Dean C. Wadsworth

VIBRATIONAL FAVORING EFFECT IN DSMC DISSOCIATION MODELS

Several common models for dissociation reactions in direct simulation Monte Carlo calculations are analyzed quantitatively under general equilibrium and nonequilibrium conditions. The models differ in the degree to which the internal energy of the colliding particles contributes to the probability of dissociation. Test calculations in an equilibrium bath show that the temperature dependence of the predicted equilibrium rate constant, a commonly used measure of accuracy, is dominated by the collision selection algorithm, rather than the details of the dissociation model, and is thus a poor measure of physical validity or accuracy. The distribution of internal energy states of molecules selected for dissociation under the bath conditions, as used for analysis here, is a preferred means to assess accuracy, and is available qualitatively from existing theory. Recent state-specific quasi-classical trajectory calculations allow for quantitative assessment for certain molecules. Certain singularities present in ...

Two-dimensional hybrid continuum/particle approach for rarefied flows

A hybrid numerical technique previously developed for one-dimensional rarefied gas flows is generalized to two dimensions. The method is based on the fact that the flowfield that develops near a body in a rarefied gas typically contains local regions of continuum, transitional and free-molecular flow. By utilizing the solution technique most appropriate for each region and coupling the techniques in an interface region where both are applicable, more computationally efficient solutions can be obtained, or equivalently, more complex flowfields can be analyzed. The present method combines finite difference solution of the Navier-Stokes equations in the continuum regions, with Direct Simulation Monte Carlo in the more rarefied regions. The two schemes are coupled interactively via a general conservative flux boundary condition. The method is tested by application to the model problem of pressure-driven rarefied flow through a slit. Results show the hybrid scheme offers a speedup of a factor of nearly two for the nominal conditions considered, due to the decrease in the size of the Direct Simulation domain.

Slip effects in a confined rarefied gas. I: Temperature slip

The model problem of a gas confined between stationary heated plates is analyzed numerically to quantify slip effects for gas densities ranging from near‐continuum to highly rarefied conditions. Calculations are performed with the direct simulation Monte Carlo technique using two gas–surface interaction models and with a finite‐difference Navier–Stokes method using slip boundary conditions. Comparisons are made with experimental density profiles and with previous discrete ordinate calculations using a model form of the Boltzmann equation. To the resolution of the experimental data, both direct simulation Monte Carlo and the Navier–Stokes equations with slip provide very good agreement for essentially all cases considered, while the discrete ordinate results are less accurate. The calculated pressure and temperature profiles show slightly larger differences. The details of the gas–surface interaction model have a relatively small effect on calculated density profiles. The direct simulation Monte Carlo and ...

Transient motion of a confined rarefied gas due to wall heating or cooling

The transient motion that arises in a confined rarefied gas as a container wall is rapidly heated or cooled is simulated numerically. The Knudsen number based on nominal gas density and characteristic container dimension is varied from nearcontinuum to highly rarefied conditions. Solutions are generated with the direct simulation Monte Carlo method. Comparisons are made with finite-difference solutions of the Navier-Stokes equations, the limiting free-molecular values, and (continuum) results based on a small perturbation analysis. The wall heating and cooling scenarios considered induce relatively large acoustic disturbances in the gas, with characteristic flow speeds on the order of 20 % of the local sound speed. Steadystate conditions are reached after on the order of 5 to 10 acoustic time units, here based on the initial speed of sound in the gas and the container dimension. As rarefaction increases, the initial gas response time is decreased. For the case of a rapid increase in wall temperature, transient rarefaction effects near the wall greatly alter gas response compared to the continuum predictions, even at relatively small nominal Knudsen number. For wall cooling, the continuum solution agrees well with direct simulation at that same Knudsen number. A local Knudsen number, based on the mean free path and the scale length of the temperature gradient, is found to be a more suitable indicator of transient rarefaction effects.

Numerical simulation of rarefied flow through a slit. Part I: Direct simulation Monte Carlo results

The pressure‐driven flow of a rarefied monatomic gas through a two‐dimensional slit is simulated using the direct simulation Monte Carlo technique. Of particular interest is the change in flow field structure as pressure ratio and Knudsen number are varied. Comparisons are made to quantify the limits of validity of free‐molecular theory and approximate, nearly free‐molecular iterative methods. Also addressed is the sensitivity of the numerical solutions to grid structure and boundary conditions. The free‐molecular theory is found to predict quantitative flow field properties (e.g., centerline velocities or downstream flux profiles) reasonably well for large finite Knudsen number with the error dependent on the pressure ratio. The nearly free‐molecular corrections are shown to have limited range of applicability. A previously derived parameter is found to correlate total mass flux well as a function of pressure ratio and Knudsen number over a large portion of the transitional regime.

Influence of Gas-Surface Interaction Models on Predicted Performance of a Micro-Resistojet

Abstract : The Free Molecule Micro-Resistojet was designed as a micropropulsion system capable of performing attitude control and primary maneuvers for nanospacecraft with a mass of less than or equal to 10 kg. The details of gas-surface interactions between propellant molecules and surfaces held at elevated temperature are critical in predicting the propulsion systems performance and efficiency. The aim of this study is to parametrically assess the performance of a typical thruster geometry using a general Maxwell scattering model and two versions of the Cercignani-Lampis-Lord model. The models are incorporated into a Direct Simulation Monte Carlo numerical code and are used to bound the predicted performance characteristics of the thruster. The total specific impulse varies by approximately 20% over range of accommodation coefficients from specular to diffuse surface scattering. However, there was only a maximum difference of about 5% between the models for a given accommodation coefficient. Other more microscopic parameters, such as axial velocity distribution functions, appear to depend more on the scattering model used.