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:20260416T115438Z UID:a5a224a7ce6937c1ac2921ddb28d2cef84f50468b1e4eb82c9a168fe CATEGORIES:Conferences - Seminars DESCRIPTION:Prof. Thomas Leweke\, IRPHE\, CNRS\, Marseille\, France\nSumma ry\nVortical structures generated by fixed or rotating lifting surfaces pl ay a central role in many aerodynamic and hydrodynamic flows. Their stabil ity and eventual breakdown influence wake dynamics\, mixing processes\, an d overall flow performance. This seminar presents two recent studies explo ring vortex stability and vortex breakdown.\nPart I – Short-wave instabi lity of a helical vortex\nThe first part of the seminar discusses the shor t-wave instability of a helical vortex generated by a rotating blade. Comb ining experimental dye visualisations\, numerical simulations\, and theore tical analysis\, the study identifies displacement perturbations whose wav elengths are small relative to the helix radius and pitch but may remain l arge compared with the vortex core size. Stability analysis based on exper imentally measured vortex profiles reveals broad bands of unstable wavenum bers for several vortex bending modes. These results differ from predictio ns of existing theories for short-wave vortex instability. Similar instabi lity modes are also observed in arrays of straight vortices\, indicating t hat the phenomenon is not related to vortex curvature. A theoretical exami nation of the dispersion relation of Kelvin modes for the measured vortex profiles uncovers a previously unidentified family of modes associated wit h the specific vorticity distribution. Their non-resonant interaction thro ugh the strain field provides a plausible explanation for the experimental ly observed instability features.\nPart II – Two-phase wing-tip vortex b reakdown\nThe second part presents the discovery of a new flow feature obs erved in the wake of a rectangular wing in water: the breakdown of the win g-tip vortex triggered by the injection of air into the vortex core downst ream of the wing. Experiments show that\, for certain combinations of Reyn olds number and angle of attack\, a stationary air bubble becomes trapped within the vortex core at a finite distance behind the wing and can persis t for several minutes even after the air injection is stopped. Under diffe rent conditions\, the bubble may drift upstream or downstream\, or it may disintegrate immediately. Measurements of bubble properties and vortex cha racteristics reveal that the breakdown behaviour depends primarily on the vortex circulation and on the axial flow component within the core. The fo rmation of a stable breakdown bubble occurs only when a velocity excess re lative to the free stream is present.\nThe two studies highlight new mecha nisms governing the stability and transformation of vortical flows\, offer ing insight into the dynamics of vortex instabilities and vortex-core modi fications in fluid systems.\n\n\nBiography\nThomas Leweke graduated from R WTH Aachen University in Germany in 1990 with a “Diplom” (Master) in P hysics. He completed his PhD in 1994 at the Université de Provence in Mar seille\, on the experimental study and modelling of bluff-body wakes. Afte r a post-doctoral stay at Cornell University on vortex instabilities\, he joined the IRPHE institute in Marseille in 1996 as a CNRS Researcher and b ecame a Senior Researcher in 2007. His research focusses on the experiment al 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 Journa l 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