Thierry Magin

Rovibrational internal energy transfer and dissociation of N2(1Σg+)−N(4Su) system in hypersonic flows

A rovibrational collisional model is developed to study energy transfer and dissociation of N(2)((1)Σ(g)(+)) molecules interacting with N((4)S(u)) atoms in an ideal isochoric and isothermal chemical reactor. The system examined is a mixture of molecular nitrogen and a small amount of atomic nitrogen. This mixture, initially at room temperature, is heated by several thousands of degrees Kelvin, driving the system toward a strong non-equilibrium condition. The evolution of the population densities of each individual rovibrational level is explicitly determined via the numerical solution of the master equation for temperatures ranging from 5000 to 50,000 K. The reaction rate coefficients are taken from an ab initio database developed at NASA Ames Research Center. The macroscopic relaxation times, energy transfer rates, and dissociation rate coefficients are extracted from the solution of the master equation. The computed rotational-translational (RT) and vibrational-translational (VT) relaxation times are different at low heat bath temperatures (e.g., RT is about two orders of magnitude faster than VT at T = 5000 K), but they converge to a common limiting value at high temperature. This is contrary to the conventional interpretation of thermal relaxation in which translational and rotational relaxation timescales are assumed comparable with vibrational relaxation being considerable slower. Thus, this assumption is questionable under high temperature non-equilibrium conditions. The exchange reaction plays a very significant role in determining the dynamics of the population densities. The macroscopic energy transfer and dissociation rates are found to be slower when exchange processes are neglected. A macroscopic dissociation rate coefficient based on the quasi-stationary distribution, exhibits excellent agreement with experimental data of Appleton et al. [J. Chem. Phys. 48, 599-608 (1968)]. However, at higher temperatures, only about 50% of dissociation is found to take place under quasi-stationary state conditions. This suggest the necessity of explicitly including some rovibrational levels, when solving a global kinetic rate equation.

Fire II Flight Experiment Analysis by Means of a Collisional-Radiative Model

We study the behavior of the excited electronic states of atoms in the relaxation zone of one-dimensional airflows obtained in shock-tube facilities. A collisional-radiative model is developed, accounting for thermal nonequilibrium between the translational energy mode of the gas and the vibrational energy mode of individual molecules. The electronic states of atoms are treated as separate species, allowing for non-Boltzmann distributions of their populations. Relaxation of the free-electron energy is also accounted for by using a separate conservation equation. We apply the model to three trajectory points of the Fire II flight experiment. In the rapidly ionizing regime behind strong shock waves, the electronic energy level populations depart from Boltzmann distributions because the high-lying bound electronic states are depleted. To quantify the extent of this nonequilibrium effect, we compare the results obtained by means of the collisional-radiative model with those based on Boltzmann distributions. For the earliest trajectory point, we show that the quasi-steady-state assumption is only valid for the high-lying excited states and cannot be extended to the metastable states.

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

Electronic Excitation of Atoms and Molecules for the FIRE II Flight Experiment

An accurate investigation of the behavior of electronically excited states of atoms and molecules in the postshock relaxation zone of a trajectory point of the Flight Investigation of ReentryEnvironment 2 (FIRE II) flight experiment is carried out bymeans of a one-dimensional flow solver coupled to a collisional-radiativemodel. Themodel accounts for thermal nonequilibrium between the translational energy mode of the gas and the vibrational energy mode of individualmolecules. Furthermore, electronic states of atoms andmolecules are treated as separate species, allowing for non-Boltzmann distributions of their populations. In the rapidly ionizing regime behind a strong shockwave, the high-lying bound electronic states of atoms are depleted. This leads to the electronic energy level populations of atoms departing from the Boltzmann distributions. For molecular species, departures from Boltzmann equilibrium are limited to a narrow zone close to the shock front. A comparison with the recent model derived by Park (Park, C., “Parameters for Electronic Excitation of Diatomic Molecules 1. Electron-Impact Processes,” 46th AIAAAerospace Sciences Meeting and Exhibit, Reno, NV, AIAA Paper 2008-1206, 2008.) shows adequate agreement for predictions involving molecules. However, the predictions of the electronic level populations of atoms differ significantly. Based on the detailed collisional-radiative model developed, a reduced kinetic mechanism has been designed for implementation into two-dimensional or three-dimensional flow codes.

