MechE Colloquium : Instability and breakdown phenomena in vortical flows
Event details
| Date | 28.04.2026 |
| Hour | 12:00 › 13:00 |
| Speaker | Prof. Thomas Leweque, IRPHE-CNRS, Marseille, France |
| Location | Online |
| Category | Conferences - Seminars |
| Event Language | English |
Summary
Vortical structures generated by fixed or rotating lifting surfaces play a central role in many aerodynamic and hydrodynamic flows. Their stability and eventual breakdown influence wake dynamics, mixing processes, and overall flow performance. This seminar presents two recent studies exploring vortex stability and vortex breakdown.
Part I – Short-wave instability of a helical vortex
The first part of the seminar discusses the short-wave instability of a helical vortex generated by a rotating blade. Combining experimental dye visualisations, numerical simulations, and theoretical analysis, the study identifies displacement perturbations whose wavelengths are small relative to the helix radius and pitch but may remain large compared with the vortex core size. Stability analysis based on experimentally measured vortex profiles reveals broad bands of unstable wavenumbers for several vortex bending modes. These results differ from predictions of existing theories for short-wave vortex instability. Similar instability modes are also observed in arrays of straight vortices, indicating that the phenomenon is not related to vortex curvature. A theoretical examination of the dispersion relation of Kelvin modes for the measured vortex profiles uncovers a previously unidentified family of modes associated with the specific vorticity distribution. Their non-resonant interaction through the strain field provides a plausible explanation for the experimentally observed instability features.
Part II – Two-phase wing-tip vortex breakdown
The second part presents the discovery of a new flow feature observed in the wake of a rectangular wing in water: the breakdown of the wing-tip vortex triggered by the injection of air into the vortex core downstream of the wing. Experiments show that, for certain combinations of Reynolds number and angle of attack, a stationary air bubble becomes trapped within the vortex core at a finite distance behind the wing and can persist for several minutes even after the air injection is stopped. Under different conditions, the bubble may drift upstream or downstream, or it may disintegrate immediately. Measurements of bubble properties and vortex characteristics reveal that the breakdown behaviour depends primarily on the vortex circulation and on the axial flow component within the core. The formation of a stable breakdown bubble occurs only when a velocity excess relative to the free stream is present.
The two studies highlight new mechanisms governing the stability and transformation of vortical flows, offering insight into the dynamics of vortex instabilities and vortex-core modifications in fluid systems.
Biography
Thomas Leweke graduated from RWTH Aachen University in Germany in 1990 with a “Diplom” (Master) in Physics. He completed his PhD in 1994 at the Université de Provence in Marseille, on the experimental study and modelling of bluff-body wakes. After a post-doctoral stay at Cornell University on vortex instabilities, he joined the IRPHE institute in Marseille in 1996 as a CNRS Researcher and became a Senior Researcher in 2007. His research focusses on the experimental study of fundamental aspects of fluid mechanics, especially in vortex dynamics and fluid-structure interactions, with relevance to applications. He was the co-organiser of a conference series on Bluff-Boddy Wakes and Vortex-Induced Vibrations (BBVIV), and an associate editor for the Journal of Fluids and Structures and the Journal of Visualization
Vortical structures generated by fixed or rotating lifting surfaces play a central role in many aerodynamic and hydrodynamic flows. Their stability and eventual breakdown influence wake dynamics, mixing processes, and overall flow performance. This seminar presents two recent studies exploring vortex stability and vortex breakdown.
Part I – Short-wave instability of a helical vortex
The first part of the seminar discusses the short-wave instability of a helical vortex generated by a rotating blade. Combining experimental dye visualisations, numerical simulations, and theoretical analysis, the study identifies displacement perturbations whose wavelengths are small relative to the helix radius and pitch but may remain large compared with the vortex core size. Stability analysis based on experimentally measured vortex profiles reveals broad bands of unstable wavenumbers for several vortex bending modes. These results differ from predictions of existing theories for short-wave vortex instability. Similar instability modes are also observed in arrays of straight vortices, indicating that the phenomenon is not related to vortex curvature. A theoretical examination of the dispersion relation of Kelvin modes for the measured vortex profiles uncovers a previously unidentified family of modes associated with the specific vorticity distribution. Their non-resonant interaction through the strain field provides a plausible explanation for the experimentally observed instability features.
Part II – Two-phase wing-tip vortex breakdown
The second part presents the discovery of a new flow feature observed in the wake of a rectangular wing in water: the breakdown of the wing-tip vortex triggered by the injection of air into the vortex core downstream of the wing. Experiments show that, for certain combinations of Reynolds number and angle of attack, a stationary air bubble becomes trapped within the vortex core at a finite distance behind the wing and can persist for several minutes even after the air injection is stopped. Under different conditions, the bubble may drift upstream or downstream, or it may disintegrate immediately. Measurements of bubble properties and vortex characteristics reveal that the breakdown behaviour depends primarily on the vortex circulation and on the axial flow component within the core. The formation of a stable breakdown bubble occurs only when a velocity excess relative to the free stream is present.
The two studies highlight new mechanisms governing the stability and transformation of vortical flows, offering insight into the dynamics of vortex instabilities and vortex-core modifications in fluid systems.
Biography
Thomas Leweke graduated from RWTH Aachen University in Germany in 1990 with a “Diplom” (Master) in Physics. He completed his PhD in 1994 at the Université de Provence in Marseille, on the experimental study and modelling of bluff-body wakes. After a post-doctoral stay at Cornell University on vortex instabilities, he joined the IRPHE institute in Marseille in 1996 as a CNRS Researcher and became a Senior Researcher in 2007. His research focusses on the experimental study of fundamental aspects of fluid mechanics, especially in vortex dynamics and fluid-structure interactions, with relevance to applications. He was the co-organiser of a conference series on Bluff-Boddy Wakes and Vortex-Induced Vibrations (BBVIV), and an associate editor for the Journal of Fluids and Structures and the Journal of Visualization
Practical information
- General public
- Free
Organizer
- MechE Colloquium
Contact
- Prof. Eunok Yim