A Nested Grid Particle-Mesh Code for High Resolution Simulations of Gravitational Instability in Cosmology

I describe a nested-grid particle-mesh (NGPM) code designed to study gravitational instability in three-dimensions. The code is based upon a standard PM code. Within the parent grid I am able to define smaller sub-grids allowing us to substantially extend the dynamical range in mass and length. I treat the fields on the parent grid as background fields and utilize a one-way interactive meshing. Waves on the coarse parent grid are allowed to enter and exit the subgrid, but waves from the subgrid are precluded from effecting the dynamics of the parent grid. On the parent grid the potential is computed using a standard multiple Fourier transform technique. On the subgrid I use a Fourier transform technique to compute the subgrid potential at high resolution. I impose quasi-isolated boundary conditions on the subgrid using the standard method for generating isolated boundary conditions, but rather than using the isolated Green function I use the Ewald method to compute a Green function on the subgrid which possesses the full periodicity of the parent grid. I present a detailed discussion of my methodology and a series of code tests.Comment: 13 pages, 10 figures included, uses mn.sty & epsf.sty. Accepted by MNRAS. This is the final refereed versio

The Superiority of the Minimal Spanning Tree in Percolation Analyses of Cosmological Datasets

In this work we demonstrate the ability of the Minimal Spanning Tree to duplicate the information contained within a percolation analysis for a point dataset. We show how to construct the percolation properties from the Minimal Spanning Tree, finding roughly an order of magnitude improvement in the computer time required. We apply these statistics to Particle-Mesh simulations of large-scale structure formation. We consider purely scale-free Gaussian initial conditions ($P(k) \propto k^n$, with $n = -2, -1, 0 \ \& +1$) in a critical density universe. We find in general the mass of the percolating cluster is a much better quantity by which to judge the onset of percolation than the length of the percolating cluster.Comment: 7 pages, 4 figures included, uses mn.sty & epsf.sty. Accepted by MNRA

A Test of the Particle Paradigm in N-Body Simulations

We present results of tests of the evolution of small ``fluid elements'' in cosmological N--body simulations, to examine the validity of their treatment as particles. We find that even very small elements typically collapse along one axis while expanding along another, often to twice or more their initial comoving diameter. This represents a possible problem for high--resolution uses of such simulations.Comment: Uses aasms4.sty; accepted for publication in ApJ Letters. Files available also at ftp://kusmos.phsx.ukans.edu/preprints/ates

Heat Sponge: A Concept for Mass-Efficient Heat Storage

The heat sponge is a device for mass-efficient storage of heat. It was developed to be incorporated in the substructure of a re-entry vehicle to reduce thermal- protection-system requirements. The heat sponge consists of a liquid/vapor mixture contained within a number of miniature pressure vessels that can be embedded within a variety of different types of structures. As temperature is increased, pressure in the miniature pressure vessels also increases so that heat absorbed through vaporization of the liquid is spread over a relatively large temperature range. Using water as a working fluid, the heat-storage capacity of the liquid/vapor mixture is many times higher than that of typical structural materials and is well above that of common phase change materials over a temperature range of 200 F to 700 F. The use of pure ammonia as the working fluid provides a range of application between 432 deg R and 730 deg R, or the use of the more practical water-ammonia solution provides a range of application between 432 deg R and 1160 deg R or in between that of water and pure ammonia. Prototype heat sponges were fabricated and characterized. These heat sponges consisted of 1.0-inch-diameter, hollow, stainless-steel spheres with a wall thickness of 0.020 inches which had varying percentages of their interior volumes filled with water and a water-ammonia solution. An apparatus to measure the heat stored in these prototype heat sponges was designed, fabricated, and verified. The heat-storage capacity calculated from measured temperature histories is compared to numerical predictions

Fundamental Discreteness Limitations of Cosmological N-Body Clustering Simulations

We explore some of the effects that discreteness and two-body scattering may have on N-body simulations with ``realistic'' cosmological initial conditions. We use an identical subset of particles from the initial conditions for a $128^3$ Particle-Mesh (PM) calculation as the initial conditions for a variety P$^3$M and Tree code runs. We investigate the effect of mass resolution (the mean interparticle separation) since most ``high resolution'' codes only have high resolution in gravitational force. The phase-insensitive two--point statistics, such as the power spectrum (autocorrelation) are somewhat affected by these variations, but phase-sensitive statistics show greater differences. Results converge at the mean interparticle separation scale of the lowest mass-resolution code. As more particles are added, but the force resolution is held constant, the P$^3$M and the Tree runs agree more and more strongly with each other and with the PM run which had the same initial conditions. This shows high particle density is necessary for correct time evolution, since many different results cannot all be correct. However, they do not so converge to a PM run which continued the fluctuations to small scales. Our results show that ignoring them is a major source of error on comoving scales of the missing wavelengths. This can be resolved by putting in a high particle density. Since the codes never agree well on scales below the mean comoving interparticle separation, we find little justification for quantitative predictions on this scale. Some measures vary by 50%, but others can be off by a factor of three or more. Our results suggest possible problems with the density of galaxy halos, formation of early generation objects such as QSO absorber clouds, etc.Comment: Revised version to be published in Astrophysical Journal. One figure changed; expanded discussion, more information on code parameters. Latex, 44 pages, including 19 figures. Higher resolution versions of Figures 10-15 available at: ftp://kusmos.phsx.ukans.edu/preprints/nbod

Demonstrating Discreteness and Collision Error in Cosmological N-body Simulations of Dark Matter Gravitational Clustering

Two-body scattering and other discreteness effects are unimportant in cosmological gravitational clustering in most scenarios, since the dark matter has a small particle mass. The collective field should determine evolution: Two-body scattering in simulations violates the Poisson-Vlasov equations. We test this in PM, P$^3$M, Tree, and NGPM codes, noting that a collisionless code will preserve the one-dimensional character of plane wave collapse. We find collisionality vanishing as the softening parameter approaches the mean interparticle separation. Solutions for the problem are suggested, involving greater computer power, PM-based nested grid codes, and a more conservative approach to resolution claims.Comment: Final version accepted for ApJ Letters. Minor revisions, including due to bug fix in tree code. Uses aasms4.sty. 15 pages. Higher resolution figures available at ftp://kusmos.phsx.ukans.edu/preprints/discret

De zoekende ondernemer; Een studie naar het netwerken in de agrosector

Het doel van het project 'het ontwikkelen en toetsen van interventietrajecten om het ondernemerschap in de agrarische sector te versterken, met speciale aandacht voor de versterking van netwerkcompetenties' is in vier stappen uitgewerkt. Allereerst is een theoretische verkenning uitgevoerd rond ondernemerschap(scompetenties), strategisch management en het netwerken. Vervolgens zijn diepte-interviews gehouden en is een uitgebreide interventiestudie uitgevoerd. Ten slotte zijn de ervaringen bij de opzet en uitvoering èn de bevindingen bediscussieerd met vertegenwoordigers van adviesorganisaties en kennisinstellingen. The aim of the project 'the development and testing of intervention trajectories aimed at strengthening entrepreneurship in the agricultural sector, with special focus on strengthening networking competences' was developed in four steps. First of all, a theoretical study was conducted into entrepreneurship and entrepreneurial competences, strategic management and networking activities. This was followed by in-depth interviews and a detailed intervention study. Finally, the experiences in setting up and implementing the project as well as the findings were discussed with representatives of advisory organisations and educational institutions