Rick Wagner

The Statistics of Supersonic Isothermal Turbulence

We present results of large-scale three-dimensional simulations of supersonic Euler turbulence with the piecewise parabolic method and multiple grid resolutions up to 2048 3 points. Our numerical experiments describe non-magnetized driven turbulent o ws with an isothermal equation of state and an rms Mach number of 6. We discuss numerical resolution issues and demonstrate convergence, in a statistical sense, of the inertial range dynamics in simulations on grids larger than 512 3 points. The simulations allowed us to measure the absolute velocity scaling exponents for the rst time. The inertial range velocity scaling in this strongly compressible regime deviates substantially from the incompressible Kolmogorov laws. The slope of the velocity power spectrum, for instance, is -1:95 compared to -5=3 in the incompressible case. The exponent of the third-order velocity structure function is 1:28, while in incompressible turbulence it is known to be unity. We propose a natural extension of Kolmogorov’s phenomenology that takes into account compressibility by mixing the velocity and density statistics and preserves the Kolmogorov scaling of the power spectrum and structure functions of the density-weighted velocity v 1=3 u. The low-order statistics of v appear to be invariant with respect to changes in the Mach number. For instance, at Mach 6 the slope of the power spectrum of v v is -1:69, and the exponent of the third-order structure function of v v is unity. We also directly measure the mass dimension of the ifractali density distribution in the inertial subrange, Dm 2:4, which is similar to the observed fractal dimension of molecular clouds and agrees well with the cascade phenomenology. Subject headings: hydrodynamics o instabilities o ISM: structure o methods: numerical o turbulence

On the Density Distribution in Star-forming Interstellar Clouds

We use deep adaptive mesh refinement simulations of isothermal self-gravitating supersonic turbulence to study the imprints of gravity on the mass density distribution in molecular clouds. The simulations show that the density distribution in self-gravitating clouds develops an extended power-law tail at high densities on top of the usual lognormal. We associate the origin of the tail with self-similar collapse solutions and predict the power index values in the range from ?7/4 to ?3/2 that agree with both simulations and observations of star-forming molecular clouds.

Flux correlations in supersonic isothermal turbulence

Using data from a large-scale three-dimensional simulation of supersonic isothermal turbulence, we have tested the validity of an exact flux relation derived analytically from the Navier--Stokes equation by Falkovich, Fouxon and Oz [2010 New relations for correlation functions in Navier--Stokes turbulence. J. Fluid Mech. 644, 465]. That relation, for compressible barotropic fluids, was derived assuming turbulence generated by a large-scale force. However, compressible turbulence in simulations is usually initialized and maintained by a large-scale acceleration, as in gravity-driven astrophysical flows. We present a new approximate flux relation for isothermal turbulence driven by a large-scale acceleration, and find it in reasonable agreement with the simulation results.

Scaling laws and intermittency in highly compressible turbulence

We use large‐scale three‐dimensional simulations of supersonic Euler turbulence to get a better understanding of the physics of a highly compressible cascade. Our numerical experiments describe non‐magnetized driven turbulent flows with an isothermal equation of state and an rms Mach number of 6. We find that the inertial range velocity scaling deviates strongly from the incompressible Kolmogorov laws. We propose an extension of Kolmogorov’s K41 phenomenology that takes into account compressibility by mixing the velocity and density statistics and preserves the K41 scaling of the density‐weighted velocity ν ≡ ρ1/3u. We show that low‐order statistics of ν are invariant with respect to changes in the Mach number. For instance, at Mach 6 the slope of the power spectrum of ν is −1.69 and the third‐order structure function of ν scales linearly with separation. We directly measure the mass dimension of the “fractal” density distribution in the inertial subrange, Dm ≈ 2.4, which is similar to the observed fracta...

QUANTIFYING THE COLLISIONLESS NATURE OF DARK MATTER AND GALAXIES IN A1689

We use extensive measurements of the cluster A1689 to assess the expected similarity in the dynamics of galaxies and dark matter (DM) in their motion as collisionless ‘particles’ in the cluster gravitational potential. To do so we derive the radial profile of the specific kinetic energy of the cluster galaxies from the Jeans equation and observational data. Assuming that the specific kinetic energies of galaxies and DM are roughly equal, we obtain the mean value of the DM velocity anisotropy parameter, and the DM density profile. Since this deduced profile has a scale radius that is higher than inferred from lensing observations, we tested the validity of the assumption by repeating the analysis using results of simulations for the profile of the DM velocity anisotropy. Results of both analyses indicate a significant difference between the kinematics of galaxies and DM within r . 0.3rvir. This finding is reflected also in the shape of the galaxy number density profile, which flattens markedly with respect to the steadily rising DM profile at small radii. Thus, r � 0.3rvir seems to be a transition region interior to which collisional effects significantly modify the dynamical properties of the galaxy population with respect to those of DM in A1689 Subject headings: clusters: galaxies – clusters: individual: A1689

