Ingrid J. Wysong

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 ...

Vibrational energy transfer of NO (X 2Π, v=2 and 1)

A two‐laser (ir overtone pump and uv laser‐induced fluorescence probe) technique has been used to measure vibrational relaxation rate coefficients for NO (X 2Π, v=2 and 1) with various collision partners at room temperature. The relaxation of v=2 by NO, H2, and NH3 has also been measured at 240 K. The relaxation of v=2 by NO is dominated by vibration‐to‐vibration (V–V) transfer and this process has not previously been measured below room temperature. The probability for V–V transfer increases with decreasing temperature, indicating that attractive forces play a key role in this process. Different physical mechanisms for NO vibrational energy transfer are discussed in light of the available data.

On the use of chemical reaction rates with discrete internal energies in the direct simulation Monte Carlo method

The conventional chemical reaction models of the direct simulation Monte Carlo method developed with the assumption of continuous rotational or vibrational modes that are shown to exhibit systematic errors when used with discrete energy modes. A reaction model is proposed that is consistent with the use of discrete energy distributions of rotational and vibrational modes, and is equally applicable to diatomic and polyatomic systems. The sensitivity of the model to variations of different reaction rate parameters is examined. The revised chemical reaction model is then applied to the modeling of hypersonic flows over spacecraft in the Martian and Earth atmospheres.

Nonequilibrium numerical model of homogeneous condensation in argon and water vapor expansions

A computational approach capable of modeling homogeneous condensation in nonequilibrium environments is presented. The approach is based on the direct simulation Monte Carlo (DSMC) method, extended as appropriate to include the most important processes of cluster nucleation and evolution at the microscopic level. The approach uses a recombination-reaction energy-dependent mechanism of the DSMC method for the characterization of dimer formation, and the RRK model for the cluster evaporation. Three-step testing and validation of the model is conducted by (i) comparison of clusterization rates in an equilibrium heat bath with theoretical predictions for argon and water vapor and adjustment of the model parameters, (ii) comparison of the nonequilibrium argon cluster size distributions with experimental data, and (iii) comparison of the nonequilibrium water cluster size distributions with experimental measurements. Reasonable agreement was observed for all three parts of the validation.

Numerical and experimental investigation of microchannel flows with rough surfaces

A conical surface roughness model applicable to particle simulations has been developed. The model has been experimentally validated for channel flows using helium and nitrogen gases at Reynolds numbers from 0.01 to 10 based on inlet conditions. To efficiently simulate gas-surface interaction, molecular collisions with the actual rough surface are simulated by collisions with a randomly positioned conical hole having a fixed opening angle. This model requires only one surface parameter, average surface roughness angle. This model has also been linked to the Cercignani-Lampis scattering kernel as a required reference for use in deterministic kinetic solvers. Experiments were conducted on transitional flows through a 150μm tall, 1cm wide, 1.5cm long microchannel where the mean free path is on the order of the roughness size. The channel walls were made of silicon with: (i) polished smooth surfaces, (ii) regular triangular roughness, and (iii) regular square roughness with characteristic roughness scales of ...

Reaction cross sections for two direct simulation Monte Carlo models: Accuracy and sensitivity analysis

The quantum kinetic chemical reaction model proposed by Bird for the direct simulation Monte Carlo method is based on collision kinetics with no assumed Arrhenius-related parameters. It demonstrates an excellent agreement with the best estimates for thermal reaction rates coefficients and with two-temperature nonequilibrium rate coefficients for high-temperature air reactions. This paper investigates this model further, concentrating on the non-thermal reaction cross sections as a function of collision energy, and compares its predictions with those of the earlier total collision energy model, also by Bird, as well as with available quasi-classical trajectory cross section predictions (this paper also publishes for the first time a table of these computed reaction cross sections). A rarefied hypersonic flow over a cylinder is used to examine the sensitivity of the number of exchange reactions to the differences in the two models under a strongly nonequilibrium velocity distribution.

Direct Simulation Monte Carlo Dissociation Model Evaluation: Comparison to Measured Cross Sections

Recent measurements of collision-induced dissociation (CID) cross sections for Ar 2 + -Ar collisions for vibrationally cold and hot cases are utilized to test and compare several CID models that have been proposed for the direct simulation Monte Carlo (DSMC) technique. The idea that the CID process is strongly favored by vibrational energy is discussed relative to the various models. The Ar 2 + data do not show any vibrational favoring of the CID cross sections. The predictions of the DSMC models are examined using values of their adjustable parameters suggested in the literature

Laser induced fluorescence of ground state hydrogen atoms at nozzle exit of an arcjet thruster

Abstract : We report the first observation of a two-photon laser-induced fluorescence (LIF) technique in an arcjet plume. Ground state hydrogen atoms are detected with high spatial resolution near the thruster nozzle exit. Number density, axial and radial velocity, and translational temperature distributions are obtained in the expansion plume of a 1 kW arcjet operating on hydrogen propellant. Comparison of the ground state properties with previously measured excited state hydrogen data and recent computational data is discussed.

Chemical Reaction Modeling for Hypervelocity Collisions between O and HCl

The sensitivity of a rarefied-to-transitional flow to the fidelity of the chemical reaction model is investigated for a new molecular dynamics/quasiclassical trajectory (MD/QCT)-derived model and compared with the widely used total collision energy (TCE) model of Bird. For hypervelocity collisions that occur in the space environment, it is not clear, a priori, that the TCE model will provide reasonable results for the required high energy range and, particularly, if strong favoring of the reaction among different forms of reactant energy occurs. In fact, in previous work, the TCE model, using available Arrhenius parameters, has been found, for these flow conditions, to give unphysical probabilities. A chemical reaction model, suitable for use in the direct simulation Monte Carlo (DSMC) method, is developed to simulate the hypervelocity collisions of O(P3)+HCl(Σ+1)→OH(Π2)+Cl(P2), an example of an important reaction in high-altitude atmospheric-jet interactions. The model utilizes the MD/QCT method with a n...

Comparison of direct simulation Monte Carlo chemistry and vibrational models applied to oxygen shock measurements

Validation of three direct simulation Monte Carlo chemistry models—total collision energy, Quantum Kinetic, and Kuznetsov state specific (KSS)—is conducted through the comparison of calculated vibrational temperatures of molecular oxygen with measured values inside a normal shock wave. First, the 2D geometry and numerical approach used to simulate the shock experiments is verified. Next, two different vibrational relaxation models are validated by comparison with data for the M = 9.3 case where dissociation is small in the nonequilibrium region of the shock and with newly obtained thermal rates. Finally, the three chemistry model results are compared for M = 9.3 and 13.4 in the region where the vibrational temperature is greatly different from the rotational and translational temperature, and thus nonequilibrium dissociation is important. It is shown that the peak vibrational temperature is very sensitive to the initial nonequilibrium rate of reaction in the chemistry model and that the vibrationally favored KSS model is much closer to the measured peak, but the post-peak behavior indicates that some details of the model still need improvement.