Abstract

*Spallation is a phenomenon in which solid particles are ejected off the surface of an ablative material in a high-enthalpy, high-shear flow field. The main contributor to this phenomenon in carbon-based heat shields is the mechanical erosion of carbon fibers weakened by oxidation decomposition. The dynamics of this phenomenon, which are poorly characterized in the literature, strongly affect the ablation rate of the material. In state-of-the-art codes, ablation by spallation is modeled using a “failure” ablation rate that is empirically determined. The present study aims at understanding the rate of ablation of low-density carbon materials. Results from a test campaign at the NASA Langley Hypersonic Materials Environmental Test System (HYMETS) arc jet facility are used to examine spallation. High-speed multi-camera imagery at 44,000 fps is used to generate velocity vectors of spalled particles emitted from carbon-fiber samples exposed to an arc jet airflow. The imagery recorded approximately 4×106 unique particles, indicating that spallation is a potentially non-trivial process. The velocities of the particles ejected from the surface were found to be between 10 m/s and 20 m/s, accelerating to velocities as high as 250 m/s further away from the sample surface. Although the particle diameters were not directly observable, estimates suggest anywhere from 0.06% to 5.6% of the mass loss from the sample occurred due to spallation.*

Borchetta, C. G., Martin, A., and Bailey, S. C. C., “Examination of the effect of blowing on the near-surface flow structure over a dimpled surface,” *Experiments in Fluids*, vol. 59, no. 3, Article 36, 2017.

doi: 10.1007/s00348-018-2498-z.

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Abstract

*Spallation is a phenomenon in which solid particles are ejected off the surface of an ablative material in a high-enthalpy, high-shear flow field. The main contributor to this phenomenon in carbon-based heat shields is the mechanical erosion of carbon fibers weakened by oxidation decomposition. The dynamics of this phenomenon, which are poorly characterized in the literature, strongly affect the ablation rate of the material. In state-of-the-art codes, ablation by spallation is modeled using a “failure” ablation rate that is empirically determined. The present study aims at understanding the rate of ablation of low-density carbon materials. Results from a test campaign at the NASA Langley Hypersonic Materials Environmental Test System (HYMETS) arc jet facility are used to examine spallation. High-speed multi-camera imagery at 44,000 fps is used to generate velocity vectors of spalled particles emitted from carbon-fiber samples exposed to an arc jet airflow. The imagery recorded approximately 4×106 unique particles, indicating that spallation is a potentially non-trivial process. The velocities of the particles ejected from the surface were found to be between 10 m/s and 20 m/s, accelerating to velocities as high as 250 m/s further away from the sample surface. Although the particle diameters were not directly observable, estimates suggest anywhere from 0.06% to 5.6% of the mass loss from the sample occurred due to spallation.*

Bailey, S. C. C., Bauer, D., Panerai, F., Splinter, S. C., Danehy, P. M., Hardy, J. M., and Martin, A., “Experimental analysis of spallation particle trajectories in an arc-jet environment,” *Experimental Thermal and Fluid Science*, 2018.

doi: 10.1016/j.expthermflusci.2018.01.005

Sparks, J. D., Whitmer, E. C., Myers, G. I., Montague, C. C., Dietz, C. J., Khouri, N., Nichols, J. T., Smith, S. W., and Martin, A., “Overview of the first test-flight of the Kentucky re-entry universal payload system (KRUPS),” *56th AIAA Aerospace Sciences Meeting,* AIAA Paper 2018-1720, Kissimmee, FL, January 2018.

doi: 10.2514/6.2018-1720

Duzel, U., Schroeder, O. M., and Martin, A., “Computational prediction of nasa langley hymets arc jet flow with KATS,” *56th AIAA Aerospace Sciences Meeting*, AIAA 2018-1719, Kissimmee, FL, January 2018.

doi: 10.2514/6.2018-1719

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*This paper presents a data-driven computational model for simulating unsteady turbulent flows, where sparse measurement data is available. The model uses the retrospective cost adaptation (RCA) algorithm to automatically adjust the closure coefficients of the Reynolds-averaged Navier-Stokes (RANS) k – ω turbulence equations to improve agreement between the simulated flow and the measurements. The RCA-RANS k – ωmodel is verified for steady flow using a pipe-flow test case and for unsteady flow using a surface-mounted-cube test case. Measurements used for adaptation of the verification cases are obtained from baseline simulations with known closure coefficients. These verification test cases demonstrate that the RCA-RANS k – ω model can successfully adapt the closure coefficients to improve agreement between the simulated flow field and a set of sparse flow-field measurements. Furthermore, the RCA-RANS k – ω model improves agreement between the simulated flow and the baseline flow at locations at which measurements do not exist. The RCA-RANS k – ω model is also validated with experimental data from 2 test cases: steady pipe flow, and unsteady flow past a square cylinder. In both test cases, the adaptation improves agreement with experimental data in comparison to the results from a non-adaptive RANS k – ω model that uses the standard values of the k – ω closure coefficients. For the steady pipe flow, adaptation is driven by mean stream-wise velocity measurements at 24 locations along the pipe radius. The RCA-RANS k – ω model reduces the average velocity error at these *locations* by over 35%. For the unsteady flow over a square cylinder, adaptation is driven by time-varying surface pressure measurements at 2 locations on the square cylinder. The RCA-RANS k – ω model reduces the average surface-pressure error at these locations by 88.8%.*

[1] Li, Z., Bailey, S. C. C., Hoagg, J. B., and Martin, A., “A retrospective cost adaptive Reynolds-averaged Navier-Stokes k-w model for data-driven unsteady turbulent simulation,” *Journal of Computational Physics*, 2018.

