演讲摘要:

The strikingly regular stripe patterns found in transitional Couette flow prominently feature in Feynman’s lecture notes to exemplify humans’ inability to capture the qualitative content of equations. The origin of these laminar turbulent patterns has been widely debated in recent years and is commonly suggested to result from a linear instability of turbulence. Turbulence is here considered as a symmetric and homogeneous state that undergoes a symmetry breaking bifurcation with decreasing Reynolds number.  In the present talk I am demonstrating that the perceived stripe patterns arise bottom up, i.e. with increasing Reynolds number. Instead of an ordered pattern that arises top down from a wavelength instability, stripe patterns are shown to self-assemble bottom up, directly from the directed percolation transition. Notably the latter is an intrinsically stochastic process, incapable of giving rise to a persistent global wavelength. I will further show that as the fully turbulent state is approached, the perceived stripe patterns continue to lack a global wavelength. Moreover the dynamics of the underlying stripes remain inherently stochastic. In particular there is no critical point at higher Re where patterns would transition to homogeneous turbulence.

演讲人简介：

Prof. Björn Hof obtained his PhD in 2001 from the University of Manchester, UK. From 2003 to 2005, he was a Research Associate at the Delft University of Technology, The Netherlands, and from 2005 to 2007, a lecturer at the University of Manchester. From 2007 to 2013, he led a research group at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany. Since 2013, he has been Professor at the Institute of Science and Technology Austria (ISTA). In 2017, he was elected a Fellow of the American Physical Society (APS).

Prof. Hof and his research group investigate how complex chaotic dynamics develop in nonlinear systems, of which fluid turbulence is probably the most common and at the same time most relevant example. Their approach is to investigate turbulence when it first arises from laminar motion. By combining detailed laboratory experiments with highly resolved computer simulations and apply methods from nonlinear dynamics and statistical physics, they were able to decipher key aspects of the transition scenario and identify universal features shared with out-of-equilibrium systems in many other areas of physics. Their hope is that their approach could eventually lead to a more fundamental understanding of the nature of turbulence going beyond a purely statistical view.

Prof. Hof has made important contributions to understanding the transition from laminar flow to turbulence, as evidenced by publications in several high-impact journals, including three in Science, three in Nature, three in Annual Review of Fluid Mechanics, three in Nature Physics, two in Nature Communications, three in PNAS, and thirteen in Physical Review Letters.

审核：刘有晟、游小清