11 avril 2024

Webinaire Hassan M. Nagib

Hassan M. Nagib is the John T. Rettaliata Endowed Professor of Mechanical and Aerospace Engineering at the Illinois Institute of Technology, and was the founding director of the institute’s Fluid Dynamics Research Center. His field of specialty is in fluid mechanics, turbulent flow, and flow management and control. At Illinois Tech, he served as MMAE department Chair, dean of Armour College, academic vice president, and chief scientist for IIT Research Institute (IITRI). Nagib is the recipient of several prestigious honors including being a Fellow of the American Physical Society, the American Association of Advancement of Science, the American Institute of Aeronautics and Astronautics, and the American Society of Mechanical Engineers. From his base institute for more than half a century, he has been a visiting faculty on several occasions at Stanford University, California Institute of Technology, KTH Royal Institute of Technology, École Polytechnique Fédérale de Lausanne, Friedrich-Alexander University, Erlangen, and a Tewkesbury Fellow in the Department of Mechanical Engineering at University of Melbourne.
Wall-Bounded Turbulence: Recent Lessons from Experiments-Asymptotics-Computation

Utilizing the three-pronged approach of experimental measurements, computational results (DNS), and matched asymptotic analysis (MAE), we have reexamined the three canonical wall-bounded turbulent flows of pipe, channel, and pressure gradient boundary layers (FPG, ZPG and APG). Detailed and systematic study confirmed the non-universality of the Kármán constant (κ) reported in 2008 by Nagib, H. M. and Chauhan, K. A. in Variations of Von Kármán Coefficient in Canonical Flows, Phys. Fluids 20, 101518. Recently, a new matching approach also revealed an inner-outer overlap consisting of a superposition of log-law and a linear term, as detailed in a paper by Monkewitz, P. A. and Nagib, H. M., The Hunt for the Kármán “Constant” Revisited, J. Fluid Mechanics, Vol. 967, A15, 2023. A similar linear term was suggested by Afzal and Yajnik, J. Fluid Mech. (1973 and 1970), Lee and Moser (2015), J. Fluid Mech, 774, pp. 395-414, and Luchini (2017) Phys. Rev. Lett. 118, 224501. In our results, we find that the coefficients of both terms are dependent on the pressure gradient of the flow. A new and robust method is devised to simultaneously determine the coefficients of the log and linear terms, and the constant term in pressure driven flows at currently accessible Reynolds numbers, and yields κ values that are consistent with the κ values deduced from the Reynolds number dependence of centerline velocities. After many decades of experience with “canonical” wall-bounded turbulent flows, we recognize fully developed pipe flow as the ideal flow to compare computations and experiments. With collaborators at several universities, we have conducted experiments, with Re_τ up to 33,000, and DNS for pipe flow at Re_τ = 550 and 1,000, with several resolutions, and extended Eddy Turnover Times (ETT) of up to 200. New criteria for resolution and an existing criterion for convergence of DNS are being developed and confirmed, respectively. We find that higher resolution DNS and longer computational times are required for wall-bounded turbulence, compared to values commonly used. Finally, the impact of the new combined log-law and linear term on textbooks, lecture notes, RANS codes, and turbulence models will be highlighted.

11 avril 2024, 16h3017h30
Webinar (please contact F. Romano for the link)