Jiaqiang Zhong

Development of kinetic-based energy exchange models for noncontinuum, ionized hypersonic flows

Ultrahigh Mach number re-entry vehicles create sufficiently energetic flow conditions with substantial ionization occurring in the noncontinuum flow regime. To model these noncontinuum, ionized, and thermochemically nonequilibrium flows, a direct simulation Monte Carlo (DSMC) approach is investigated. Energy exchange models that have been developed for Navier–Stokes computational fluid dynamics computations are examined and revised for application to the DSMC method. Since the electron–heavy particle collision rate is approximately two orders of magnitude higher than that between heavy particles, a new model is developed for electron scattering collision processes and electron-vibrational energy exchange based on the electron-molecule shape resonance phenomena. It is found that the flow electron and vibrational temperatures are sensitive to the electron-vibrational relaxation model because the relaxation time changes by orders of magnitude. The DSMC calculations of the bow-shock region of a blunt body wer...

Modeling of Stardust Reentry Ablation Flows in the Near-Continuum Flight Regime

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.

Direct Simulation Monte Carlo Modeling of Homogeneous Condensation in Supersonic Plumes

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 multispecies 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.

Kinetic Model of Condensation in a Free Argon Expanding Jet

The direct-simulation Monte Carlo (DSMC) method has recently been developed to simulate homogeneous condensation in a free-expansion rocket plume. However, cluster‐monomer and cluster‐cluster collision models as well as the determination of cluster size were simplified in the previous work, and the effect on the accuracy of the numerical simulation results was not quantified. In this work, the molecular-dynamics (MD) method is used to simulate collision and sticking probabilities for argon clusters and the results are compared with the hard-sphere model. These improved models are then integrated into a DSMC code to predict the Rayleigh scattering intensity in a free-expanding argon condensation plume, and numerical results are compared with experimental data along the plume centerline. Nomenclature A = constant B = constant b = impact parameter c =v elocity d = diameter E = evaporation rate or energy I = intensity i = number of atoms J = nucleation rate K = intensity constant k B = Boltzmann’s constant L = specific latent heat M = cluster mass m = molecular mass N = number density nc = number of simulated nuclei particles ps = saturation pressure q = sticking coefficient R = ideal gas constant r = distance or radius T = temperature V = interaction potential or specific volume α = species polarizability � t = time step � V = cell volume � = potential constant λ0 =w avelength ρ = density σ = surface tension or potential constant Subscripts

Simulation of droplet heating and desolvation in inductively coupled plasma—part II: coalescence in the plasma

A numerical model is developed to consider for the first time droplet coalescence along with transport, heating and desolvation in an argon inductively coupled plasma (Ar ICP). The direct simulation Monte Carlo (DSMC) method and the Ashgriz–Poo model are used, respectively, to compute droplet–droplet interactions and to determine the outcome of droplet collisions. Molecular dynamics (MD) simulations support the use of the Ashgriz–Poo coalescence model for small droplet coalescence. Simulations predict spatial maps of droplet number and mass densities within an Ar ICP for a conventional nebulizer-spray chamber arrangement, a direct injection high efficiency nebulizer (DIHEN), and a large bore DIHEN (LB-DIHEN). The primary findings are: (1) even at 1500 W, the collisions of the droplets in the plasma lead primarily to coalescence, particularly for direct aerosol injection; (2) the importance of coalescence in a spray simulation exhibits a complex relationship with the gas temperature and droplet size; (3) DIHEN droplets penetrate further into the Ar ICP when coalescence is considered; and (4) droplets from a spray chamber or the LB-DIHEN coalesce less frequently than those from a DIHEN. The implications of these predictions in spectrochemical analysis in ICP spectrometry are discussed. 2003 Elsevier B.V. All rights reserved.

