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Simplified plasma models based on reduced kinetics - Aurélie Bellemans - Ph.D. Thesis - Free download

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VKI PHDT 2018-02, Aurélie Bellemans, Simplified plasma models based on reduced kinetics, ISBN 978-2-87516-126-0, 168 pgs


Simplified plasma models based on reduced kinetics  - Aurélie Bellemans - Ph.D. Thesis - Free download

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Simplified plasma models based on reduced kinetics
By Aurélie Bellemans

PhD Thesis from the von Karman Institute / Université Libre de Bruxelles, Belgium, December 2017, ISBN 978-2-87516-126-0, 168 pgs



Performing high-fidelity plasma simulations remains computationally expensive because of their large dimension and complex chemistry. Atmospheric re-entry plasmas for instance, involve hundreds of species in thousands of re- actions used in detailed physical models. These models are very complex as they describe the non-equilibrium phenomena due to finite-rate processes in the flow. Chemical non-equilibrium arises because of the many dissociation, ionization and excitation reaction at various time-scales. Vibrational, rotational, electronic and translational temperatures characterize the flow and exchange energy between species, which leads to thermal non-equilibrium. With the current computational resources, detailed three-dimensional simulations are still out of reach. Detailed calculations using the full dynamics are often restricted to a zero- or one-dimensional description. A trade-off has to be made between the level of accuracy of the model and its computational cost.
This thesis presents various methods to develop accurate reduced kinetic models for plasma flows. Starting from detailed chemistry, high-fidelity reductions are achieved through the application of either physics-based techniques, such as presented by the binning methods and time-scale based reductions, either empirical techniques given by principal component analysis. As an original contribution to the existing methods, the physics-based techniques are combined with principal component analysis uniting both com- munities. The different techniques are trained on a 34 species collisional- radiative model for argon plasma by comparing shock relaxation simulations. The best performing method is applied on the large N-N2 mechanism containing 9391 species and 23 million reactions calculated by the NASA Ames Research Center.
As a preliminary step, the system dynamics is analyzed to improve our understanding of the various processes occurring in plasma flows. The re- actions are analyzed and classified according to their importance. A deep investigation of the kinetics enables finding the main variables and parameters characterizing the plasma, which can thereafter be used to develop or improve existing reductions.
As a result, a novel coarse grain model has been developed for argon by binning the electronic excited levels and the ionized species into 2 Boltzmann averaged energy bins. The ground state is solved individually together with the free electrons, reducing the species mass conservation equations from 34 to 4.
Principal component analysis has been transferred from the combustion community to plasma flows by investigating the Manifold-Generated and Score-PCA techniques. PCA identifies low-dimensional manifolds empirically, projecting the full kinetics to its base of principal components. A novel approach combines the binning techniques with PCA, finding an optimized model for reducing the N3 rovibrational collisional model.

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