DOI:10.1016/j.jcp.2017.11.037

[1] Fu, R., Weng, H., Wenk, J. F., and Martin, A., “Thermo-mechanical coupling for charring ablators,” *Journal of Thermophysics and Heat Transfer*, 2017. doi: 10.2514/1.T5194.

https://uknow.uky.edu/research/after-developing-small-spacecraft-uk-students-help-launch-it-nasa-wallops ]]>

Martin, A., Zhang, H., and Tagavi, K. A., “An introduction to a systematic derivation of surface balance equations without the excruciating pain,” *International Journal of Heat and Mass Transfer*, Vol. 115, Part A, December 2017, pp. 992–999.

DOI: 10.1016/j.ijheatmasstransfer.2017.07.078

*Analyzing complex fluid flow problems that involve multiple coupled domains, each with their respective set of governing equations, is not a trivial undertaking. Even more complicated is the elaborate and tedious task of specifying the interface and boundary conditions between various domains. This paper provides an elegant, straightforward and universal method that considers the nature of those shared boundaries and derives the appropriate conditions at the interface, irrespective of the governing equations being solved. As a first example, a well-known interface condition is derived using this method. For a second example, the set of boundary conditions necessary to solve a baseline aerothermodynamics coupled plain/porous flow problem is derived. Finally, the method is applied to two more flow configurations, one consisting of an impermeable adiabatic wall and the other an ablating surface.*

[1] Irvan, M. L., Barrow, C., Keen, A., Maddox, J. F., and Martin, A., “Physics Based Modeling of Fibrous High Porosity Insulation Materials Using Comparative Cut-Bar Experimentation,” *24th AIAA Aerodynamic Decelerator Systems Technology Conference*, AIAA Paper 2017-3887, Denver, CO, June 2017.

doi: 10.2514/6.2017-3887

[2] Weng, H. and Martin, A., “Development of a Universal Solver and Its Application to Ablation Problems,” *47th AIAA Thermophysics Conference*, AIAA Paper 2017-3355, Denver, CO, June 2017.

doi: 10.2514/6.2017-3355

[3] Omidy, A. D., Weng, H., Martin, A., and Gran ̃a-Otero, J. C., “Modeling Gasification of Carbon Fiber Preform in Oxygen-Rich Environments,” *47th AIAA Thermophysics Conference*, AIAA Paper 2017-3686, Denver, CO, June 2017.

doi: 10.2514/6.2017-3686

[4] Omidy, A. D., Cooper, J. M., Fu, R., Weng, H., and Martin, A., “Development Of An Open-Source Avcoat Material Database, VISTA,” *47th AIAA Thermophysics Conference*, AIAA Paper 2017-3356, Denver, CO, June 2017.

doi: 10.2514/6.2017-3356

This paper presents a new data-driven adaptive computational model for simulating turbulent flow, where partial-but-incomplete measurement data is available. The model automatically adjusts the closure coefficients of the Reynolds-averaged Navier–Stokes (RANS) k–ω turbulence equations to improve agreement between the simulated flow and the measurements. This data-driven adaptive RANS k–ω (D-DARK) model is validated with 3 canonical flow geometries: pipe flow, backward-facing step, and flow around an airfoil. For all test cases, the D-DARK model improves agreement with experimental data in comparison to the results from a non-adaptive RANS k–ω model that uses standard values of the closure coefficients. For the pipe flow, adaptation is driven by mean stream-wise velocity data from 42 measurement locations along the pipe radius, and the D-DARK model reduces the average error from 5.2% to 1.1%. For the 2-dimensional backward-facing step, adaptation is driven by mean stream-wise velocity data from 100 measurement locations at 4 cross-sections of the flow. In this case, D-DARK reduces the average error from 40% to 12%. For the NACA 0012 airfoil, adaptation is driven by surface-pressure data at 25 measurement locations. The D-DARK model reduces the average error in surface-pressure coefficients from 45% to 12%.

Li, Z., Zhang, H., Bailey, S. C., Hoagg, J. B., and Martin, A., “A Data-Driven RANS k-ω approach for modeling turbulent flows,” *Journal of Computational Physics,* vol. 345, 2017, pp. 111–131.

doi:10.1016/j.jcp.2017.05.009

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