Speaker
Description
- Introduction
Meteoroids largely disintegrate during their entry in the atmosphere, contributing significantly to the input of cosmic material to Earth. Yet, their atmospheric entry is not well understood. Among the daily material delivered into our planet, the Chelyabinsk event in 2013 [1] renewed awareness of potential hazards motivating the planning of deflection and mitigation strategies of incoming asteroids. These strategies rely on the knowledge of the physical properties of the material and structure of the incoming object. At the range of velocities typical of meteor phenomena, radiation becomes highly significant, the intense light observed during a meteor entry is mainly due to the radiation of the air and metallic species, where the latter are coming from ablation products. The current models to study meteor entries are focus in the ablation of small particles in the upper atmosphere. They usually rely on a zero-dimensional approach providing lack of an accurate treatment of the particle interaction with the atmosphere, from the fluid dynamic point of view.
Moreover, experimental studies on meteoroid material degradation in high-enthalpy facilities are scarce and when the material is recovered after testing, it rarely provides sufficient quantitative data for the validation of simulation tools.
In the work of [2], the author investigated the thermo-chemical degradation mechanism of a meteorite in a high-enthalpy ground facility able to reproduce atmospheric entry conditions. Time resolved optical emission spectroscopy data of ablated species allowed to identify the main radiating atoms and ions of potassium, sodium, magnesium, and iron. Three HR4000 spectrometers were used covering a wide spectral range (200 nm to 1000 nm) with a 2 mm distance from each other being the closest 1mm from the surface.
A H5 chondrite sample was cut into a cylindrical shape and it was embedded in a hemispherical holder of 50 mm diameter and 45 mm length made out of cork-composite ablative material and exposed to 1 MW/m2 heat flux.
The objective of this work is to simulate the plasmatron experiment by means of computational tools and to compare the spectral and total intensity with the measurements performed during the experiment.
Solving the RTE by integrating the spectrum computed with the Line-by-Line (LBL) method becomes computationally very expensive for complex molecular spectra. In this work we use a hybrid statistical narrow band model (HSNB) [3,4] which has the feature of presenting an accurate description of the radiative flux with low CPU by dividing the spectra into narrow bands and compute the intensity in terms of averages. We extended the database to alkali and metallic atoms such as Fe, Mg, Si, Na, Al and many others. Due to the weak spectral density of the atomic lines, the LBL method is used to compute their contribution to the total intensity. - Methodology
The plasmatron flow field is reproduced by using a 1D Stagnation-Line solver where the physico-chemical properties are provided by the Mutation++ library built at the VKI [2]. The evaporation/condensation of a molten layer is estimated by solving a surface mass balance where the production terms are computed with the Hertz-Knudsen law. Also, oxidation and nitridation reactions were included to simulate the presence of carbonaceous species such as CN and CO due to the presence of the cork holder.
Once the flow field solution is obtained one can build spherical caps around the sample assuming constant properties, leading to a 2D representation of the boundary layer. Moreover, the shape of the torch jet and the full plasmatron chamber is also included.
This representation of the flow field is essential for the generation of the line of sight on which the Radiative Transport Equation (RTE) can be integrated and directly compared (as a post-processing) with the spectrometer measurements. Several optically thick and thin systems are considered including the CO and CN mechanism due to the presence of cork.
Due to the saturation of the Na line (586 nm) during the experiment, this line has been removed from the experimental spectra. -
Results & Conclusions
Several experimental time steps were chosen for the comparison with the numerical results. The measured surface temperature is imposed as a boundary condition for the numerical simulations. From the recorded spectra it is observed the appearance of a Planck continuum which might be due to the swelling of the cork and/or the boiling of the meteorite material. Based on that a swelling function in time is considered and the spectrometers which point directly to a surface are disregarded.
It is observed a strong effect of CN (violet) due to the presence of cork. This mechanism also coincides with the region where the second most important ablated species, Fe, shows the most important lines. It is also observed a significant emission of the N2(1+) and N2(2+) systems.
Moreover, the small presence of potassium, K, around 10-6 mole fraction shows a strong impact on the spectral intensity.
A good agreement was observed in the spectral intensity of Fe by choosing and evaporation coefficient equal to the unity and a condensation coefficient around 0.3.
Even though there cannot be a direct comparison of the strongest ablated species, Na, due to its experimental saturation, when included, the simulated spectral intensity becomes 10 times higher than the experimental, suggesting the importance of such element.
The mass loss of the sample predicted by the Hertz-Knudsen law is around 5mg which agrees reasonably well with the measurement after the experiment, 3 ± 1mg.
The use of cork as a sample holder complicates the comparison between the experimental and numerical results. Mostly due to swelling effects and presence of the carbonaceous species produced during ablation. -
References
- Popova, O. P., Jenniskens, P., Emel’yanenko, V., Kartashova, A., Biryukov, E., Khaibrakhmanov, S., Shuvalov, V., Rybnov, Y., Dudorov, A., Grokhovsky, V. I., et al. Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization. Science 342, 6162 (2013), 1069–1073.
- Helber, B., Dias, B., Bariselli, F., Zavalan, L. F., Pittarello, L., Goderis, S., Soens, B., McKibbin, S. J., Claeys, Ph., Magin T. E. (in prep.) Analysis of meteoroid ablation based on plasma wind-tunnel experiments, surface characterization, and numerical simulations. Astrophysical Journal
- Scoggins, J. B.; Magin, T. E. Development of mutation++: Multicomponent thermodynamic and transport properties for ionized plasmas written in c++. In 11th AIAA/ASME Joint Thermophysics and Heat Transfer Conference (2014), Atlanta,GA. AIAA 2014-2966.
- Lamet, J., Rivière, P., Perrin, M., and Soufiani, A. Narrow-band model for nonequilibrium air plasma radiation. Journal of Quantitative Spectroscopy and Radiative Transfer 111, 1 (2010), 87–104.
- Soucasse, L.; Scoggins, J. B.; Rivière, P.; Magin, T. E.; Soufiani, A.; Flow-radiation coupling for atmospheric entries using a Hybrid Statistical Narrow Band model, Journal of Quantitative Spectroscopy and Radiative Transfer,Volume 180,2016,Pages 55-69,
Summary
Meteor entry is characterized by complex shock layer physics such as radiation, evaporation of the meteoroid surface and the resulting chemistry process with the air constituents. Moreover, several meteor observations include spectral measurements from which their composition can be inferred. Recently a ground facility experiment of an H5 chondrite was performed at the VKI plasmatron facility which includes the measurement of time resolved optical emission spectroscopy data of ablated species. In this work we present a model able to reproduce the ablation of meteors and the comparison of the numerical radiative field with the one observed in the experiments. It is observed that the numerical results agree generally well with the experimental data when nitridation and oxidation gas-surface reactions are included due to the presence of a cork sample holder.