Kinetic nucleation model for free expanding water condensation plume simulations

Recent direct simulation Monte Carlo (DSMC) simulations of homogeneous condensation in free expansion water plumes [Z. Li, J. Zhong, D. A. Levin, and B. Garrison, AIAA J. 47, 1241 (2009)] show that the nucleation rate is a key factor for accurately modeling condensation phenomenon. In this work, we use molecular dynamics (MD) simulations of a free expansion to explore the microscopic mechanisms of water dimer formation and develop collision models required by DSMC. Bimolecular and termolecular dimer cluster formation mechanisms are considered and the former is found to be the main mechanism in expanding flows to vacuum. MD simulations between two water molecules using the simple point charge intermolecular potential were performed to predict the bimolecular dimer formation probability and the probability was found to decrease with collision energy. The formation probabilities and postcollisional velocity and energy distributions were then integrated into DSMC simulations of a free expansion of an orifice condensation plume with different chamber stagnation temperatures and pressures. The dimer mole fraction was found to increase with distance from the orifice and become constant after a distance of about two orifice diameters. Similar to experiment, the terminal dimer mole fraction was found to decrease with chamber stagnation temperatures and increase linearly with chamber stagnation pressures which is consistent with a bimolecular nucleation mechanism.

Development of a Kinetic Nucleation Model for a Free-Expanding Argon Condensation Flow

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. 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 High-Altitude Hypersonic Wake Flows of Slender and Blunt Bodies

The gas dynamic features of the laminar, near-wake flow behind slender and blunt hypersonic vehicles are studied using the direct simulation Monte Carlo method. Near-wake flows are characterized by features of low density, low Reynolds number, high temperature, thermal nonequilibrium, species separation, and recirculation. The impact of freestream number density and velocity on the near-wake flowfield is considered and compared for slender and blunt bodies. The near-wake structure postulated by theory and observed in numerical continuum calculations is also observed in the kinetic simulations, which are more accurate in the high-altitude, rarefied near-wake flow. The paper discusses the validation of the direct simulation Monte Carlo computational tool with experimental data for slender and blunt shapes and a previously published blunt direct simulation Monte Carlo geometry case. Then, the near-wake flows generated by a 10 deg slender cone and a 70 deg blunt body are analyzed. The near-wake flows behind slender and blunt bodies are similar in that the freestream Mach number has little impact on the near-wake flow structure and the recirculation length is not found to be related to the local Reynolds number. For both geometries, the base radius was found to be the characteristic length in the near-wake flow. Significant differences in the near-wake flow for the two geometries were observed in the spatial distribution of gas temperatures, the degree of chemical dissociation, and the sensitivity of recirculation length to freestream number density.

Modeling of the Stardust Reentry Flows with Ionization in DSMC

Ionized hypersonic o ws for the Stardust blunt body at 80 km altitude were simulated using the direct simulation Monte Carlo (DSMC) method, at a speed of 12.8 km/s, the fastest man-made object to enter the Earth’s atmosphere. The modeling of ionization processes and energy exchange between translational and internal modes in DSMC is discussed. Eleven species including 5 ions and electron and chemical reactions are considered in the o weld. It was found that the charge neutrality assumption held in the bow-shock region and using this assumption, ionization processes were modeled. The electron-vibration energy exchange model is important for the prediction of electron and vibrational temperature proles, and this process was modeled by Lee’s relaxation time for the rst time in DSMC. DSMC results were compared with CFD(DPLR) results, with the main result that DSMC predicted lower energy exchange rates between translational and internal modes than CFD. The lower energy exchange rates resulted in lower dissociation rates and a lower degree of ionization in DSMC. Furthermore, radiation was calculated using the NEQAIR code from the CFD(DPLR) and DSMC o welds along the stagnation line for NO, N + , N, and O. The development of the DSMC models signican tly aected the N radiation and O radiation in ultraviolet range.

Sensitivity of Water Condensation in a Supersonic Plume to the Nucleation Rate

The direct simulation Monte Carlo (DSMC) method has recently been developed to simulate homogeneous water condensation in a free expansion rocket plume. A nucleation rate, based on the classical nucleation theory (CNT), was used in the DSMC simulation to predict initial nuclei in the condensation region. However, recent experimental research suggests that the CNT nucleation rate should be corrected and the magnitude of the correction is a function of the vapor temperature. Because the nucleation rate is one of the most important factors that impacts the accuracy of the numerical simulation results for a condensation plume, the impact is investigated of the corrected nucleation rate on cluster growth processes and flow macroparameters in an expanding flow using the DSMC method to model both the gas and condensate flow.