**INHALTE**

## Forschung

# Thermal Convection

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 cylindrical Rayleigh-Benard convection cell, one of the simplest convection systems. The streamlines of the instantaneous velocity field are plotted. They indicate a complex large-scale circulation pattern which evolves 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:**

Janet D. Scheel (Occidental College Los Angeles),

Holger Siebert (Leibniz-Institut für Troposphärenforschung Leipzig),

Raymond A. Shaw (Michigan Technological University Houghton),

Olivier Pauluis (Courant Institute for Mathematical Sciences, New York University)

# Magnetohydrodynamics

In metallurgical applications, magnetic fields can be used to brake, stir and control the flow of liquid metals. Examples include the continuous casting of steel or crystal growth, where both static and alternating fields are routinely applied. An understanding of the physics of such magnetohydrodynamic flows can be obtained by accurate numerical simulations. We are interested in the transition to turbulence in simple wall-bounded flows in channels, ducts and pipes, which we study by stability analyses and direct numerical simulations. Simulations are also used to investigate properties of developed turbulence in such flows. For most cases, the magnetic field has no independent dynamics because magnetic diffusion is dominant (quasistatic limit).

In the near future we shall also employ the full induction equation for flows with rapid transients. For the simulations we use our own spectral- and finite-difference methods on structured meshes that allow for an efficient parallelization.

**Current Collaborations:**Oleg Zikanov (University of Michigan Dearborn)

Bernard Knaepen (Universite Libre Bruxelles)

Leo Bühler (Karlsruher Institut für Technologie)

# Marangoni Convection

Solutal Marangoni convection is based on the dependence of interfacial tension on the concentration of a solute. It can significantly enhance the mass transfer rate through a liquid-liquid and liquid-gas interface and is therefore important for extraction processes in chemical engineering. It also produces complex flow patterns at the interface between immiscible liquids with different concentrations of the solute, which are known as interfacial turbulence. We study these phenomena in solutal Marangoni convection by high-resolution direct numerical simulations with a pseudospectral method for a system of two liquid layers separated by a planar interface. Because the system is closed, i.e. there is no solute added or removed, the evolution is transient, and progresses through different, distinct stages.

Simulations allow us to analyze the physical mechanisms responsible for the particular flow structures and and their transformations.

**Current Collaborations:**Kerstin Eckert (Technische Universität Dresden)

Maurice Rossi (Universite Pierre et Marie Curie Paris(VI))

Frederic Doumenc (Universite Pierre et Marie Curie Paris (VI))

# Turbulence

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 "Rapid Groth Events of Enstrophy in Fluid Turbulence" on a computational grid with 2048 mesh points in each direction of a square box.

**Current Collaborations:**

Bruno Eckhardt (Philipps Universität Marburg),

Katepalli R. Sreenivasan (New York University),

Victor Yakhot (Boston University)