Rotational and Vibrational Nonequilibrium in a Low Diffusion Particle Method for Continuum Flow Simulation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77353/1/AIAA-2009-3743-194.pd

Cold atoms and molecules

The ability to cool and trap atoms has revolutionized atomic and ultra-cold physics. Molecular physics is currently undergoing a similar transformation. This thesis aims to research a general cooling method that will be applicable to wide range of molecular and atomic species and other particles. We studied the dynamics of molecules in optical fields, focusing in particular on exploring the molecular self-organisation phenomena in optical cavities to cool molecular ensembles to sub-mK temperatures. Firstly, the general model of cavity cooling from atoms to molecules and the dynamics of a particle in a single cavity mode were discussed. We extended the existing cooling scheme for two-level atoms to an ensemble of multi-level molecules. Then we studied the spatial dynamics of molecules in the new parameter conditions, focusing in particular on exploring the molecular self-organisation phenomena in optical cavities to cool molecular ensembles to sub-mK temperatures. The scheme complements well with our present experimental work on the deceleration and focusing of cold molecules and can extend our present capability to simultaneously cool and trap a large cold molecular ensemble. For simulation of a large ensemble of molecules, we proposed a new statistical model based on the Boltzmann equation beside the traditional discrete model and studied two solution methods. The comparison of the theory and numerical simulations between discrete model and statistical model showed a good agreement, which validated this new model. We then explored the scaling laws with a view to the self-organization and cooling of a large ensemble of species. We studied the cooling of a CN molecular cloud of the density 1013/cm3, with an initial temperature at 10 mK in an optical cavity. We found that more than a third of the molecules are stably trapped by the intracavity field and the final temperature is below 1mK. We discussed the scaling laws in the case when a large ensemble of species is involved. Finally we argue that cavity cooling using a far off-resonant laser source can be a general cooling method that is applicable to any particles and studied the probability and conditions

Modelling of nitrogen and oxygen gas mixture with a novel diatomic kinetic model

Practical applications involve flows that often have more than one constituent. Therefore, the capability to model a gas mixture flow is important. Extending kinetic model equations of the Bhatnagar–Gross–Krook type from a single-species gas to multi-species gas mixtures presents a number of important challenges. This challenge is further pronounced when diatomic gas mixtures are considered due to the addition of internal energy modes. In this paper, a novel diatomic binary mixture model with separate translational, rotational, and vibrational temperatures is derived. The species drift-velocity and diffusion are considered by introducing separate species velocities and accounting for their relationship. The derivation is detailed as a logical build-up with a multi-step approach from a diatomic model for a single gas, known in the literature. Transport properties are obtained through the Chapman–Enskog type expansion. The diatomic mixture model is numerically evaluated for a gas mixture of nitrogen and oxygen. The model is validated against Monte Carlo results for normal shocks, showing good agreement for species density and temperature profiles. A parametric study demonstrates the variation in flow properties for different Mach numbers, vibrational collision numbers, and concentrations. Interesting results for the mixture properties are shown when the physics of the flow is discussed in greater detail. The effect of the different levels of vibrational excitation in the different species emphasizes the importance of modeling the flow as a mixture. The newly introduced diatomic gas mixture model demonstrates promising computational results for relevant applications

Optical measurement methods in thermogasdynamics

A review is presented of a number of optical methods of flow measurements. Consideration is given to such spectroscopic methods as emission and absorption techniques, electron beam-stimulated fluorescence, and light scattering - Rayleigh, Raman and Mie - methods. The following visualization methods are also discussed: shadow photography, schlieren photography, interferometry, holographic interferometry, laser anemometry, particle holography, and electron-excitation imaging. A large bibliography is presented and the work is copiously illustrated with figures and photographs

