Seminar "Survival of Localized Turbulence in Wall-Bounded Flows" by Prof. Takahiro Tsukahara

[Speaker]

Prof. Takahiro Tsukahara
Professor
Department of Mechanical and Aerospace Engineering
Tokyo University of Science

[Abstract]

The subcritical transition of wall-bounded shear flows is a long-standing problem in fluid dynamics, characterized by localized turbulent structures such as puffs, oblique bands, stripes, and ring-shaped turbulence. These structures arise in pipe, planar Couette/Poiseuille, Taylor–Couette–Poiseuille, and annular flows, and are often accompanied by secondary large-scale flows that appear to play an important role in turbulence localization and sustenance. Their sensitivity to geometry, confinement, surface conditions, and other perturbations highlights the complexity of determining the global critical Reynolds number and the mechanisms governing laminar–turbulence coexistence. This talk summarizes recent investigations based mainly on direct numerical simulations, with some supporting experiments, across canonical planar shear flows and annular geometries. Particular attention is paid to the formation, persistence, and decay of localized turbulence, their associated large-scale flows, and their implications for turbulent heat transfer in engineering systems such as annular coolant channels. Recently, large-scale DNS has also been performed in which the Reynolds number is temporally reduced from a fully turbulent state. These simulations were used to investigate the characteristic time and conditions required for turbulent-band formation. The results indicate that sustained band formation is governed by the presence of localized turbulent “cores” that can develop into mature bands. As the rate of Reynolds-number reduction increases, the number density of such cores decreases, making successful band formation less likely and promoting relaminarization. These findings provide a dynamical picture of how localized turbulence survives transient changes in operating conditions and further clarify the critical dynamics of subcritical transition.

[Biography]

Takahiro Tsukahara is a professor in the Department of Mechanical and Aerospace Engineering at Tokyo University of Science (TUS). He received his PhD in Engineering from TUS in 2007, after which he served as a visiting researcher at KTH in Sweden. He joined TUS as faculty in 2008 and has been a professor since 2022. In 2011, he was also a visiting researcher at the TU Darmstadt in Germany. His research employs DNS to study subcritical transition and turbulent heat transfer in wall-bounded shear flows of Newtonian and viscoelastic fluids. His interests include flow instability, wall turbulence, drag reduction, elasto-inertial turbulence, scalar transport, interfacial and multiphase flows, and machine-learning-based approaches to turbulence modeling. He has authored more than 70 peer-reviewed journal papers. He serves as an Associate Editor of the International Journal of Heat and Fluid Flow and as an Editor of the Journal of Fluid Science and Technology. He has received multiple awards, including the JACM Fellows Award (2024), Outstanding Reviewer recognition from the Journal of Fluid Mechanics (2024), the Frontier Award of JSME (2024), and JSME Medal for Outstanding Paper (2025).