Virtual MEchanics GAthering -MEGA- Seminar: Talk 1 - Scaling and modelling of vortex rings behind cones; Talk 2- Discrete shedding of secondary vortices along a modified Kaden spiral
Event details
| Date | 18.03.2021 |
| Hour | 16:15 › 17:30 |
| Speaker | Guillaume de Guyon & Diego Francescangeli (UNFOLD, EPFL) |
| Location |
https://epfl.zoom.us/s/84678428267 Passcode: 174387
Online
|
| Category | Conferences - Seminars |
Talk 1: Scaling and modelling of vortex rings behind cones, by Guillaume de Guyon (UNFOLD, EPFL)
Abstract Ring vortices are efficient at transporting fluid across long distances. They are observed in nature in various ways: they propel squids, inject blood in the heart, and entertain dolphins. These vortices are generally produced by ejecting a volume of fluid through a circular orifice and have been widely studied and characterised. After four convective times, three events happen simultaneously: the vortex moves faster than the shear layer it originates from, it separates from the shear layer, and the circulation and non-dimensional energy of the vortex converge. The simultaneity of these three events obfuscates the causality between them. To analyse the temporal evolution of the vortex independently of the separation, we analyse the development of vortices generated in the wake of cones. The vortex rings that form behind the cones have a self-induced velocity that causes them to follow the cone. They continue to grow as the cone travels well beyond the limiting vortex formation times observed for vortices generated by pistons. The non-dimensional circulation, based on the vortex diameter, and the non-dimensional energy of the vortex rings converge after three convective times. This result proves that the convergence of non-dimensional quantities is not just a consequence of the separation. In addition, the evolution of the vortex is modelled with an axisymmetric discrete vortex method. The model predicts accurately the evolution of the vortex.
Bio Guillaume de Guyon is a PhD student in the Unsteady flow diagnostics Laboratory, under the scientific supervision of Prof. K. Mulleners. He obtained his M.Sc. in Mechanics in the Sorbonne university (Paris, France) in 2016. During his studies, he got interested in theoretical fluid mechanics, which brought him to his PhD devoted to the study of vortex formation.
Talk 2: Discrete shedding of secondary vortices along a modified Kaden spiral, Diego Francescangeli (UNFOLD, EPFL)
Abstract When an object is accelerated in a fluid, a primary vortex is formed through the roll-up of a shear layer. This primary vortex does not grow indefinitely and will reach a limiting size and strength. Additional vorticity beyond the critical limit will end up in a trailing shear layer and accumulate into secondary vortices. The secondary vortices are typically considerably smaller than the primary vortex. In this paper, we focus on the formation, shedding, and trajectory of secondary vortices generated by a rotating rectangular plate in a quiescent fluid using time-resolved particle image velocimetry. The Reynolds number (Re) based on the maximum rotational velocity of the plate and the distance between the centre of rotation and the tip of the plate is varied from 840 - 11150. At low Re, the shear layer is a continuous uninterrupted layer of vorticity that rolls up into a single coherent primary vortex. At Re = 1955, the shear layer becomes unstable and secondary vortices emerge and subsequently move away from the tip of the plate. For Re = 4000, secondary vortices are discretely released from the plate tip and are not generated from the stretching of an unstable shear layer. First, we demonstrate that the roll-up of the shear layer, the trajectory of the primary vortex, and the path of secondary vortices can be predicted by a modified Kaden spiral for the entire Re range considered. Second, the timing of the secondary vortex shedding is analysed using the swirling strength criterion. The separation time of each secondary vortex is identified as a local maximum in the temporal evolution of the average swirling strength close to the plate tip. The time interval between the release of successive secondary vortices is not constant during the rotation but increases the more vortices have been shed. The shedding time interval also increases with decreasing Reynolds number. The increased time interval under both conditions is due to a reduced circulation feeding rate.
