BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Memento EPFL// BEGIN:VEVENT SUMMARY:MechE Colloquium : Instability and breakdown phenomena in vortical flows DTSTART:20260428T120000 DTEND:20260428T130000 DTSTAMP:20260403T211741Z UID:ff62217a38b88efe7a1f600fde93a0de2bb357559b36c1e937ffc4de CATEGORIES:Conferences - Seminars DESCRIPTION:Prof. Thomas Leweke\, IRPHE-CNRS\, Marseille\, France\nSummary \nVortical structures generated by fixed or rotating lifting surfaces play a central role in many aerodynamic and hydrodynamic flows. Their stabilit y and eventual breakdown influence wake dynamics\, mixing processes\, and overall flow performance. This seminar presents two recent studies explori ng vortex stability and vortex breakdown.\nPart I – Short-wave instabili ty of a helical vortex\nThe first part of the seminar discusses the short- wave instability of a helical vortex generated by a rotating blade. Combin ing experimental dye visualisations\, numerical simulations\, and theoreti cal analysis\, the study identifies displacement perturbations whose wavel engths are small relative to the helix radius and pitch but may remain lar ge compared with the vortex core size. Stability analysis based on experim entally measured vortex profiles reveals broad bands of unstable wavenumbe rs for several vortex bending modes. These results differ from predictions of existing theories for short-wave vortex instability. Similar instabili ty modes are also observed in arrays of straight vortices\, indicating tha t the phenomenon is not related to vortex curvature. A theoretical examina tion of the dispersion relation of Kelvin modes for the measured vortex pr ofiles uncovers a previously unidentified family of modes associated with the specific vorticity distribution. Their non-resonant interaction throug h the strain field provides a plausible explanation for the experimentally observed instability features.\nPart II – Two-phase wing-tip vortex bre akdown\nThe second part presents the discovery of a new flow feature obser ved 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 downstre am of the wing. Experiments show that\, for certain combinations of Reynol ds number and angle of attack\, a stationary air bubble becomes trapped wi thin the vortex core at a finite distance behind the wing and can persist for several minutes even after the air injection is stopped. Under differe nt conditions\, the bubble may drift upstream or downstream\, or it may di sintegrate immediately. Measurements of bubble properties and vortex chara cteristics reveal that the breakdown behaviour depends primarily on the vo rtex circulation and on the axial flow component within the core. The form ation of a stable breakdown bubble occurs only when a velocity excess rela tive to the free stream is present.\nThe two studies highlight new mechani sms governing the stability and transformation of vortical flows\, offerin g insight into the dynamics of vortex instabilities and vortex-core modifi cations in fluid systems.\n\n\nBiography\nThomas Leweke graduated from RWT H Aachen University in Germany in 1990 with a “Diplom” (Master) in Phy sics. He completed his PhD in 1994 at the Université de Provence in Marse ille\, on the experimental study and modelling of bluff-body wakes. After a post-doctoral stay at Cornell University on vortex instabilities\, he jo ined the IRPHE institute in Marseille in 1996 as a CNRS Researcher and bec ame a Senior Researcher in 2007. His research focusses on the experimental study of fundamental aspects of fluid mechanics\, especially in vortex dy namics and fluid-structure interactions\, with relevance to applications. He was the co-organiser of a conference series on Bluff-Boddy Wakes and Vo rtex-Induced Vibrations (BBVIV)\, and an associate editor for the Journal of Fluids and Structures and the Journal of Visualization LOCATION:MED 0 1418 https://plan.epfl.ch/?room==MED%200%201418 https://epf l.zoom.us/j/61360740951 STATUS:CONFIRMED END:VEVENT END:VCALENDAR