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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.08.027 Byline: Sergey L. Gratiy (a), Deborah A. Levin (a), Andrew C. Walker (b) Keywords: Radiative transfer; Io Abstract: Validation of global numerical models of planetary atmospheres requires simulating images and spectra from the IR to UV spectral regions in order to compare them with remote observations. This paper describes Rassvet, a 3-D spherical-shell backward Monte Carlo radiative transfer model developed for such simulations. It utilizes a new methodology for calculating atmospheric brightness in scattered sunlight by introducing the concept of an 'effective emission source'. This allows for the accumulation of the scattered contribution along the entire path of a ray and the calculation of the atmospheric radiation when both scattered sunlight and thermal emission contribute to the remote measurement - which was not possible in previous models. A 'polychromatic' algorithm is extended for applications with the backward Monte Carlo method and implemented in the model. It allows for the calculation of radiative intensity for several wavelengths simultaneously, resulting in improved efficiency. The capabilities of the model are demonstrated by simulating remote measurements from the atmosphere of Io. Author Affiliation: (a) Department of Aerospace Engineering, Pennsylvania State University, University Park, PA 16802, USA (b) Department of Aerospace Engineering, University of Texas at Austin, Austin, TX 78712, USA Article History: Received 9 March 2009; Revised 14 August 2009; Accepted 31 August 2009

A condensation phenomenon in free expansion plumes has been observed and studied in the past several decades. To model homogeneous condensation of small, polar molecules produced in the combustion products of chemical propellants, such as water, accurate microscopic cluster models need to be developed. In this work, the molecular dynamics method is used to develop a model of water duster sizes, cluster--monomer collisions, and sticking probabilities necessary for the study of water homogeneous condensation in a plume expanding to low pressure, space conditions. The molecular dynamics results are integrated into a direct-simulation Monte Carlo simulation and the simulated Rayleigh scattering intensifies are compared with experimental data to test the models and direct-simulation Monte Carlo simulation results.

The ablation process of the Stardust thermal protection material is designed to reduce aerodynamic heating during reentry for extreme conditions. The coupling of ablation species with the flowfield is investigated in this work using the direct simulation Monte Carlo method for transitional to near-continuum flows. To model surface thermal and chemical ablation processes, a variable surface temperature wall is obtained assuming a radiative heat flux balanced by convective heat flux. It is found that chemical ablation due to the reaction between thermal protection system carbon materials and gaseous oxygen and nitrogen atoms is dominant compared with thermal ablation. As the altitude decreases, the forebody surface temperature increases, the ablation process becomes more intensive, and the influence of ionization reactions on the flowfield becomes more important due to denser freestream conditions.

Using classical nucleation theory (CNT), a homogeneous condensation model has been developed in previous work to simulate condensation in a free-expanding plume using the direct simulation Monte Carlo (DSMC) method. However, the accuracy of the CNT nucleation theory is questionable due to unphysical assumptions such as the use of macroscopic cluster surface tension for small clusters and the lack of an incubation time for nucleation. Molecular dynamics (MD) simulations performed in previous work confirm that the fundamental mechanism for the initiation of condensation is through dimer formation in two-stage ternary collisions of monomers. In this work we propose a kinetic nucleation model based on the mechanism whereby stable dimers are created from triple collisions. The new hybrid MD-DSMC kinetic nucleation model is implemented in a DSMC simulation of cluster growth processes, starting from dimers, in a condensation plume. Comparison of the distributions of steady-state plume cluster number density and size using the kinetic and CNT nucleation models shows that the condensation physics is significantly different for the two nucleation models investigated. The use of a physically more realistic nucleation model is shown to generate terminal cluster properties consistent with experiments in free-expanding jets. DOI: 10.2514/1.28234

Time-dependent performance of a high-temperature MEMS-based thruster is studied in detail by a coupled thermal-fluid analysis. The material thermal response governed by the transient heat conduction equation is obtained using the finite element method. The low-Reynolds number gas flow in the microthruster is modeled by the direct simulation Monte Carlo (DSMC) approach. The temporal variation of the thruster material temperature and gas flowfields are obtained as well as the thruster operational time limits for thermally insulated and convectively cooled thrusters. The predicted thrust and mass discharge coefficient of both two-dimensional (2-D) and three-dimensional (3-D) micronozzles decreases in time as the viscous losses increase for higher wall temperatures. [1046] Index Terms--Fluid flow, kinetic methods, microfluidics, semiconductor device, space vehicle propulsion.

