by a similar amount as observed in the drag crises. Including a larger gas-pocket around the solid sphere it forms a streamlined fairing which eliminates the form and viscous drag at the shear-stress-free boundary, resulting in near-zero drag-coefficient. In this seminar we will show a surprising connection between drag-reducing air-layers to generation of “singular” jets during drop impacts onto a pool surface. These fine 10-micron jets can emerge at over 100 m/s, during the rebounding of the impact craters. High-speed video shows the fastest jets emerge from a dimple at the bottom of the crater. Using axisymmetric numerical simulations with extreme grid refinement, we reveal how the presence of air layers guides the fastest jets
Siggi Thoroddsen is a Professor in Mechanical Engineering at KAUST, having joined in 2009, as a founding faculty. Before that he was Associate Professor at NUS in Singapore and Assistant Professor in Theoretical and Applied Mechanics at University of Illinois, Urbana-Champaign. He received his PhD in 1992 from UC San Diego in Applied Mechanics, MS from Colorado State University and BS from his home country at University of Iceland. Thoroddsen’s research focuses on the use of ultra-high-speed video imaging to study the dynamics of multi-phase flows. This includes coalescence and breakup of drops and bubbles, splashing and spray formation, as well as the dynamics of fine jets of relevance to inkjet printers. He is also interested in coating flows, the dynamics of granular media, laser-produced cavitation and volumetric measurements in turbulence. His research has been published in 181 scientific papers which have received over 6800 citations (Web of Knowledge). In 2012 he became a Fellow of the American Physical Society’s Division of Fluid Dynamics.