Time: 1:20pm, December 17 (Thursday)
Title: The emergence of collective modes, ecological collapse and directed percolation at the laminar-turbulent transition
Speaker: Dr. Hong-Yan Shih
(Institute of Physics, Academia Sinica)

Place: Science Building III 1F SC110

Abstract: 

How a laminar flow becomes turbulence has been an unsolved problem for more than a century and is important in various industrial applications. Recently precise measurements in pipe flow experiments showed non-trivial spatiotemporal complexity at the onset of turbulence where lifetime and splitting time of metastable turbulence do not diverge asymptotically as would have been expected in a sharp transition.
Based on numerical evidence from the hydrodynamics equations, we discovered the surprising fact that the fluid behavior at the transition is governed by the emergent predator-prey dynamics of the important long-wavelength mode, leading to the mathematical prediction that the laminar-turbulent transition is analogous to an ecosystem on the edge of extinction. This prediction demonstrates that the laminar-turbulent transition is a non-equilibrium phase transition in the directed percolation universality class, and provides a unified picture of transition to turbulence emerging in systems ranging from turbulent convection to magnetohydrodynamics.

Reference:
[1] Hong-Yan Shih, Tsung-Lin Hsieh and Nigel Goldenfeld, Nature Physics 12, 245 (2016)
[2] https://www.nature.com/collections/rxsztdqblr