Bio Diego Francescangeli is a PhD student in the Unsteady flow diagnostics Laboratory, UNFoLD under the supervision of Prof. K. Mulleners. He obtained his M.Sc. in Mechanical engineering in the Marche polytechnic university (Ancona, Italy) in 2017. For his Master thesis, he had an intership in Belgium at von Karman Institute, to study the cavitation. From this experience, he developed a growing interest in fluid dynamics, which leads him to start a PhD about vortex dynamics.
Abstract Ring vortices are efficient at transporting fluid across long distances. They are observed in nature in various ways: they propel squids, inject blood in the heart, and entertain dolphins. These vortices are generally produced by ejecting a volume of fluid through a circular orifice and have been widely studied and characterised. After four convective times, three events happen simultaneously: the vortex moves faster than the shear layer it originates from, it separates from the shear layer, and the circulation and non-dimensional energy of the vortex converge. The simultaneity of these three events obfuscates the causality between them. To analyse the temporal evolution of the vortex independently of the separation, we analyse the development of vortices generated in the wake of cones. The vortex rings that form behind the cones have a self-induced velocity that causes them to follow the cone. They continue to grow as the cone travels well beyond the limiting vortex formation times observed for vortices generated by pistons. The non-dimensional circulation, based on the vortex diameter, and the non-dimensional energy of the vortex rings converge after three convective times. This result proves that the convergence of non-dimensional quantities is not just a consequence of the separation. In addition, the evolution of the vortex is modelled with an axisymmetric discrete vortex method. The model predicts accurately the evolution of the vortex.
Bio Guillaume de Guyon is a PhD student in the Unsteady flow diagnostics Laboratory, under the scientific supervision of Prof. K. Mulleners. He obtained his M.Sc. in Mechanics in the Sorbonne university (Paris, France) in 2016. During his studies, he got interested in theoretical fluid mechanics, which brought him to his PhD devoted to the study of vortex formation.
Talk 2: Discrete shedding of secondary vortices along a modified Kaden spiral, Diego Francescangeli (UNFOLD, EPFL)
Abstract When an object is accelerated in a fluid, a primary vortex is formed through the roll-up of a shear layer. This primary vortex does not grow indefinitely and will reach a limiting size and strength. Additional vorticity beyond the critical limit will end up in a trailing shear layer and accumulate into secondary vortices. The secondary vortices are typically considerably smaller than the primary vortex. In this paper, we focus on the formation, shedding, and trajectory of secondary vortices generated by a rotating rectangular plate in a quiescent fluid using time-resolved particle image velocimetry. The Reynolds number (Re) based on the maximum rotational velocity of the plate and the distance between the centre of rotation and the tip of the plate is varied from 840 - 11150. At low Re, the shear layer is a continuous uninterrupted layer of vorticity that rolls up into a single coherent primary vortex. At Re = 1955, the shear layer becomes unstable and secondary vortices emerge and subsequently move away from the tip of the plate. For Re = 4000, secondary vortices are discretely released from the plate tip and are not generated from the stretching of an unstable shear layer. First, we demonstrate that the roll-up of the shear layer, the trajectory of the primary vortex, and the path of secondary vortices can be predicted by a modified Kaden spiral for the entire Re range considered. Second, the timing of the secondary vortex shedding is analysed using the swirling strength criterion. The separation time of each secondary vortex is identified as a local maximum in the temporal evolution of the average swirling strength close to the plate tip. The time interval between the release of successive secondary vortices is not constant during the rotation but increases the more vortices have been shed. The shedding time interval also increases with decreasing Reynolds number. The increased time interval under both conditions is due to a reduced circulation feeding rate.
Bio Diego Francescangeli is a PhD student in the Unsteady flow diagnostics Laboratory, UNFoLD under the supervision of Prof. K. Mulleners. He obtained his M.Sc. in Mechanical engineering in the Marche polytechnic university (Ancona, Italy) in 2017. For his Master thesis, he had an intership in Belgium at von Karman Institute, to study the cavitation. From this experience, he developed a growing interest in fluid dynamics, which leads him to start a PhD about vortex dynamics.
Practical information
- General public
- Free
Organizer
- MEGA.Seminar Organizing Committee