KINETIC THEORY OF PLASMAS: TRANSLATIONAL ENERGY

In the present contribution, we derive from kinetic theory a unified fluid model for multicomponent plasmas by accounting for the electromagnetic field influence. We deal with a possible thermal nonequilibrium of the translational energy of the particles, neglecting their internal energy and the reactive collisions. Given the strong disparity of mass between the electrons and heavy particles, such as molecules, atoms, and ions, we conduct a dimensional analysis of the Boltzmann equation. We then generalize the Chapman-Enskog method, emphasizing the role of a multiscale perturbation parameter on the collisional operator, the streaming operator, and the collisional invariants of the Boltzmann equation. The system is examined at successive orders of approximation, each of which corresponding to a physical time scale. The multicomponent Navier-Stokes regime is reached for the heavy particles, which follow a hyperbolic scaling, and is coupled to first order drift-diffusion equations for the electrons, which follow a parabolic scaling. The transport coefficients exhibit an anisotropic behavior when the magnetic field is strong enough. We also give a complete description of the Kolesnikov effect, i.e., the crossed contributions to the mass and energy transport fluxes coupling the electrons and heavy particles. Finally, the first and second principles of thermodynamics are proved to be satisfied by deriving a total energy equation and an entropy equation. Moreover, the system of equations is shown to be conservative and the purely convective system hyperbolic, thus leading to a well-defined structure.

Analysis of the FIRE II Flight Experiment by Means of a Collisional Radiative Model

An accurate investigation of the behavior of electronically excited states of atoms and molecules in the post shock relaxation area is carried out by means of the Collisionalradiative model. The model is applied to a 1D shock tube code and the operating conditions are taken from three points in the trajectory of the FIRE II flight experiment. We account for thermal nonequilibrium between the translational and vibrational energy modes of individual molecular species and treat the electronic states of atoms and molecules as separate species. Relaxation of free-electrons is also accounted for by making use of a separate conservation equation for their energy. Non-Boltzmann distributions of the electronic state populations of atoms and molecules are allowed. Deviations from Boltzmann distributions are expected to occur in a rapidly ionizing regime behind a strong shock wave, due to the depletion of the high lying bound electronic states. In order to quantify the extent of departure from equilibrium of the electronic state populations, results are compared with those obtained assuming a Boltzmann distribution.

A Code Calibration Study for Huygens Entry Aeroheating

A preliminary code calibration study is presented for Huygens entry aeroheating. New aeroheating calculations are performed using state-of-the-art codes from NASA, AOES, EADS Space Transportation, and Ecole Centrale in Paris. The codes and methods employed for convective heating are based on new models developed in the last three years by the NASA In-Space Propulsion program to explore possible Titan aerocapture missions, while those for radiative heating are a mix of heritage and new NASA and ESA sponsored models, including three new collisional-radiative models developed in support of a recent international Huygens entry risk assessment. The calculations are carried out on a Monte- Carlo predicted worst-case peak heating trajectory assuming a standard minimum density atmospheric profile. The results show that the three computational fluid dynamics codes employed are all in excellent agreement (within 3%) in their predictions of both laminar and turbulent convective aeroheating on the capsule forebody over the entire trajectory. The shock layer radiation predictions show more variation, with the peak radiative heating rates ranging from 45-70 W/cm 2 when a Boltzmann assumption is employed, and 12-38 W/cm 2 using a nonequilibrium collisional-radiative model.

