Entry Modeling for Asteroid Threat Assessment

Transcription

1 Presenter: Eric Stern Entry Modeling for Asteroid Threat Assessment Asteroid Threat Assessment Project (ATAP), Entry Modeling Lead Team: Parul Agrawal, Dinesh Prabhu, Francesco Panerai, Justin Haskins, Joseph Brock, Greg Gonzalez, Tane Boghozian, Aaron Brandis Analytical Mechanics Associates, Inc. Susan M. White, Y-K. Chen, Ethiraj Venkatapathy, and James O. Arnold NASA Ames Research Center Chris Johnston NASA Langley Research Center and Peter Jenniskens SETI Institute 61st Course of Hypersonic Meteoroid Entry Physics

2 Asteroid Threat Assessment Project (ATAP) Asteroid Properties Characterization Measurements Inference Data aggregation Property database website Entry Physics Entry Simulations & Testing Ablation & radiation modeling Ground testing Fracture, Fragmentation, and Break-up Surface Hazards Damage & Risk Hazard Simulations 3D blast simulations Impact crater simulations Tsunami simulations Thermal radiation models Global effects Probabilistic Risk Assessment Analytic physics-based entry and damage models Probabilistic Monte Carlo simulation using uncertainty distributions 2

3 Heating and Ablation in Threat Assessment Asteroid Entry Equation of Motion * Airburst height ab = C H Q Heat Transfer Coefficient Heat of Ablation * Wheeler et al.,

4 Entry Modeling in Threat Assessment Asteroid Entry Equation of Motion * Airburst height ab = C H Q = 0.1 = 8 MJ/kg Nominal Values NASA Asteroid Threat Assessment Project working to improve models for these phenomena ** Prediction of fragmentation onset, and fragment spread rate is an important driver of energy deposition rate. While not a focus of this talk, it is an active area of research * Wheeler et al.,

5 Large Meteoroid Entry Environment Ablation products mix with shock-heated gas in the wake and emit radiation, producing observed light curves and spectra (on-going work) Flow of melted material Strong radiative heat flux to the surface Shock layer radiation out to the surroundings Massive ablation from vaporization produces thick layer of ablation products 5

6 Large Meteoroid Entry Environment Outline for the talk: Flow of melted material Strong radiative heat flux to the surface Massive ablation from vaporization produces thick layer of ablation products Focus will be on efforts to improve heating and ablation models for large meteoroids Coupled CFD-Radiation-Ablation calculations to assess heating High-energy ground experiments to investigate meteoroid ablation Brief highlights of other entry modeling research Tomography of meteorite fracture Characterization of meteoritic melts Experimental/numerical studies of fragment spreading 6

7 Aerothermodynamic Analysis of Meteoroid Heating 7

8 Coupled Aerothermal Environment Modeling For trajectory simulations of meteoroid entries, the heat transfer coefficient is often assumed to be a constant 0.1 Based on inviscid stagnation-line analyses by Page et al. (AIAA ) as correlated by Baldwin and Sheaffer (JGR, 1971). Includes coupled radiation Coupled ablation not included Current work focuses on development of new heat transfer coefficient model for large meteoroids, using fully coupled axisymmetric aerothermal calculations For axisymmetric spherical meteoroid, the heat transfer coefficient is defined as : For large meteoroids, convective heating is negligible compared with radiation 8

9 Coupled Aerothermal Environment Modeling CFD (LAURA) q conv T ve N i Surface S rad Radiation Code (HARA) q rad Meteor Surface Ablation Products Air Species More detail on methodology, analysis of coupled radiation only, precursor radiation, and other results can be found in: - Johnston and Stern, AIAA Paper, June Johnston et al., Icarus, in-review 9

10 Coupled Ablation Details 26 species added to 13 species air for flowfield model: Fe, Fe+, FeO, Mg, Mg+, MgO, Si, Si+, SiO, SiO2, Fe++, Mg++, Si++, S, S+ SO, SO2, Al, Al+, AlO, Ca, Ca+, CaO, Na, Na+, NaO. Flowfield kinetic rates obtained from literature. Ablation rate and wall temperature computed from the surface energy balance and steady-state, equilibrium ablation assumption. Elemental mass fractions assumed similar to LL-Chondrite: O Si Mg Fe S Al Ca Na

11 Coupled Ablation Details p = 70 atm, T = 10,000 K Ablation Products Air For the same conditions, meteoritic ablation products absorb significantly more than air Relatively low ionization energies of Mg and Ca (~6-8eV) relative to air species, result in stronger absorption 11

12 Coupled Ablation Results Diameter = 20m Velocity = 20 km/s Altitude = 50 km Resulting ablation rate at the stagnation point = 50% of freestream mass flux Stagnation Line Temperatures 12

13 Coupled Ablation Results Mole M ole Fraction Si Si+ Mg Mg+ Fe Fe+ S S+ Na Na+ Ca Ca+ Al Al+ SiO MgO FeO CaO distance along stagnation line (cm) distance along stagnation line [cm] 88% reduction No Ablation With Ablation Both molecular and atomic species present in ablation layer at the stagnation point Very significant reduction in wall-directed heat flux due to presence of ablation products 13

14 Impact of Turbulence Cebeci-Smith turbulence model applied to coupled radiation and ablation cases. This low-fidelity turbulence model provides robust approach for assessing qualitative impact of turbulence on coupled flowfield. Increased radiative heating downstream of stagnation point due to effect of turbulence Depletion of molecular species due to higher temperatures near the wall, results in less blockage Body Normal Line Radiative Heating (W/cm [W/cm2] 2 ) Laminar Turbulent radial distance (m) Vertical distance from centerline [m] MgO Mole Fraction Distance from surface [cm] 14

