Extreme events in fluid flows from atmospheric to engineering scales have a strong social and economic impact, and hence there is a need to develop models for their early prediction. The difficulty here stems from the intrinsic sensitivity to initial conditions of the governing equations and from the fact that, by definition, extreme events occur rarely. This makes their characterization, let alone their prediction, challenging. In this talk, the mechanisms and the prediction of extreme events in fluid flows are examined from a dynamical-system standpoint. Specifically, massive ensembles of direct numerical simulations of the Navier—Stokes equations are used to exhaust the phase space of the system. This approach will be exemplified for three fundamental fluid flows.

We will begin with the transition to turbulence in pipe flow, which has been an open problem since the groundbreaking experiments of Osborne Reynolds. Here, the competition of extreme events of opposite nature, turbulence proliferation and extinction, leads to a well-defined critical point for the onset of turbulence. Second, the dynamics of dilute turbulent emulsions will be addressed. It will be shown that extreme drop breakup events determine the asymptotic evolution of the emulsion. The breakup process is driven by outer eddies that can generate a large strain at the drop surface. Finally, the occurrence of extreme dissipation events in a two-dimensional turbulent flow will be studied. In this case, the large-scale circulation patterns set the predictability limit of extreme dissipation events.

**Bio**

Marc Avila studied Mathematics at the Universitat Autònoma de Barcelona and at the University of Glasgow and got his PhD in Applied Physics and Scientific Computing from the Universitat Politècnica de Catalunya in 2008. During his PhD, he was a research scholar at Arizona State University and subsequently a postdoc at the Max-Planck-Institute for Dynamics and Self-Organization. Prior to becoming Professor of fluid mechanics at the University of Bremen, and Director of its Center of Applied Space Technology and Microgravity (ZARM), he was Professor at the University of Erlangen-Nuremberg. For his work on the transition to turbulence in pipe flow, he received the Euromech Young Scientist Award in 2009 and the Richard-von-Mises Prize of the GAMM in 2018. He enjoys applying dynamical-systems approaches to understand and predict the dynamics of transitional and turbulent flows, and more recently of multiphase flows.

https://www.uni-bremen.de/en/humans-on-mars-initiative/team/coordination-team/prof-dr-marc-avila

**Colloquium
Wednesday, March 6
1:30pm
WXLR A206**

Marc Avila

Director

Center of Applied Space Technology and Microgravity

Professor of fluid mechanics

University of Bremen