Gaseous flow through a microchannel is treated numerically and analytically in order to assess pressure and mass flow losses due to a constriction of a finite length. Numerical modeling of two-dimensional (2-D) microchannel flow in the slip and transitional regimes is carried out using the direct simulation Monte Carlo (DSMC) method. The prediction of pressure losses and mass flow based on a simple analytic model for constriction microchannel flow are found to be in excellent agreement with DSMC simulations. Constriction has a dramatic effect on pressure loss and mass flow rate for the considered cases. The DSMC results indicate that the flow in the constriction microchannel separates in the transition section, but the separation does not significantly impact pressure distributions. [1189] Index Terms--Fluid flow, gases, losses, microfluidics, modeling, Monte Carlo methods, numerical analysis, pressure effects, separation. NOMENCLATURE a Speed of sound, m/s. m Mass flow rate, kg/s. [mu] Viscosity coefficient, kg/m-s. p Pressure, Pa. P Inlet to outlet pressure ratio. u X-component of velocity, m/s. v Y-component of velocity, m/s. R Gas constant, J/kg x K. T Temperamre, K. [rho] Density, kg/[m.sup.3]. [sigma] Accommodation coefficient. Subscripts int Interior cell. e Boundary. in Inlet. out Outlet.

A particle simulation method to model water condensation process in a supersonic rocket plume is proposed and developed. Classic nucleation theory is used to predict nucleation, condensation, and the evaporation rates for water clusters. Microscopic kinetic models are developed to simulate collision processes between water clusters and monomers, between water clusters and foreign molecules, and evaporation of monomers from water clusters. These models are integrated into the direct simulation Monte Carlo method to simulate an axisymmetric mullispecies gas expansion coupled with condensation. The developed computational scheme, first verified by empirical scaling laws of condensation in supersonic microjets, is then applied to predict the spatial distributions of water cluster number density, size, and temperature in a rocket exhaust plume. An empirical equation is used to correct classical nucleation rate, condensation results are compared between the original and corrected nucleation rate, and the impact of the nucleation rate on a flow with condensation is discussed in detail.

The relationship between deficits in affect regulation and Adult Antisocial Behavior (ASB) in primary crack/cocaine-using women was explored in a sample of 80 inner-city women. Narrative early memories were coded for two components of affect regulation, Affect Tolerance and Affect Expression, using the Epigenetic Assessment Rating Scale (EARS; Wilson, Passik, & Kuras, 1989). ASB was measured by the adult criteria of Antisocial Personality Disorder on the SCID-SAC (Spitzer, Williams, Gibbon, & First, 1993). Analyses compared primary crack/cocaine-using women with and without ASB on the affect regulation measures. Findings using memories of primary caretakers revealed that women with ASB had significantly poorer capacity for Affect Tolerance and Affect Expression than women without ASB, suggesting that ASB is significantly associated with differences in the capacity to regulate emotional experience among primary crack/cocaine-using women.

A numerical study of three-dimensional effects on the performance of a micronozzle fabricated from flat silicon wafers is performed by use of both continuum and kinetic approaches. The nozzle operates in a low-Reynolds-number regime, and viscous effects dominate the gas expansion. Thrust losses occur because the shear on the wall is greater in a flat nozzle configuration than in an axisymmetric conical nozzle. Therefore, the prediction of the micronozzle performance based on axisymmetric or two-dimensional modeling can lead to significant design errors. Comparison of simulation with recent data shows good agreement in terms of thrust predictions for cold-gas thrusters at Reynolds numbers of approximately 2 x [10.sub.2].

The overlay method is an effective method for calculation of excited state species and chemical species present in trace amounts in hypersonic flows. The method involves solving the Navier-Stokes equations, simulating the flowfield and overlaying the mass conversion equations for the traces of chemical species on the computed flowfield. Conditions such as CO(a) kinetics in an expanding flow and CO(a) formation in a rocket motor nozzle demonstrate the usefulness of the method. The method enables sensitivity analysis of kinetics modeling and comparison with radiance data obtained in flight.