A kinetic Fokker-Planck algorithm for simulating multiscale gas flows

Numerical, aerodynamic analysis of spacecraft requires the modeling of rarefied hypersonic flows. Such flow regimes are usually dominated by broad shock waves and strong expansion flows. In such areas of the flow the gas is far from its equilibrium state and therefore conventional modeling approaches such as the Euler or Navier-Stokes equations cannot be used. Instead, non-equilibrium modeling approaches must be applied. While most non-equilibrium flow solvers are computationally expensive, a recently introduced kinetic Fokker-Planck (FP) method shows the potential of describing non-equilibrium flows with satisfactory accuracy and, at the same time, significantly reducing computational costs. However, the application of kinetic FP solvers was so far still limited to simple, single species gases. The aim of this study is to extend the capabilities of the kinetic FP approach for describing complex gas flows. Particular attention is paid to the modeling of non-equilibrium aerodynamics, as it is relevant for describing spacecraft related gas flows. Methods for describing polyatomic species as well as gas mixtures within the kinetic FP framework are constructed. All models are intensively validated by comparison to already established numerical methods, as well as in comparison to experimental studies. Excited energy states are modeled by a stochastic jump process described by a master equation. This approach allows the description of both continuous and discrete energy levels. Gas mixtures are modeled based on the hard-sphere and variable hard-sphere collision potentials. For both cases, FP models are constructed for an arbitrary number of species. The efficiency of the described models is investigated and different strategies are proposed to use kinetic FP methods efficiently. The expansion of synthetic air from an axially symmetric orifice is numerically reproduced using the developed models and results are compared with experimental measurements. Although the numerical simulations capture several magnitudes of Knudsen numbers, from the continuum flow in the reservoir up to the free-molecular far field, good agreement between simulation and experiment is seen

Computation of Hypersonic Flows Using the Direct Simulation Monte Carlo Method

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140680/1/1.A32767.pd

Ultraviolet Absorption Properties of Diatomic Sulfur

Diatomic Sulfur, S2, is present in the atmospheres of Jupiter and its moon, Io. It is very difficult to obtain an S2 spectrum in the laboratory; when heated, elemental sulfur forms many different gaseous species including S2, S3, and S4. We use a two-temperature furnace and absorption cell to create conditions of high temperature and low pressure, which favor pure S2 formation. Using high resolution measurements from the National Institute of Standards and Technology in Gaithersburg, Maryland, we quantified oscillator strengths for atmospheric modeling purposes. This work is generously supported by the Jerome A. Schiff Fellowship

Molecular-Based Optical Measurement Techniques for Transition and Turbulence in High-Speed Flow

High-speed laminar-to-turbulent transition and turbulence affect the control of flight vehicles, the heat transfer rate to a flight vehicle's surface, the material selected to protect such vehicles from high heating loads, the ultimate weight of a flight vehicle due to the presence of thermal protection systems, the efficiency of fuel-air mixing processes in high-speed combustion applications, etc. Gaining a fundamental understanding of the physical mechanisms involved in the transition process will lead to the development of predictive capabilities that can identify transition location and its impact on parameters like surface heating. Currently, there is no general theory that can completely describe the transition-to-turbulence process. However, transition research has led to the identification of the predominant pathways by which this process occurs. For a truly physics-based model of transition to be developed, the individual stages in the paths leading to the onset of fully turbulent flow must be well understood. This requires that each pathway be computationally modeled and experimentally characterized and validated. This may also lead to the discovery of new physical pathways. This document is intended to describe molecular based measurement techniques that have been developed, addressing the needs of the high-speed transition-to-turbulence and high-speed turbulence research fields. In particular, we focus on techniques that have either been used to study high speed transition and turbulence or techniques that show promise for studying these flows. This review is not exhaustive. In addition to the probe-based techniques described in the previous paragraph, several other classes of measurement techniques that are, or could be, used to study high speed transition and turbulence are excluded from this manuscript. For example, surface measurement techniques such as pressure and temperature paint, phosphor thermography, skin friction measurements and photogrammetry (for model attitude and deformation measurement) are excluded to limit the scope of this report. Other physical probes such as heat flux gauges, total temperature probes are also excluded. We further exclude measurement techniques that require particle seeding though particle based methods may still be useful in many high speed flow applications. This manuscript details some of the more widely used molecular-based measurement techniques for studying transition and turbulence: laser-induced fluorescence (LIF), Rayleigh and Raman Scattering and coherent anti-Stokes Raman scattering (CARS). These techniques are emphasized, in part, because of the prior experience of the authors. Additional molecular based techniques are described, albeit in less detail. Where possible, an effort is made to compare the relative advantages and disadvantages of the various measurement techniques, although these comparisons can be subjective views of the authors. Finally, the manuscript concludes by evaluating the different measurement techniques in view of the precision requirements described in this chapter. Additional requirements and considerations are discussed to assist with choosing an optical measurement technique for a given application