Internal Energy Excitation and Dissociation of Molecular Nitrogen in a Compressing Flow

Prediction of the radiative heat-flux to the surface of a spacecraft entering a planetary atmosphere strongly depends on the completeness and accuracy of the physical model used to describe the non-equilibrium phenomena in the flow. During an atmospheric entry, the translational energy of the fluid particles drastically rises through a shock. Depending on the intensity of the shock, different physico-chemical processes may take place, such as excitation of the internal energy modes, dissociation of the molecules, ionization of the atoms and molecules. These non-equilibrium phenomena are strongly coupled to each other. For re-entry velocities >10 km/s, a significant portion of the heating experienced by the heat shield can be due to radiation and is highly influenced by the shape of the internal energy distribution function. Understanding thermo-chemical non-equilibrium effects is also important for a correct interpretation of experimental measurements in flight and in ground wind-tunnels. Concentration of the gas species and distribution of their internal energy level populations can be estimated by means of either multi-temperature models (Park 1990) or collisional radiative (CR) models (Laux 2002; Bultel et al. 2006; Magin et al. 2006; Panesi et al. 2009). In multi-temperature models, the physico-chemical properties of the air flow are obtained by assuming that, for all the species, the population of each internal energy mode follows a Boltzmann distribution at its own temperature (Tr rotational, Tv vibrational or Te electronic temperature, respectively). These models have been developed based on experimental data obtained in flight and also in high-enthalpy facilities representative of specific flight conditions, such as in arc-jet and shock-tube wind-tunnels (Appleton et al. 1968). The problem with this approach is that the models may contain many uncertainties that can be extremely difficult to quantify. Moreover, there is no detailed information about the specific state of the gas since these data are highly averaged (e.g., stagnation point heat-flux measurement). Park (2006) has worked extensively on multi-temperature models for air and has also shown that the use of these models, even if very efficient from a computational point of view, can be justified only when the departure from the Boltzmann population is small, i.e., for low-velocity and high-pressure re-entry conditions. Collisional radiative models take into account all relevant collisional and radiative mechanisms between the internal energy levels of the different species in the flow. They constitute a valid alternative to the multi-temperature models since they exhibit a wider range of applicability. By increasing order of complexity and computational time, three kinds of CR models can be distinguished for air: electronic, vibrational and rovibrational. In electronic CR models, transitions between the electronic states are considered and the rovibrational levels of the molecules are populated according to Boltzmann distributions

Numerical Simulation of Nonequilibrium Stagnation-Line CO2 Flows with Catalyzed Surface Reactions

A methodology developed at the Institute for Problems in Mechanics of Moscow is used for the analysis of the catalytic properties of thermal protection materials in a CO 2 environment. The method relies on a combination of 1) heat-transfer and pitot-pressure measurements in a subsonic plasma jet and 2) numerical flow simulations. The simulated environments are typical of Mars entry conditions. In particular, this work is focused on the finite-rate chemistry part of the flow description. The extension of numerical tools developed at the von Karman Institute, required within the methodology for the determination of catalycity properties for thermal protection system materials, has been completed for CO 2 flows. Nonequilibrium stagnation-line computations have been performed for several outer edge conditions in order to analyze the influence of the chemical models for bulk reactions. Moreover, wall surface reactions have been examined, and the importance of several recombination processes has been discussed

Modeling of stagnation-line nonequilibrium flows by means of quantum based collisional models

The stagnation-line flow over re-entry bodies is analyzed by means of a quantum based collisional model which accounts for dissociation and energy transfer in N2-N interactions. The physical model is based on a kinetic database developed at NASA Ames Research Center. The reduction of the kinetic mechanism is achieved by lumping the rovibrational energy levels of the N2 molecule in energy bins. The energy bins are treated as separate species, thus allowing for non-Boltzmann distributions of their populations. The governing equations are discretized in space by means of the Finite Volume method. A fully implicit time-integration is used to obtain steady-state solutions. The results show that the population of the energy bins strongly deviate from a Boltzmann distribution close to the shock wave and across the boundary layer. The sensitivity analysis to the number of energy bins reveals that accurate estimation of flow quantities (such as chemical composition and wall heat flux) can be obtained by using only 10 energy bins. A comparison with the predictions obtained by means of conventional multi-temperature models indicates that the former can lead to an overestimation of the wall heat flux, due to an inaccurate modeling of recombination in the boundary layer.