Convection processes can be found almost everywhere. Think of air condition systems in buildings, the generation of magnetic fields in the Earth and Sun, or the formation of clouds in the atmosphere. The figure is a view from the top onto a flat cylindrical Rayleigh-Benard convection cell, one of the simplest convection systems. The streamlines of the time-averaged velocity field are plotted in red. They indicate a complex large-scale circulation pattern consisting of pentagons which evolve in convective turbulence. One of the big miracles in convection is how these patterns grow out of the stochastic turbulent fluid motion and how they affect the turbulent heat transport through the cell. A way to attempt this problem is by a drastic reduction of the degrees of freedom in the underlying mathematical model. Such a so-called low-dimensional system can then be analysed with very powerful instruments of the theory of dynamical systems. Furthermore, we study the impact of moisture on thermal convection which allows for additional condensation and evaporation. This aspect is important for a better understanding of the formation, shape and lifetime of atmospheric clouds.
Current Collaborations:
Andre Thess Ilmenau (Department of Mechanical Engineering, University of Technology)
Eberhard Bodenschatz (Max Planck Institute for Dynamics and Self-Organization Goettingen)
Holger Siebert (Institute for Tropospheric Research Leipzig)
Raymond A. Shaw (Physics Department,Michigan Technological University Houghton)
Olivier Pauluis (Courant Institute for Mathematical Sciences, New York University)

The figure shows an isovolume plot of the vorticity magnitude in a direct numerical simulation of homogeneous isotropic turbulence on a computational grid of about 10 billion grid points. Visible are elongated structures, so-called vortex tubes. One fundamental question is related to the growth rates of such vortex filaments. In addition, we study the mixing of active and passive scalar fields in turbulence and the decay of large-scale anisotropies in a turbulent flow. The latter is important for the development of subgrid-scale models of turbulence that are frequently necessary in computational fluid dynamics. The massively parallel numerical computations are carried out at the Sun Cluster of the University Computing Center and on the JUMP IBM Power 6 Cluster of the Juelich Supercomputing Centre in Germany. The animation shows results of such a supercomputer simulation of homogeneous isotropic turbulence on a computational grid with 1 billion mesh points.
Current Collaborations:
Bruno Eckhardt (Physics Department, Philipps University Marburg)
Katepalli R. Sreenivasan (ICTP Trieste),
Victor Yakhot (Aerospace and Mechanical Engineering Department, Boston University),
Werner J. A. Dahm (Aerospace Engineering Department, The University of Michigan)

Minute amounts of polymers can change the properties of fluids dramatically. The fluid looses its Newtonian character, it becomes a viscoelastic fluid. An important example is the reduction of the turbulent drag by adding minute amounts of long-chained polymers to a turbulent flow. The figure shows a snapshot of a model simulation on the stretching behaviour of polymers in a turbulent shear flow. Polymers are modelled as 2 beads that are connected by a nonlinear spring force. A new class of so-called elastic instabilities has numerous potential applications, e.g. for the mixing in microfluidic devices.
Current Collaborations:
Burkhard Dünweg (Max Planck Institute for Polymer Research Mainz)