Research
FEARLESS ( Fluid mEchanics with Adaptively Refined Large- Eddy SimulationS )
Overview
Vorticity of compressively driven
isothermal turbulence
We plan to develop, implement, and apply a new numerical scheme for modeling turbulent, multiphase astrophysical flows such as galaxy cluster cores and star forming regions. The method combines the capabilities of adaptive mesh refinement (AMR) and large-eddy simulations (LES) to capture localized features and to represent unresolved turbulence, respectively; it will therefore be referred to as Fluid mEchanics with Adaptively Refined Large-Eddy SimulationS or FEARLESS. Recent advances in the field of dynamical subgrid-scale (SGS) models for LES of thermonuclear supernovae made by our group enable us for the first time to formulate a self-consistent SGS model on adaptive meshes based on local similarity arguments for turbulent transport. We are currently testing and verifying the implementation of the full dynamical SGS model into the existing AMR hydrocode Enzo. Meanwhile, our group continues working on some of the hottest astrophysical problems and is ready to apply FEARLESS in simulations of turbulence regulated star formation in galaxy scale simulations and galaxy cluster turbulence simulations in collaboration with international experts in these fields.
Projects & Research Goals
Our main research goals and associated projects:
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Numerical implementation and validation of FEARLESS. [ A. Maier, W. Schmidt, J. Niemeyer ]
The implementation uses the existing public domain framework Enzo, an AMR hydrocode coupled to an N-body solver for the gravitational field of cold dark matter. The benchmark test for this phase of the project is a simulation of forced turbulence with a controlled sequence of refinements and derefinements, compared to the case with fixed resolution at the highest refinement level.
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Development and test of new AMR criteria, designed for refining turbulent flows [ L. Iapichino, J. Adamek, W. Schmidt, J. Niemeyer ]
As a preparatory work for FEARLESS, the problem of the resolution of turbulent flows AMR simulations is investigated by means of 3D hydrodynamical simulations in an idealised setup, representing a moving subcluster during a merger event. We demonstrate that the newly developed AMR criteria provide a better resolution of the flow past the subcluster, allowing us to follow the onset of the shear instability, the evolution of the turbulent wake and the subsequent back-reaction on the subcluster core morphology. We further investigate some implications for the modelling of cluster cold fronts. MNRAS 388 (2008) 1079
Slices (xz plane) showing density (left) and square of the vorticity modulus with the mesh superimposed (right), at t = 2 Gyr, in an idealised subcluster simulation. Grids of AMR levels from 0 to 3 are rendered as mesh structures, whereas for ease of visualisation grids of level 4 and 5 are only rendered with colours green and blue, respectively.
Application of FEARLESS to the following astrophysical problems:- forced supersonic self-gravitating turbulence [ W. Schmidt, M. Hupp, J. Niemeyer ]
please refer to your DECI Report - AMR cosmological simulations of galaxy clusters and study of turbulence in the intra-cluster medium [ L. Iapichino, J. Niemeyer ]
The development of turbulent gas flows in the ICM and in the core of a galaxy cluster is studied by means of AMR cosmological simulations. A series of six runs was performed, with the same simulation parameters but with different criteria for triggering the mesh refinement. In runs using the newly developed AMR criteria, or a more accurate tracking of the overdense subclumps, we get an improved resolution of the turbulent flow in the ICM. We show that the turbulent flow is not highly volume-filling, but has a large area-covering factor, in agreement with previous theoretical expectations. The measured turbulent velocity in the cluster core is larger than 200 km/s and the level of turbulent pressure contribution to the cluster hydrostatic equilibrium is increased by using the improved AMR criteria. MNRAS 388 (2008) 1089
Slice (xz plane) at z = 0, showing a detail of the simulated cluster and its
outskirts, with an ongoing minor mergers at (0.498; 0.502), in a simulation
using the new AMR criteria. The coordinates are in code units (1 = 128
Mpc/h; Rvir ~ 0.01). The square of the vorticity modulus is colour coded,
with baryon density contours superimposed in black.
- turbulence regulated star formation in isolated disk galaxies [ M. Hupp, W. Schmidt, J. Niemeyer ]
Taking advantage of the unique capabilities of the FEARLESS code currently developed in our group, we intend to implement a theory for turbulence regulated star formation recently proposed by Krumholz and McKee (2005) as a subgrid-scale (SGS) model in galaxy simulations. FEARLESS combines SGS modeling of unresolved turbulence with adaptive mesh refinement (AMR) in the cosmological hydrocode ENZO. It provides a local measure for the kinetic energy of unresolved turbulence which is required to calculate the local star formation rate if star formation occurs in virialized, supersonically turbulent gas clouds. read more
Ongoing star formation in an isolated galaxy
- forced supersonic self-gravitating turbulence [ W. Schmidt, M. Hupp, J. Niemeyer ]
Mean turbulent energy over time for four static grid simulations of driven turbulence compared to one AMR simulation with four levels of refinement.
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Comparison of the numerical results with previous simulations, analysis of the performance and utility of FEARLESS, and appropriate follow-up projects.
Hydrodynamics & Associated Projects
Furthermore, our current endeavors comprise :
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Turbulence emerging from colliding
gas flows in an adiabatic simulation
In our simulations we use the advantages of AMR to ease the study of turbulent converging gas flows. Here neutral atomic hydrogen (HI) as a thermally bistable medium condenses from a warm diffuse phase into a cold and dense phase. Out of that cold dense phase structures like molecular clouds and later on galaxies and stars can form.
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Application of CLUMPFIND to a simulation
of driven isothermal turbulence
The simulation of supersonic turbulent flows makes it possible to study the effects of turbulence in star forming regions. An implementation of the CLUMPFIND algorithm (see Williams et al. (1994)) is used to identify overdense regions in simulations of turbulent molecular clouds in order to calculate the core mass function (CMF) which is thought to be related to the stellar initial mass function (IMF).
The FEARLESS Team
Universität Würzburg:
Jens Niemeyer (PI )
Christian Klingenberg
Wolfram Schmidt
Markus Hupp
Andreas Maier
Sebastian Kern
Markus Niklaus
Universität Heidelberg (ITA):
Luigi Iapichino
Christoph Federrath
Collaborators
Enzo group (Laboratory for Computational Astrophysics, University of California, San Diego)