Virtual MEchanics GAthering -MEGA- Seminar: Coating flow instabilities: to pearl or to whirl on fibers and sculpting draperies in limestone caves
Event details
| Date | 19.11.2020 |
| Hour | 16:15 › 17:30 |
| Speaker | Shahab Eghbali & Pier Giuseppe Ledda (LFMI, EPFL) |
| Location |
Zoom: epfl.zoom.us/s/98393329833 Room Passcode: 349948
Online
|
| Category | Conferences - Seminars |
Abstract The study of the dynamics of coating flows is of undeniable interest owing to the wide presence in natural environments and the large number of applications in industrial and manufacturing processes. Here, we bring two examples of coating flows showing the broad variety of patterns, i.e., the destabilization of a gravity-driven viscous flow coating a vertical fiber, and the draperies formation in the context of karst structures when the coating fluid flows below an inclined substrate.
Numerous studies have focused on the transition of a liquid thread into a downward-traveling train of beads along a fiber, a phenomenon known as Rayleigh-Plateau instability, in the limit of small Bond number, where the surface tension dominates over gravity. Here, we explore the limit of large Bond number, i.e. centimetric radial sized liquid column. The experiments are carried out using highly viscous silicone oil to focus on inertialess flows. We observe the formation of a helical interface, coiling around the thin fiber. We address theoretically the physical mechanism underlying the observed interface coiling by the linear stability analysis of a unidirectional flow along a rigid eccentric fiber. The asymmetry of the drainage velocity, above a certain threshold, induces coiling. Overall, small fiber radius and large eccentricity tend to promote the coiling of the interface, while small Bond numbers tend to preserve the axisymmetric interface.
We then move to the study of the role of hydrodynamic instabilities in the morphogenesis of typical draperies structures encountered among other speleothems in limestone caves. The problem is tackled using the lubrication approximation for the fluid film flowing under an inclined plane, in the presence of substrate perturbations that grow according to a classical deposition law. We generalize to the two-dimensional case the spatio-temporal analysis of the linear impulse response resulting from linear simulations. We exploit the concepts of Riesz transform and monogenic signal, the multi-dimensional complex continuation of a real signal, to retrieve the asymptotic properties of the wavepacket. The isotropy of the pure hydrodynamic solution is broken and the deposition process selects predominant streamwise structures on the substrate as the response is advected away. Furthermore, the presence of an initial localized perturbation on the substrate produces a quasi-steady region characterized by streamwise structures both in the fluid film and on the substrate. We suggest that these linear selection mechanisms contribute to the formation of draperies under inclined cave ceilings.
Bio Shahab Eghbali did his bachelor's in Mechanical Engineering at the University of Tehran, Iran, in 2014. In the same year, he joined EPFL to pursue his studies in the Master's program in the Institute of Mechanical Engineering with the Aero-and-Hydrodynamics orientation. Since 2018, he is a doctoral assistant in the Laboratory of Fluid Mechanics and Instabilities (LFMI) where he is currently working on the instability of complex interfacial flows.
PG Ledda is a Ph.D. student at the Laboratory of Fluid Mechanics and Instabilities at the EPFL. He obtained his B.sc and M.sc in Aerospace and Aeronautical Engineering at the University of Pisa. He joined EPFL during his master thesis, focused on the pattern characterization when a porous bluff body is invested by an asymptotic flow. He is now focused on the pattern formation in karst environments and the flow modelization using lubrication models. His main interests involve the flow control of aerodynamic flows by porosity variations and of thin films by textured substrates.
Numerous studies have focused on the transition of a liquid thread into a downward-traveling train of beads along a fiber, a phenomenon known as Rayleigh-Plateau instability, in the limit of small Bond number, where the surface tension dominates over gravity. Here, we explore the limit of large Bond number, i.e. centimetric radial sized liquid column. The experiments are carried out using highly viscous silicone oil to focus on inertialess flows. We observe the formation of a helical interface, coiling around the thin fiber. We address theoretically the physical mechanism underlying the observed interface coiling by the linear stability analysis of a unidirectional flow along a rigid eccentric fiber. The asymmetry of the drainage velocity, above a certain threshold, induces coiling. Overall, small fiber radius and large eccentricity tend to promote the coiling of the interface, while small Bond numbers tend to preserve the axisymmetric interface.
We then move to the study of the role of hydrodynamic instabilities in the morphogenesis of typical draperies structures encountered among other speleothems in limestone caves. The problem is tackled using the lubrication approximation for the fluid film flowing under an inclined plane, in the presence of substrate perturbations that grow according to a classical deposition law. We generalize to the two-dimensional case the spatio-temporal analysis of the linear impulse response resulting from linear simulations. We exploit the concepts of Riesz transform and monogenic signal, the multi-dimensional complex continuation of a real signal, to retrieve the asymptotic properties of the wavepacket. The isotropy of the pure hydrodynamic solution is broken and the deposition process selects predominant streamwise structures on the substrate as the response is advected away. Furthermore, the presence of an initial localized perturbation on the substrate produces a quasi-steady region characterized by streamwise structures both in the fluid film and on the substrate. We suggest that these linear selection mechanisms contribute to the formation of draperies under inclined cave ceilings.
Bio Shahab Eghbali did his bachelor's in Mechanical Engineering at the University of Tehran, Iran, in 2014. In the same year, he joined EPFL to pursue his studies in the Master's program in the Institute of Mechanical Engineering with the Aero-and-Hydrodynamics orientation. Since 2018, he is a doctoral assistant in the Laboratory of Fluid Mechanics and Instabilities (LFMI) where he is currently working on the instability of complex interfacial flows.
PG Ledda is a Ph.D. student at the Laboratory of Fluid Mechanics and Instabilities at the EPFL. He obtained his B.sc and M.sc in Aerospace and Aeronautical Engineering at the University of Pisa. He joined EPFL during his master thesis, focused on the pattern characterization when a porous bluff body is invested by an asymptotic flow. He is now focused on the pattern formation in karst environments and the flow modelization using lubrication models. His main interests involve the flow control of aerodynamic flows by porosity variations and of thin films by textured substrates.
Practical information
- General public
- Free
Organizer
- MEGA.Seminar Organizing Committee