Abstract: How turbulence arises in simple shear flows, such as pipes and channels has been an open question for over a century. In these cases turbulence arises despite the linear stability of the laminar flow and transition is caused by finite amplitude perturbations. Despite numerous experimental and theoretical studies it has not been possible to determine a well-defined critical point nor to clarify the nature of the transition. It will be shown for the examples of pipe and Couette flow that the onset of sustained turbulence is a nonequilibrium phase transition. The critical point is determined by resolving the extremely long time scales of the underlying growth and decay processes. By detailed numerical simulations and experimental measurements close to the transition point we determine the critical exponents and show that this transition falls into the directed percolation universality class. 

About the speaker: Prof. Björn Hof obtained his Ph.D in 2001 from University of Manchester, UK. He then successively took the positions as a Research Associate in Delft University of Technology, The Netherlands and a Lecturer in University of Manchester, UK, and a Research Group Leader at the Max Planck Institute for Dynamics and Self-Organization, Germany. In 2013, he accepted the offer as a Professor in IST Austria and stayed there since.

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 the understanding of the transition from laminar flow to turbulence, which is reflected in publications in several high-impact journals, including 3 in Science, 2 in Nature, 1 in Nature Physics, 1 in PNAS, and 7 in Physical Review Letters.