Radiation and ablation coupling applied to the study of the Lost City bolide.
Authors: Dias Bruno , Magin Thierry 
Published: October 2020

IntroductionMeteor phenomena involve a series of complex aspects, from multiphase physics of the meteoroid (melting and evaporation) to non-equilibrium effects within the flow.The current meteor physics equations (single-body theory), rely on a zero-dimensional method and lack a precise treatment of the particle interaction with the atmosphere from the fluid dynamics point of view.Moreover, the study of the material response (melting and possible material removal) is often neglected.Another approach involves detailed computational simulations of the phenomena. Although these simulations are computationally expensive, they provide physical features of the flow that the single body theory cannot.The complexity of these detailed simulations significantly increases when one tries to couple all physical aspects, where Golub et al. [1], Johnston and Stern [2], Johnston et al. [3], Shuvalov and Artemieva [4], Svettsov et al. [5] show some examples.This abstract aims to simulate Lost City entry with a quasi-1D approach employing high-fidelity models by coupling a material solver with a flow solver, where the latter includes radiative features.Finally, we compare the mass loss from the numerical simulations with the dynamic mass derived from the observations of Ceplecha and ReVelle [6].&#160;MethodologyWe describe the coupling procedure to study Lost City ablation, which involves three solvers.Stagnation-line solver: It solves the discretized Navier-Stokes equations employing the Finite-Volume method. It includes an evaporation boundary condition based on the Hertz-Knudsen model. The open-source library Mutation++ [7] provides the necessary thermodynamic, transport, and kinetic closure to the Navier-Stokes equations.Radiation solver: It solves the Radiative Transport Equation (RTE) using the tangent slab method allowing the computation of radiative fluxes, radiative powers, and the mass production rate due to photochemistry.&#160;Chemical and energy source terms due to radiation are included in the Navier-Stokes equations (stagnation-line solver) by following the work of Soucasse et al. [8] and Dias et al. [9].Material solver: It solves the material phase-change and the removal of the liquid layer by shear forces. The solver takes as boundary conditions from the stagnation-line solver the heat flux, aerodynamic forces, and evaporation rate.&#160; The reader is referred to Dias et al. [10] for a complete description of the material solver.Finally, we use an implicit approach to couple the material and flow solver [11, 12].&#160;Results &amp; ConclusionWe simulate a trajectory between 60 km and 53 km &#8211; with an interval of 1 km from a point to another &#8211; with a constant velocity of 14.15 km/s [6].A fair comparison has been obtained at the first trajectory points between the numerical results and the dynamic mass derived from observations.Bellow 57 km, the numerical results underestimate the ablation.The removal of mass due to shear forces is the primary source of ablation above 54 km, whereas the evaporation rate becomes dominant at 53 km. The evaporation rate increases at lower altitudes due to a surface temperature rise owed to the rise of the radiative heat flux at the surface.The average shear forces, which causes molten layer removal, increase along the trajectory due to an increase of the free-stream pressure. Despite the rise of the aerodynamic forces, the melting mass removal tends to an asymptotic value. This effect is owed to a decrease of the molten thickness caused by a more substantial evaporation rate.Regarding the in-dept material, one observes a large temperature gradient close to the surface while the core remains cold.This large temperature gradient at the surface is a combination of the low material thermal diffusivity and the large mass removal.&#160;Due to the coupling between the flow and material, we have observed a much lower evaporation rate than in the results shown in Dias et al. [9].&#160;These results support the importance of material/flow coupling for this type of bolide. Moreover, the liquid layer removal is the dominant source of ablation for most of the trajectory. Additional processes might be missing from our analysis, such as the mass removal due to the inertial forces that might explain the small discrepancy between our results and the observations.&#160;References&#160;[1] A. P. Golub, I. B. Kosarev, I. V. Nemchinov, and V. V. Shuvalov. Emission and Ablation of a Large Meteoroid in the Course of Its Motion through the Earth&#8217;s Atmosphere. Solar System Research, 30:183, 1996.[2] C. O. Johnston and E. C. Stern. A model for thermal radiation from the Tunguska airburst. Icarus, 327:48&#8211;59, Jul 2019.[3] C. O. Johnston, E. C. Stern, and L. F. Wheeler. Radiative heating of large meteoroids during atmospheric entry. Icarus, 309:25 &#8211; 44, 2018.[4] V. Shuvalov and N. Artemieva. Numerical modeling of Tunguska like impacts. Planetary and Space Science, 50(2):181 &#8211; 192, 2002.[5] V. V. Svettsov, V. V. Shuvalov, and O. P. Popova. Radiation from a superbolide. Solar System Research, 52(3):195&#8211;205, May 2018.[6] Z. Ceplecha and D. 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