SR-IOV: Performance Benefits for Virtualized Interconnects

The demand for virtualization within high-performance computing is rapidly growing as new communities, driven by both new application stacks and new computing modalities, continue to grow and expand. While virtualization has traditionally come with significant penalties in I/O performance that have precluded its use in mainstream large-scale computing environments, new standards such as Single Root I/O Virtualization (SR-IOV) are emerging that promise to diminish the performance gap and make high-performance virtualization possible. To this end, we have evaluated SR-IOV in the context of both virtualized InfiniBand and virtualized 10 gigabit Ethernet (GbE) using micro-benchmarks and real-world applications. We compare the performance of these interconnects on non-virtualized environments, Amazons SR-IOV-enabled C3 instances, and our own SR-IOV-enabled InfiniBand cluster and show that SR-IOV significantly reduces the performance losses caused by virtualization. InfiniBand demonstrates less than 2% loss of bandwidth and less than 10% increase in latency when virtualized with SR-IOV. Ethernet also benefits, although less dramatically, when SR-IOV is enabled on Amazons cloud.

Exploring large data over wide area networks

Simulations running on the top supercomputers are routinely producing multi-terabyte data sets. Enabling scientists, at their home institutions, to analyze, visualize and interact with these data sets as they are produced is imperative to the scientific discovery process. We report on interactive visualizations of large simulations performed on Kraken at the National Institute for Computational Sciences using the parallel cosmology code Enzo, with grid sizes ranging from 10243 to 64003. In addition to the asynchronous rendering of over 570 timesteps of a 40963 simulation (150 TB in total), we developed the ability to stream the rendering result to multipanel display walls, with full interactive control of the renderer(s).

Using Gordon to accelerate LHC science

The discovery of the Higgs boson by the Large Hadron Collider (LHC) has garnered international attention. In addition to this singular result, the LHC may also uncover other fundamental particles, including dark matter. Much of this research is being done on data from one of the LHC experiments, the Compact Muon Solenoid (CMS). The CMS experiment was able to capture data at higher sampling frequencies than planned during the 2012 LHC operational period. The resulting data had been parked, waiting to be processed on CMS computers. While CMS has significant compute resources, by partnering with SDSC to incorporate Gordon into the CMS workflow, analysis of the parked data was completed months ahead of schedule. This allows scientists to review the results more quickly, and could guide future plans for the LHC.

Storage utilization in the long tail of science

The increasing expansion of computations in non-traditional domain sciences has resulted in an increasing demand for research cyberinfrastructure that is suitable for small- and mid-scale job sizes. The computational aspects of these emerging communities are coming into focus and being addressed through the deployment of several new XSEDE resources that feature easy on-ramps, customizable software environments through virtualization, and interconnects optimized for jobs that only use hundreds or thousands of cores; however, the data storage requirements for these emerging communities remains much less well characterized. To this end, we examined the distribution of file sizes on two of the Lustre file systems within the Data Oasis storage system at the San Diego Supercomputer Center (SDSC). We found that there is a very strong preference for small files among SDSCs users, with 90% of all files being less than 2 MB in size. Furthermore, 50% of all file system capacity is consumed by files under 2 GB in size, and these distributions are consistent on both scratch and projects storage file systems. Because parallel file systems like Lustre and GPFS are optimized for parallel IO to large, widestripe files, these findings suggest that parallel file systems may not be the most suitable storage solutions when designing cyberinfrastructure to meet the needs of emerging communities.

Direct Numerical Simulations of Cosmological Reionization: Field Comparison: Density

The light from early galaxies had a dramatic impact on the gasses filling the universe. This video highlights the spatial structure of the lights effect, by comparing two simulations: one with a self-consistent radiation field (radiative), and one without (non-radiative), each with a very high dynamic range. Looking at the simulations side-by-side its hard to see any difference. However, because the simulations have the same initial conditions, we can directly compare them, by looking at the relative difference of the density. The coral-like blobs are regions where light has radiated out, heating the gas, and raising the pressure. The red regions show where the density is much higher in the radiative simulation, while the yellow regions are where the non-radiative has more density, showing where gravity was able to pull the filaments into tighter cylinders, without having to work against pressure from stellar heating. This is the first known visualization of this process, known as Jeans smoothing.