Catálogo de publicaciones - libros

We have simulated the interaction of jets with a galactic wind at high resolution using the magnetohydrodynamics code NIRVANA on the NEC SX-6 at the HLRS. This setup may describe a typical situation for the starbursting radio galaxies of the early universe. The results show a clear resolution dependence in the expected way, but the formed clumps are denser than expected from linear extrapolation. We also report our recent progress in the adaptation of the magnetic part of NIRVANA to the SX-6. The code is now fully tuned to the machine and reached more than 3 Gflops. We plan to use this new code version to extend our study of magnetized jets down to very low jet densities. This should be especially applicable to the conditions in the young universe.

In this project we addresses the nature of host galaxies of gammaray bursts (GRBs) through numerical simulations of galaxy formation. GRBs are the most energetic events in the universe and those longer than about 2 seconds are thought to result from the core collapse and explosion of massive stars [Pir05]. Because of the cosmologically short lifetime of the massive progenitors, GRBs are generally considered to be powerful tracers of the massive star formation history of the universe. They can therefore also be expected to provide useful insights into the understanding of the galaxy formation process. The current interest in GRB research started in 1997 with the discovery of their optical afterglow emission. To date over 40 optical detections have been made, but the number of GRB host galaxy observations is still rather limited with only about 20 positive detections.

We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using cluster-perturbation theory and the self-energy-functional approach, we calculate the spectral function of the Hubbard model with next-nearest neighbor electronic hopping amplitude ′ = -0.35 and on-site interaction = 8 at half filling and doping levels ranging from = 0.077 to = 0.20. We show that a comprehensive description of the single particle spectrum of the electron doped cuprates is only possible within a strongly correlated model. Weak coupling approaches that are based upon a collapse of the Mott gap by vanishing on-site interaction are ruled out.

We used the high performance systems at HLRS in the last year for two different works. One was the hybrid implementation and optimization of ParaSPH a library for the parallel computation of particle simulations. We give details of the parallelization of ParaSPH for hybrid architectures (clustered SMPs) using MPI and OpenMP and discuss performance results for single node and speedups of the code on the Opteron Cluster and the NEC SX-6. The other work we present is the enhancement of an object-oriented framework for messages passing called TPO++ to support object-oriented parallel I/O.

The range of applications using our libraries and methods is extended continuously. At present it covers many different astrophysical phenomena, basically accretion procedures, impact simulations, the simulation of brittle material and the simulation of separated two phase flows, for example the injection of diesel jets. Recently we work on a solution to simulate granular media with SPH.

A variety of quantum Monte Carlo algorithms are used to study the equilibrium properties of strongly correlated quantum systems relevant to the fields of high-T superconductivity and magnetism. Furthermore, a new exact numerical method was developed and applied to strongly correlated quantum gases to unveil their universal properties in equilibrium and new states of matter out of equilibrium.

Surface optical spectroscopies are non-destructive and capable of operation within a wide range of environments. Their potential for materials characterization can only be exploited fully, however, when the physical mechanisms giving rising to optical features are well understood. Here we use large-scale numerical simulations to investigate two highly relevant and at the same time prototypical cases from : (i) the origin of the optical anisotropy oscillations accompanying the thermal oxidation of Si(001) and (ii) the modification of the Si(001) surface optical response upon adsorption of 9,10-phenanthrenequinone. It is demonstrated to what extent strain, molecular transitions and adsorption-modified Si bulk wave functions contribute to the surface optical anisotropy.

The structural and electronic properties of atomic wires and clusters have been analysed. Structural, energy-, flow-, and elastic- properties of model colloids have been studied with particular emphasis on the effect of external fields and of long ranged dipolar interactions. In the following sections an overview is given on the results of our recent computations on quantum effects, structures and phase transitions in such systems.

This paper details a joint numerical and experimental effort to investigate a transition process in a laminar separation bubble, with the emphasis being put on the numerical contribution. A laminar separation bubble is formed if a laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition, leading to a reattached turbulent boundary layer. Development of disturbances during the transition process in such a separation bubble is studied by means of direct numerical simulation with controlled disturbance input. Focus is put on the stage of non-linear development of these perturbations, for which a detailed comparison between numerical and experimental results is given. Beside physical phenomena like shear-layer roll-up and vortex shedding, computational aspects such as the performance of the numerical code on supercomputers are treated.

Investigations on the spatial evolution of instabilities for hypersonic boundary-layer flows on a flat plate with dissociation are presented. A higher order compact numerical scheme allows for the detailed investigation of the linear and the non-linear evolution of the disturbance waves in the presence of chemical reactions. Compared to the ideal-gas case, lower temperatures in the boundary-layer are present. The three-dimensional disturbance behaviour is experiencing slight damping in the linear regime compared to equilibrium results. High-level disturbances can also lead to local shocklets that are treated with a hybrid ENO-scheme. Experiments for qualitative validation of the results at elevated Mach numbers are available by Mironov.

Since powerful computational ressources are available, numerical simulation is one of the most attractive tools to bridge the gap between the experimental and analytical description of fluid flow phenomena. One of these tools is the (DNS) technique relying on a very high spatial and temporal resolution of fluid systems. Thus, all length scales of a fluid flow, only limited by the grid size, are captured by DNS. Since investigations of many fluid systems require a very fine resolution, considerable progress can only be made by both applying sophisticated numerical methods and using high performance computers. With the inhouse 3D DNS program FS3D (Free Surface 3D) based on the method it is possible to simulate two phase flows of fundamental interest in the automotive and aerospace industry, in meteorology and agriculture but also in the oil industry or medicine.

One part of this study focuses on the numerical simulation of the physical phenomena leading to disintegration of liquid jets. Although many researchers focused in the past on primary breakup phenomena and many experimental, analytical and also numerical results are available, these processes are not well understood. Therefore it was decided to simulate this breakup process by DNS. The results presented here demonstrate the capability of DNS on modern supercomputers. The first numerical simulation results on jet breakup agree qualitatively well with present experimental results. Differences are mainly due to the inflow condition which is a crucial problem for this phenomena.

The other part of this study is dedicated to a cloud microphysical process called ‘collision-induced breakup’ limiting the maximum size of raindrops. This process comprises a binary collision of raindrops and a subsequent disintegration of the coalesced body resulting in creation of a number of smaller fragment drops. The only very few experimental investigations of this process dating from the very past are reexamined by using the advanced fluid-mechanic program FS3D. The advantage of this numerical method is its high accuracy in simulating drop collisions. First results of fragment size distributions due to collisional breakup of raindrops are presented showing a good global agreement with those from laboratory experiments.