Detached Eddy Simulations have been performed to investigate the effects of high-fidelity turbulence modeling on roughness-induced transition to turbulence during Mars entry. Chemically reacting flow solutions will be obtained for a gap filler of Mars Science Laboratory at the peak heating condition

Barnhardt, Michael D.

Candler, Graham

Sozer, Emre

Tang, Chun Y.

Yoon, Seokkwan

NASA Technical Reports Server

 
American Institute of Aeronautics and Astronautics 
 
 
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Abstract of paper for 42nd AIAA Fluid Dynamics Conference, New Orleans, June 2012 
Gap Filler Induced Transition on the Mars Science 
Laboratory Heatshield 
Seokkwan Yoon1, Michael D. Barnhardt2, Chun Y. Tang3 and Emre Sozer4 
NASA Ames Research Center, Moffett Field, California 94035 
and 
 
Graham V. Candler5  
University of Minnesota, Minneapolis, MN 55455 
Detached Eddy Simulations have been performed to investigate the effects of high-fidelity 
turbulence modeling on roughness-induced transition to turbulence during Mars entry. 
Chemically reacting flow solutions will be obtained for a gap filler of Mars Science 
Laboratory at the peak heating condition.  
I. Introduction 
  
Inaccurate prediction of aerodynamic heating may lead to mission failures.  For instance, vortices in turbulent 
boundary layers might cause uneven ablation of thermal protection materials and hence affect vehicle aerodynamics.  
Since turbulent heat transfer rates can be higher than laminar heating rates, reductions in the weight of thermal 
protection systems can be realized with an improved understanding of the physics of transition from laminar to 
turbulent flow.  The gap-filler incident of Space Shuttle mission STS-114 in 2005 was a potent reminder of the 
importance of accurate prediction of roughness-induced boundary layer transition and subsequent increase in surface 
heating. 
Next generation NASA missions will rely on larger entry systems, which will place increased emphasis on 
improved understanding of turbulent heating, transition to turbulence, shock layer radiation, and complex surface-
chemistry interactions.  These phenomena must be modeled in the context of a hypersonic chemically reacting flow 
environment. Thermal protection system materials that are chemically designed to self-heal or affect boundary layer 
transition have the potential to significantly enhance future mission performance. 
The Detached Eddy Simulation (DES) method, which employs a Reynolds-Averaged Navier-Stokes (RANS) 
turbulence model in the boundary layer while behaving like a Large Eddy Simulation (LES) model away from the 
wall, has been used to study hypersonic transition to turbulence in a perfect-gas wind tunnel test1 and in real-gas 
ballistic range experiments,2,3 as well as in high-speed reentry base flows, with favorable results.4  High-enthalpy 
flow transition experiments were conducted in the NASA Ames Ballistic Range for blunt bodies with isolated 
roughness elements in CO2 gas, simulating a Mars-like atmosphere.5  It has been shown that DES can be used to 
overcome the scaling problems associated with Direct Numerical Simulations of boundary layers and can predict 
boundary layer transition in air. 
Mars Science Laboratory (MSL) is scheduled for launch in late 2011 and is expected to land on the surface of 
Mars in August 2012.  The MSL heatshield Thermal Protection System (TPS) is constructed of a Phenolic 
Impregnated Carbon Ablator (PICA), which was used successfully on the Stardust mission.  Since the MSL 
                                                            
1 Research Scientist, NASA Advanced Supercomputing Division, Member AIAA 
2 Research Scientist, ERC, Inc., Member AIAA 
3 Research Scientist, Entry Systems and Technology Division, Member AIAA 
4 Research Scientist, STC Corp., Member AIAA 
5 Professor, Dept. of Aerospace Engineering and Mechanics, Fellow AIAA 
https://ntrs.nasa.gov/search.jsp?R=20120015909 2019-08-30T22:47:47+00:00Z
 
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aeroshell is quite large, as shown in Fig. 1, it is not possible to create a monolithic PICA heatshield. Instead, PICA is 
applied to the aeroshell in a tile pattern, and the tiles are bonded to the aeroshell structure using an adhesive 
material. A total of 113 tiles are used to form the heatshield, and gaps are left intentionally between the tiles to allow 
for thermal expansion.  To prevent hot gases from flowing between the tiles, gaps are filled with liquid RTV-560, 
which is then cured to form solid gap fillers.  Due to a difference in recession rates of PICA and RTV-560, it is 
possible that the gap fillers will recess less than the tiles during the probe’s entry into Mars. This behavior could 
result in gap fillers protruding from the surface of the heatshield.  These protuberances may induce the flowfield to 
transition earlier to turbulence and generate vortices downstream of the gap fillers. These phenomena could increase 
the heat loads on the TPS and increase the risks to the success of the mission. 
A computational study was performed using steady-state RANS on relatively coarse grids to estimate the heating 
augmentation.6 The objective of the present paper is to investigate the effects of unsteady turbulence modeling on 
roughness-induced transition in CO2 by using DES on very fine grids.  
 
