Parallel Temperatures in Supersonic Beams: Ultra Cooling of Light Atoms seeded in a Heavier Carrier Gas

We have found recently that, in a supersonic expansion of a mixture of two monoatomic gases, the parallel temperatures of the two gases can be very different. This effect is large if the seeded gas is highly diluted and if its atomic mass is considerably smaller than the one of the carrier gas. In the present paper, we present a complete derivation of our theoretical analysis of this effect. Our calculation is a natural extension of the existing theory of supersonic cooling to the case of a gas mixture, in the high dilution limit. Finally, we describe a set of temperature measurements made on a beam of lithium seeded in argon. Our experimental results are in very good agreement with the results of our calculation.Comment: 24 novembre 200

Direct Simulation Monte Carlo for astrophysical flows: I. Motivation and methodology

We describe a hybrid Direct Simulation Monte Carlo (DSMC) code for simultaneously solving the collisional Boltzmann equation for gas and the collisionless Boltzmann equation for stars and dark matter for problems important to galaxy evolution. This project is motivated by the need to understand the controlling dynamics at interfaces between gases of widely differing densities and temperature, i.e. multiphase media. While more expensive than hydrodynamics, the kinetic approach does not suffer from discontinuities and it applies when the continuum limit does not, such as in the collapse of galaxy clusters and at the interface between coronal halo gas and a thin neutral gas layer. Finally, the momentum flux is carried, self-consistently, by particles and this approach explicitly resolves and thereby captures shocks. The DSMC method splits the solution into two pieces: 1) the evolution of the phase-space flow without collisions; and 2) the evolution governed the collision term alone without phase-space flow. This splitting approach makes DSMC an ideal match to existing particle-based n-body codes. If the mean free path becomes very small compared to any scale of interest, the method abandons simulated particle collisions and simply adopts the relaxed solution in each interaction cell consistent with the overall energy and momentum fluxes. This is functionally equivalent to solving the Navier-Stokes equations on a mesh. Our implementation is tested using the Sod shock tube problem and the non-linear development of an Kelvin-Helmholtz unstable shear layer.Comment: 13 pages, 4 figures, submitted to MNRAS, revised figures, corrected typos, and incorporated comment

Worm Algorithm for Problems of Quantum and Classical Statistics

This is a chapter of the multi-author book "Understanding Quantum Phase Transitions," edited by Lincoln Carr and published by Taylor and Francis. In this chapter, we give a general introduction to the worm algorithm and present important results highlighting the power of the approachComment: 27 pages, 15 figures, chapter in a boo

MIXTURE FRACTION IMAGING BASED ON PHOTODISSOCIATION SPECTROSCOPY AND TWO PHOTON LASER INDUCED FLUORESCENCE

The study of turbulent combustion calls for new diagnostics that can measure multidimensional mixture fraction under a wide range of flame conditions. A laser diagnostic technique based on photodissociation spectroscopy (PDS) is proposed to address this need. This thesis describes the concept of the PDS-based diagnostic, reports its experimental demonstration in a non-premixed jet flame, and assesses its performance and applicable range. The two-photon laser induced fluorescence (TPLIF) technique used in conjugate with the PDS is analyzed numerically in line and planar imaging configuration. The new mixture fraction imaging technique is centered around the creative use of photodissociation (PD) for flow visualization. A carefully chosen PD precursor is seeded into the flow of interest to measure mixture fraction. The precursor is chosen such that 1) both the precursor itself and the products formed from the precursor (if it reacts) can be completely and rapidly photodissociated; thus one of the photofragments forms a conserved scalar and can be used to infer the mixture fraction, and 2) the target photofragment offers friendly spectroscopic properties (e.g., strong laser induced fluorescence signals and/or simple signal interpretation) so multidimensional imaging can be readily obtained. Molecular iodine (I2) was identified as a precursor satisfying both requirements and was seeded into a carbon monoxide (CO)/air jet flame for single-shot two-dimensional imaging of mixture fraction. This demonstration illustrates the potential of the PDS-based technique to overcome the limitations of existing techniques, and to provide multidimensional measurements of mixture fraction in a variety of reactive flows. The thesis also analyzes the imaging applications of TPLIF, which is a promising technique in the planar imaging of mixture fraction. Models are developed based on rate equation approximations and Monte Carlo simulation, with a focus on the effect of amplified spontaneous emission (ASE) on TPLIF signal interpretation. Results obtained are expected to also enhance the accuracy and applicable range of TPLIF technique in other flow imaging applications, beyond the mixture fraction imaging considered in this research