15 New Heat Transfer Coefficient Model 15

16 High-energy Meteoroid Ablation Experiments 16

17 Meteoroid Ablation Experiments (Eric Stern, Susan White, Parul Agrawal) Continuous Wave Laser Experiment Laser testing apparatus Tamdakht H5 Chondrite samples tested at heating rates from 5 to 16 kw/cm 2 Arc Jet Experiment Heating rates (~4 kw/cm 2 ) produced in the experiment comparable to 30m asteroid at 20 km/s at 65km altitude Machined sphere-cone model allows for highfidelity simulation of the test environment and material response Schematic arc jet test article Tamdakht H5 article for laser testing Tamdakht H5 model for arc jet testing 17

18 Tamdakht H5 Chondrite 5 kw/cm 2 Laser Spot High-speed video showing boiling meteorite surface 18

19 Laser Experiment Findings 8 MJ/kg 18 MJ/kg At low heat flux, effective heat of ablation value close to canonical value of 8 MJ./kg Reduction in ablative efficiency at high heat fluxes Rate limited vaporization processes Blockage of radiative flux by ablation products 19

20 Meteoroid Ablation Experiments Continuous Wave Laser Experiment Test Objectives Source of heating is radiation, which is the dominant Obtain quantitative source of heating on meteoroid for large ablation meteoroids in a Tamdakht well characterized H5 Chondrite environment samples tested at heating Assess rates viability from of 5 obtaining to 16 kw/cm high-resolution 2 emission spectra for meteoroid ablation products Laser testing apparatus Arc Current Stagnation enthalpy Cold wall heat flux Stagnation pressure 6000 Amp ~20 MJ/kg 4 kw/cm2 135 kpa Arc Jet Experiment Cold-wall Heating rates (~4 kw/cm 2 ) produced in the experiment comparable to 30m asteroid at 20 km/s at 65km altitude Machined sphere-cone model allows for highfidelity simulation of the test environment and material response Schematic arc jet test article Test report detailing methodology and results by P. Agrawal et al.to be released to public in coming months Tamdakht H5 article for laser testing Tamdakht H5 model for arc jet testing 20

21 Arc Jet Experiment 5 second exposure 5 second exposure Pure Silica Terrestrial Basalt 3 second exposure 2 second exposure FY!7 Review Campo del Cielo Iron Meteorite NASA Internal Use Only Tamdakht H5 Chondrite 21

22 Fused Silica Much lower ablation rate, owing to much higher melt viscosity 22

23 Basalt More rampant spallation observed (consistent with laser experiment) 23

24 Campo del Cielo Interesting transient heating apparent Refractory inclusions protrude into flow Preliminary analysis shows iron-rich spectrum 24

25 Tamdakht H5 Chondrite Widespread melt flow Some spallation, but fairly well-behaved 25

26 Arc Jet Experiment Findings Effective heat of ablation (Q*) from the experiment ~ 2 MJ/kg Heat is well below the canonical value of 8 MJ/kg for chondrite vaporization Indicates we are in a melt dominated regime!26

27 Arc Jet Experiment Findings High resolution emission spectra from experiment rich in more volatile elements Suggests preferential ablation processes See poster by Dias et al.!27

28 Effect of Ablation Parameter on Energy Deposition Nominal Value Range based on preceding analysis CH Q ab CH = Q

29 Effect of Ablation Parameter on Energy Deposition Strongly coupled and vaporization dominated Nominal Uncoupled and Melt Dominated 7km Increasing Size 7km 9km For 100m impactor, 9km burst height difference corresponds to 25km increase in 4psi blast footprint radius (using Glasstone and Dolan) 29

30 Conclusions on Heating and Ablation Testing Coupled Fluid Dynamics-Ablation-Radiation calculations show significant reduction in heating over canonical value, particularly at larger sizes relevant to planetary defense Ground test experiments yielding insight into ablation phenomena, and being used to develop and validate numerical models Emission spectra from arc jet experiment suggest preferential vaporization of volatile elements Reducing uncertainty in melt properties essential to accurately model meteoroid ablation Bias in ablation parameter toward the low-end results in lower altitude airburst, and therefore larger ground damage footprints Nominal ablation model results in underestimation of ground damage 30

31 neoproperties.arc.nasa.gov Properties include Density & porosity Compressive & tensile strength Elastic & shear moduli Heat capacity & thermal conductivity Preliminary meteorite class to asteroid taxonomy mappings September

32 Entry Modeling Team: Parul Agrawal, Dinesh Prabhu, Francesco Panerai, Justin Haskins, Joseph Brock, Greg Gonzalez, Tane Boghozian, Aaron Brandis Analytical Mechanics Associates, Inc. NASA Planetary Defense Coordination Officer: Lindley Johnson DLR Collaborators: Ali Guelhan, Sebastian Willems, Patrick Seltner, Ansgar Marwege, Dominik Neeb VKI Collaborators: Bruno Dias, Federico Bariselli, Thierry Magin Susan M. White, Y-K. Chen, Ethiraj Venkatapathy, and James Arnold NASA Ames Research Center Chris Johnston NASA Langley Research Center and Peter Jenniskens SETI Institute Special Thanks to NASA Ames IHF Team Thank you for your attention!