Olivier Chazot

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.

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.

Flight Extrapolation of Plasma Wind Tunnel Stagnation Region Flowfield

Development of reusable space vehicles requires a precise qualification of their thermal protection system materials. The catalytic properties are usually determined in plasma wind tunnels for test conditions relevant to the flight mission program. Therefore, for such a situation, it is important to have a methodology that allows the correct extrapolation of the ground test conditions to the real flight ones and vice-versa. The local heat transfer simulation concept presented in this paper is a possible strategy for accomplishing this task. Computational results show that the ground test conditions are indeed correctly extrapolated to the flight ones and a simple method of accounting for possible discrepancies between the two configurations is presented.

Disturbance Level Characterization of a Hypersonic Blowdown Facility

Free-stream disturbances have a dominant role on boundary layer stability and transition, especially in high speed ows. The assessment of the ow quality in a wind tunnel is therefore a key issue especially in transition to turbulence studies. With this purpose a detailed characterization of the free-stream disturbance level of the VKI H3 Mach 6 Wind Tunnel at di erent Reynolds number is carried out by means of classical intrusive measurements techniques (hot-wire anemometry and Pitot pressure probes). Issues concerning the aerodynamic heating of the probes are investigated and a special calibration technique is developed in order to compensate for these e ects. Statistical and spectral analyses are compared with the data available in the literature for other hypersonic wind tunnels, working at similar ranges of Mach and Reynolds numbers. A combined data reduction technique is also developed which allows to extract all ow quantities uctuations from those directly measurable through the above-mentioned instrumentations. By means of this technique, the validity of some physical hypotheses on the ow can be veri ed. Moreover, a Kovasznay modal analysis allows to characterize the origin of the disturbances.

Temperature Jump Phenomenon During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramics

U LTRAHIGH temperature ceramic (UHTC) materials containing hafnium diboride (HfB2) and zirconium diboride (ZrB2) with a silica former, most commonly silicon carbide (SiC), have been studied extensively over the last decade as materials for leading-edge and control surface components on hypersonic vehicles [1–3]. Such components experience extreme aerothermal heating in chemically aggressive, partially dissociated air environments. Promising aspects of diboride-based UHTC materials include the very high melting points of HfB2 and ZrB2 and their refractory oxides hafnia (HfO2) and zirconia (ZrO2), as well as the high thermal conductivities of HfB2 and ZrB2, which enables efficient heat conduction away from stagnation point regions [4]. Zirconium diboride has some advantages over hafnium diboride as an aerospace material, because it is lighter and less expensive. The oxidation of ZrB2 produces both zirconia and boron oxide (B2O3). Significant oxidation of ZrB2 in atmospheric air begins at about 1050 K. The softening temperature for amorphous B2O3 is in the range of 830–900 K [5]; below about 1500 K, the oxide scale consists of a porousZrO2 network filled with liquidB2O3 that acts as an effective oxygen diffusion barrier [6,7]. However, the vapor pressure of B2O3 increases rapidly with temperature [8], resulting in rapid loss of B2O3 above 1500 K. The residual porous zirconia scale provides little resistance to inward oxygen transport and further oxidation [9,10], making the oxidation resistance of pure ZrB2 insufficient for high-temperature hypersonic vehicle applications. The addition of a silica former to ZrB2 improves its oxidation resistance [11–14]. Compositions containing from 10 to 30% (by volume) SiC have generally been found to be optimal in this regard. The virgin ZrB2-SiC surfaces oxidize through parallel reactions that generate ZrO2,B2O3, and SiO2. LiquidB2O3 mixes with amorphous SiO2 to form a borosilicate glass that seals the ZrO2 scale [15]. With increasing temperature, boron oxide evaporates preferentially from Received 10 August 2011; revision received 13 February 2012; accepted for publication 19 February 2012. Copyright

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.

Ablation of carbon preform in the VKI Plasmatron

Ablation tests of a carbon fiber preform have been carried out in the VKI Plasmatron facility in air plasmas to study the e↵ect of test chamber static pressure on the oxidation behavior of the material and erosion of the carbon fibers. Surface temperatures at a constant cold wall heat flux of 1 MW/m 2 suggested a di↵usion-limited ablation regime for all tests. A higher mass loss and recession rate were measured for ablation tests in a low pressure environment (1.5 kPa). Fiber bundles were found in the preform, embedded between single fibers as illustrated by micrographs. High-speed images showed a strong release of particles during the ablation test. It is assumed that these fiber bundles detach during ablation as the surrounding fibers are oxidized. A tentative explanation is that a low pressures environment may lead to an enhanced release of particles due to intensified di↵usion and higher plasma flow velocities.

Modelling of high-enthalpy, high-Mach number flows

A review is made of the computational models of high-enthalpy flows developed over the past few years at the von Karman Institute and Universite Libre de Bruxelles, for the modelling of high-enthalpy hypersonic (re-)entry flows. Both flows in local thermo-chemical equilibrium (LTE) and flows in thermo-chemical non-equilibrium (TCNEQ) are considered. First, the physico-chemical models are described, i.e. the set of conservation laws, the thermodynamics, transport phenomena and chemical kinetics models. Particular attention is given to the correct modelling of elemental (LTE flows) and species (chemical non-equilibrium—CNEQ—flows) transport. The numerical algorithm, based on a state-of-the-art finite volume discretization, is then briefly described. Finally, selected examples are included to illustrate the capabilities of the developed solver.

Simulation of sub- & supersonic flows in inductive plasmatrons

Simulation of sub- and supersonic thermochemical equilibrium flows in plasmatrons is considered. A physicochemical model, numerical method, and computation results for equilibrium inductive coupled plasma flows in a plasmatron are given. An effective preconditioning technique along with an implicit total-variation-diminishing scheme is used to solve the Navier‐Stokes equations in both subsonic and supersonic regimes. The governing equations include source terms corresponding to the electromagnetic field influence: the Lorentz force components (so-called magnetic pressure) and Joule heat production. The necessary transport coefficients were calculated in advance for equilibrium air plasma as the functions of pressure and temperature. Transport properties were calculated by the precise formulas of the Chapman‐Enskog method in the temperature range 300 < ‐ T < 15,000 K. Calculations of equilibrium air plasma flows for the IPG-4 (Institute for Problems in Mechanics, Russian Academy of Science) discharge channel geometry with the channel radius Rc = 0.04 m and length Zc = 0.40 m were performed. Creation of both underexpanded and overexpanded jets exhausted from the plasmatron channel is considered. A comparison with experimental results is given.

Predictions of nonequilibrium radiation: analysis and comparison with EAST experiments

We simulate the relaxation processes behind a strong shock by means of an electronically specific collisional radiative model for atomic species. Test conditions are taken from the experimental campaign carried out in the EAST facility in the framework of the NASA Crew Exploration Vehicle Aeroscience Project. The present work is committed to the partial validation of the collisional-radiative model against the recent experimental data. A direct comparison among experimental and simulated radiative intensity profiles shows a reasonable agreement even for the low pressure cases where the flow is in strong nonequilibrium condition. In the rapidly ionizing regime behind strong shock waves, the electronic energy level populations depart from Boltzmann distributions since the high lying bound electronic states are depleted. A detailed analysis of the elementary kinetic processes occurring in the relaxation zone revealed that improvement of the agreement among simulation and experiments can be obtained using the recent rate constant compiled by Frost et al..