Investigation of a New Monte Carlo Method for the Transitional Gas Flow

Abstract. The Direct Simulation Monte Carlo method (DSMC) is well developed for rarefied gas flow in transition flow regime when 0.01<Kn<1. However, such a simulation for a complex 3D vacuum system is still a challenging task because of the huge demand on the memory and long computational time. On the other hand, if Kn>10, the gas flow is free molecular and can be simulated by the Test Particle Monte Carlo method (TPMC) without any problem even for a complex 3D vacuum system. In this paper we will investigate the approach to extend the TPMC to transition flow regime by considering the collision between gas molecules as an interaction between a probe molecule and the gas background. Recently this collision mechanism has been implemented into ProVac3D, a new TPMC simulation program developed by KIT. The preliminary simulation result shows a correct nonlinear increasing of the gas flow. However, there is still a quantitative discrepancy with the experimental data, which means further improvement is needed

Investigation of a New Monte Carlo Method for the Transitional Gas Flow

Abstract. The Direct Simulation Monte Carlo method (DSMC) is well developed for rarefied gas flow in transition flow regime when 0.01<Kn<1. However, such a simulation for a complex 3D vacuum system is still a challenging task because of the huge demand on the memory and long computational time. On the other hand, if Kn>10, the gas flow is free molecular and can be simulated by the Test Particle Monte Carlo method (TPMC) without any problem even for a complex 3D vacuum system. In this paper we will investigate the approach to extend the TPMC to the transition flow regime by considering the collision between gas molecules as an interaction between a probe molecule and the gas background. Recently this collision mechanism has been implemented into ProVac3D, a new TPMC simulation program developed by Karlsruhe Institute of Technology (KIT). The preliminary simulation result shows a correct nonlinear increasing of the gas flow. However, there is still a quantitative discrepancy with the experimental data, which means further improvement is needed

Progress in particle-based multiscale and hybrid methods for flow applications

This work focuses on the review of particle-based multiscale and hybrid methods that have surfaced in the field of fluid mechanics over the last 20 years. We consider five established particle methods: molecular dynamics, direct simulation Monte Carlo, lattice Boltzmann method, dissipative particle dynamics and smoothed-particle hydrodynamics. A general description is given on each particle method in conjunction with multiscale and hybrid applications. An analysis on the length scale separation revealed that current multiscale methods only bridge across scales which are of the order of O(102)−O(103) and that further work on complex geometries and parallel code optimisation is needed to increase the separation. Similarities between methods are highlighted and combinations discussed. Advantages, disadvantages and applications of each particle method have been tabulated as a reference

The evolution of massive black hole seeds

We investigate the evolution of high redshift seed black hole masses at late times and their observational signatures. The massive black hole seeds studied here form at extremely high redshifts from the direct collapse of pre-galactic gas discs. Populating dark matter halos with seeds formed in this way, we follow the mass assembly of these black holes to the present time using a Monte-Carlo merger tree. Using this machinery we predict the black hole mass function at high redshifts and at the present time; the integrated mass density of black holes and the luminosity function of accreting black holes as a function of redshift. These predictions are made for a set of three seed models with varying black hole formation efficiency. Given the accuracy of current observational constraints, all 3 models can be adequately fit. Discrimination between the models appears predominantly at the low mass end of the present day black hole mass function which is not observationally well constrained. However, all our models predict that low surface brightness, bulgeless galaxies with large discs are least likely to be sites for the formation of massive seed black holes at high redshifts. The efficiency of seed formation at high redshifts has a direct influence on the black hole occupation fraction in galaxies at z=0. This effect is more pronounced for low mass galaxies. This is the key discriminant between the models studied here and the Population III remnant seed model. We find that there exists a population of low mass galaxies that do not host nuclear black holes. Our prediction of the shape of the black hole mass - velocity dispersion relation at the low mass end is in agreement with the recent observational determination from the census of low mass galaxies in the Virgo cluster.Comment: MNRAS in pres