 
II. Turbulence Modeling 
 
The default turbulence modeling options in US3D are RANS-based models. While RANS methods produce 
mostly inaccurate solutions for high-speed flows with large separation regions, the DES approach of Spalart has 
predicted unsteady separated flows with improved accuracy by modeling the dynamics of the dominant length 
scales.7 
In this work, we have chosen to use the one-equation Spalart-Allmaras8 model with the compressibility 
correction of Catris and Aupoix.  The model consists of a transport equation for a variable 
! 
˜ " , which is related to the 
eddy viscosity 
! 
" t . 
 
D! %"
Dt =!" (
1
#
µ! %" )+!" ( 1
#
! %"! ! %" )+ cb2
#
! ! %" !" ! %" + cb1 %S! %" ! cw1 fw!(
%"
d )
2       (9) 
 
Here 
! 
"  is the density, 
! 
µ is the molecular viscosity, 
! 
˜ S is related to the vorticity magnitude, 
! 
fw  is a function of the 
local flow state, d is the distance to the nearest surface, and 
! 
" , 
! 
cb1, 
! 
cb2, and 
! 
cw1 are constants. The form of this 
equation is derived largely from empirical arguments, and a detailed explanation of its development can be found in 
Ref. 11. 
The first three terms on the right-hand side of the above equation represent diffusive transport of turbulent 
viscosity, the next-to-last term governs its production, and the final term is responsible for its destruction. Once the 
eddy viscosity is computed from this equation, it is used to augment the molecular viscosity. Thus, in regions of 
high turbulent viscosity the effective diffusive transport will be enhanced above the molecular level. 
Note that the destruction term depends on the inverse distance to the nearest solid surface. Thus, when the model 
is used far from solid surfaces, there is very little destruction of turbulent viscosity, and high levels of 
! 
˜ "  persist. 
This is what causes the RANS approach to over-predict the turbulent dissipation levels in separated flow regions. 
The DES solution of this problem is to replace the length scale of the model with a new one, such that the model 
behaves as a subgrid scale model in regions where the flow is separated.  Thus the new length scale should act as a 
filter, allowing the simulation to resolve eddies down to the grid scale while modeling those that are smaller. For the 
particular case of the Spalart-Allmaras model, this is accomplished in a straightforward manner by replacing d with 
a new variable that depends on some measure of the local grid cell size: 
 
! 
˜ d = min(d,CDES")                         (10) 
 
 With this definition, the modified equation is equivalent to the unchanged Spalart-Allmaras RANS model near 
walls. Away from walls, if the filter width (i.e. cell size) is chosen to lie in the inertial subrange, the production and 
destruction terms are balanced and it is easy to show that the Spalart-Allmaras model reduces to the familiar 
Smagorinksy form.  Indeed, in the context of the present study, it is perhaps more accurate to refer to our DES 
formulation as a Wall-Modeled LES because we are attempting to resolve the boundary layer itself, rather than 
simply allowing the model to operate entirely in RANS mode. 
! 
CDES  is an adjustable parameter, conventionally set 
 
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to 0.65, determined by Shur et al.9 using simulations of homogeneous turbulence. 
 
III.  Chemistry Modeling 
 
 A five-species (CO2, CO, O2, C, and O) chemical and vibrational non-equilibrium model10 for CO2 has been 
used for the chemically reacting flow computations.  Table 1 shows three dissociation and three exchange reactions 
for CO2.  Due to differences in the covalent bonding of the constituent molecules, more molecular dissociation is 
expected in CO2 than in air.  Since both O2 and CO2 have double bonds, they dissociate more easily through 
collisions than N2, which has a triple bond. 
 
