18 mars 2021

Wébinaire Joseph Katz

Joseph Katz received his B.S. degree from Tel Aviv University, and his M.S. and Ph.D. from California Institute of Technology, all in mechanical engineering. He is the William F. Ward Sr. Distinguished Professor of Engineering, and the director and co-founder of the Center for Environmental and Applied Fluid Mechanics at Johns Hopkins University. He is a Member of the National Academy of Engineering, as well as a Fellow of the American Society of Mechanical Engineers (ASME) and the American Physical Society. He has served as the Editor of the Journal of Fluids Engineering, and as the Chair of the board of journal Editors of ASME. He has co-authored more than 400 journal and conference papers. Dr. Katz research extends over a wide range of fields, with a common theme involving experimental fluid mechanics, and development of advanced optical diagnostics techniques for laboratory and field applications. His group has studied laboratory and oceanic boundary layers, flows in turbomachines, flow-structure interactions, swimming behavior of marine plankton in the laboratory and in the ocean, as well as cavitation, bubble, and droplet dynamics, the latter focusing on interfacial phenomena associated with oil spills.
Tip Leakage Flows and Stall Suppression in Axial Turbomachines

Abstract: This presentation summarizes a series of experimental studies aimed at characterizing the flow and turbulence in the tip region of a one and a half stage axial liquid turbomachine with a compressor-like geometry. The experiments have been performed using transparent blade rows in a refractive-index-matched facility and included performance tests, stereo PIV (SPIV) measurements, and flow visualization using cavitation. Data analysis follows the evolution the tip leakage flow, its rollup into a tip leakage vortex (TLV), the migration and breakup of this vortex, development of secondary flows, as well as the evolution of turbulence in the rotor passage. Experiments aimed at characterizing the precursors to rotating stall show that it involves intermittent formation of large-scale backflow vortices (BFVs) that extend diagonally upstream, from the suction side of one blade at mid-chord to the pressure side near the leading edge of the next blade. The BFVs originate form radial gradients in circumferential velocity occurring under the TLV center, at the transition between the region affected by the backward tip leakage flow and the main passage flow. When the BFVs penetrate to the next passage, either across the tip gap or by circumventing the leading edge of the next blade, they trigger a similar phenomenon there. In the stall regime, the number and size of these vortices increase. Skewed semicircular axial casing grooves, which partially overlap with the rotor blade’s leading edge and the rest extending upstream, reduce the stall flow rate by as much as 40%, but degrade the performance near the best efficiency point (BEP). These grooves entrain most of the TLV, prevent the formation of BFVs, and cause periodic variations in the incidence angle near the rotor blade leading edge. In contrast, near BEP, secondary flows generated within the grooves are entrained back into the passage by the TLV. A series of grooves with the same inlet but different outlet angles have been tested in an attempt to alleviate the performance degradation. Aligning the outflow circumferentially against the blade rotation using U-shaped grooves is very effective in suppressing the stall, but degrades the BEP performance. Conversely, aligning the outflow with the blade rotation using S-shaped grooves achieves more moderate (25%) stall suppression, but does not degrade the BEP performance. Causes for these trends are elucidated by examining the groove-passage flow interactions. For example, the better stall suppression by the U grooves is attributed to higher periodic variations in flow angle at low flow rates, while the performance degradation and increased turbulence at BEP is caused by flow jetting out from the downstream end if this groove.

18 mars 2021, 16h3017h30
Wébinaire (contacté F. Romano pour le lien)

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01 décembre 2022

Wébinaire Esteban Ferrer

New avenues in high order fluid dynamics

Esteban Ferrer est professeur de mathématiques appliquées à l'école d'aéronautique (ETSIAE-UPM). Il a obtenu son doctorat à l'université d'Oxford (Royaume-Uni) et possède 20 ans d'expérience industrielle et universitaire dans le développement de techniques numériques pour les problèmes de fluides. Il travaille activement avec l'industrie et coordonne deux projets Européens. Ses principaux intérêts sont les méthodes d'ordre élevé pour la dynamique des fluides, la modélisation de la turbulence, l'apprentissage automatique, l'aéroacoustique pour l'aéronautique et l'énergie éolienne. Il a rédigé plus de 90 articles de journaux et de conférences sur ces sujets.