 
!"#$%&'(&&)*%+,-"$&.%"-/,012&304&)56&!"#$%$&$$$!$!"$%$"$%$&$!"$%$&$$$$!$!$%$"$%$&$"#$%$&$$$$$!$"$%$"$%$&$!"$%$"$$$$!$"#$%$!$!"#$%$"$$$!$"#$%$!"$!"$%$!"$$!$!"#$%$!$$
 
IV. Preliminary Results 
Although US3D11,12 can handle both tetrahedral and hexahedral meshes, unstructured hexahedral-only grids 
have been used in the present study.  All computations are three-dimensional unsteady DES unless stated otherwise. 
Fine-grid computations have been performed using 1024 cores on the Pleiades system (a supercomputer which 
currently has 112,896 cores and 191 TB memory) at the NASA Advanced Supercomputing Division. The speed of 
each six-core Intel Xeon Westmere processor is 3.06 GHz, and the available memory is 2 GB per core. 
Our test case is a flow in CO2 over a gap filler protrusion located at the leeward side of the heatshield, where we 
expect the boundary layer to be thick and the heat flux to be high.  Figure 2 shows a partial view of the grid around 
the gap filler.  At the peak heating, the freestream velocity is 5 km/s, and the freestream temperature is 176.45K. The 
angle of attack at the peak heating is 15.49 deg. 
Three levels of grids have been generated.  The number of nodes for coarse, medium, and fine grids are 6 
million, 48 million, and 383 million, respectively.  Figures 3a and 3b show temperature and Mach number contours 
in the shock layer.  Surface density and pressure contours are shown in Fig. 4a and Fig. 4b.  Pressure gradients in 
Fig. 4b between the capsule and the outer boundary indicate a bow shock.  Fine-grid chemically-reacting unsteady 
flow DES computations will be performed to study the effect of high-fidelity turbulence modeling on the prediction 
of roughness-induced transition to turbulence in Mars entry. 
 
 
 
References 
 
1Yoon, S., Barnhardt, M.D., and Candler, G.V., “Simulations of High-Speed Flow over an Isolated Roughness,” 
AIAA Paper 2010-1573, Jan. 2010. 
2Yoon, S., Barnhardt, M.D., Sozer, E., and Candler, G.V., “Hypersonic Transition to Turbulence Triggered by 
Roughness Elements,” AIAA Paper 2011-565, Jan. 2011. 
3Yoon, S., Barnhardt, M.D., Sozer, E., and Candler, G.V., “Numerical Simulations of Transition Experiments in 
the Hypersonic Ballistic Range,” AIAA Paper 2011-3416, June 2011. 
4Barnhardt, M. D. and Candler, G. V., “Detached Eddy Simulation of the Reentry-F Flight Experiment,” AIAA 
Paper 2008-625, Jan. 2008. 
 
American Institute of Aeronautics and Astronautics 
 
 
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5Reda, D.C., Wilder, M.C., and Prabhu, D.K., “Transition Experiments on Blunt Bodies with Isolated Rough 
ness Elements in Hypersonic Free Flight,” Journal of Spacecraft and Rockets, Vol. 47, No. 5, 2010, pp. 828-835. 
6Tang, C.Y., Wright, M.J., Edquist, K.T., Sepka, S.A., and Cassell, A.M., “Numerical Simulations of Protruding 
Gap Fillers on the Mars Science Laboratory Heatshield,” AIAA Paper 2009-4077, June 2009. 
7Spalart, P.R., W-H. Jou, M. Strelets, and S.R. Allmaras, “Comments on the Feasibility of LES for Wings and on 
a Hybrid RANS/LES Approach,” Advances in DNS/LES, 1st AFOSR International Conference on DNS/LES, 1997. 
8Spalart, P.R. and Allmaras, S.R., “A One-Equation Turbulence Model for Aerodynamic Flows,” AIAA Paper 
1992-0439, Jan. 1992.  
 9Shur, M., Spalart, P.R., Strelets, M., and Travin, A., “Detached Eddy Simulation of an Airfoil at High Angle of 
attack,” 4th Int. Symp. Eng. Turb. Modeling and Measurements, 1999. 
10Park, C., Howe, J.T., Jaffe, R.J., and Candler, G.V. "Review of Chemical-Kinetic Problems of Future NASA 
Missions, I: Mars Entries," Journal of Thermophysics and Heat Transfer, Vol. 8, No. 1, Jan. 1994, pp. 9-23. 
      11Nompelis, I., Drayna, T., and Candler, G.V., “A Parallel Unstructured Implicit Solver for Hypersonic Reacting 
Flow Simulation,” AIAA Paper 2005-4867, June 2005. 
12Johnson, H.B., Drayna, T.W., Nompelis, I., and Candler, G.V., “US3D-1.0 Manual,” Oct. 2011. 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1. MSL capsule geometry 
 
Fig. 2. Surface grid around the gap filler 
 
 
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Fig. 3a. Temperature contours in the shock layer 
 
Fig. 3b. Mach number contours in the shock layer 
 
Fig. 4a. Surface density contours 
 Fig. 4b. Surface pressure contours 
 


Gap Filler Induced Transition on the Mars Science Laboratory Heatshield

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