American Institute of Aeronautics and Astronautics

1

Comparisons of Surface Roughness in Laminar and Turbulent Environments for Orion Thermal Protection

System

Antonella I. Alunni,* Michael W. Olson,† Tahir Gökçen,‡ and Kristina A. Skokova§ NASA Ames Research Center, Moffett Field, CA 94035

This paper addresses surface roughness measurements obtained from arc jet tests of Avcoat that were conducted in four different facilities at NASA Ames Research Center for Orion. Approaches and techniques for testing, measuring, and modeling fluid and material response in laminar and turbulent flows in these facilities are presented. Measured test conditions are consistent with corresponding computational simulations in each facility, and test results demonstrate that while the laminar test facilities create flow artifacts on the surfaces of the test samples, the turbulent test facility generates material roughness without the presence of flow patterns.

I. Introduction The current Orion heatshield utilizes Avcoat for the forebody thermal protection system (TPS), which is designed to protect the crew vehicle from reentry heating. Avcoat is a mid-density, glass-phenolic ablative honeycomb material with favorable mechanical properties. However, stagnation arc jet tests have demonstrated that the ablative filler material in Avcoat recesses at a faster rate than the honeycomb, resulting in varying degrees of roughness across the test surface, depending on the facility conditions. This is a concern because, during reentry, distributed roughness across ablating TPS causes perturbations in a laminar boundary layer that result in transition to turbulence, augmented heating, and ultimately higher TPS recession rates. In response, conservative approaches are taken to manage roughness in TPS design. For the Mars Science Laboratory heatshield, the maximum roughness height resulting from laminar and turbulent tests of TPS material was used to evaluate the heat flux bump factor for the heatshield design environments.1 The bump factor itself depended on the shear stress correlation Rek = ρwUτk/µw, where Rek is the roughness Reynolds number based on the roughness height, k, and ρw, Uτ, and µw, which are the density, shear velocity, and viscosity, respectively, at the wall.2 Besides assuming a fully-turbulent flow during the entire duration of the trajectory, the risks and uncertainties introduced by ablation-induced roughness are also typically mitigated by adding margin to the TPS thickness based on the expected TPS roughness under laminar and turbulent flow.3 The purpose of this paper is to quantify the roughness height of Avcoat caused by an assortment of shear arc jet tests. Prior to this study, the majority of arc jet tests for Orion were conducted in stagnation, exposing Avcoat to relevant heat flux and surface pressure but without shear stress. Ground testing limitations make it difficult to simultaneously capture desired heat flux, pressure, and shear, and the challenges involved with testing and interpreting data in shear drove the project to primarily pursue heatshield thermal response testing in stagnation.4 Very little shear testing of Avcoat had been accomplished, where a developed boundary layer environment could provide shear stress. So arc jet tests in shear were conducted in order to obtain in-depth temperature and surface response data for development of the material response model as well as to further develop a methodology for shear testing. This paper presents an investigation of surface roughness of Avcoat material tested in four facilities at NASA Ames Research Center: laminar testing in a wedge configuration in the low-enthalpy Aerodynamic Heating Facility (AHF) and high-enthalpy Interactive Heating Facility (IHF), laminar testing in the Truncated Panel Test Facility (TPTF), and turbulent testing in the Turbulent Flow Duct (TFD).

5 Ultimately, similar heat flux and pressure

* Research Scientist, Thermal Protection Materials Branch, ERC Incorporated, MS 234-1, AIAA Member † Materials Engineer, Thermal Protection Materials Branch, MS 234-1 ‡ Senior Research Scientist, Aerothermodynamics Branch, ERC Incorporated, MS 230-2, AIAA Senior Member § Senior Research Scientist, Thermal Protection Materials Branch, ERC Incorporated, MS 234-1

42nd AIAA Thermophysics Conference 27 - 30 June 2011, Honolulu, Hawaii

AIAA 2011-3776

Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner.

American Institute of Aeronautics and Astronautics

2

conditions were achieved in three of four facilities, allowing for an investigation of surface roughness across facilities and comparisons of surface roughness in laminar and turbulent environments. As a result, these shear tests that took place at Ames provide a useful comparison of laminar and turbulent tests.

II. Test Description

A. AHF and IHF Wedge Tests The 20-MW AHF and the 60-MW IHF are two of the arc jet facilities at NASA Ames that were used for laminar

testing with blunt wedge model holders. The AHF was outfitted with a conical nozzle with a 17.78-cm (7-inch) diameter nozzle exit and 3.81-cm diameter throat, and the test condition was achieved at the facility’s maximum current. The IHF, which has a previous testing history of ablative materials in a wedge configuration,6, 7 was configured with a conical nozzle with a 15.24-cm (6-inch) diameter nozzle exit and 6.03-cm diameter throat. In both facilities, cold air was mixed into the flow upstream of the nozzle throat in order to achieve higher pressure and lower enthalpy, and the wedge model holders were positioned 7.62 cm from the nozzle exit.

The model holders are 20o-half-angle water-cooled copper wedges,8 and they were mounted vertically, as shown in Figure 1a, for the test series. The wedge holder accommodates 11.15-cm x 12.17-cm test articles, also depicted in Figure 1a. The same wedge holder was used to accommodate a water-cooled calibration plate, which has a distribution of five Gardon gauges and two pressure ports, shown in Figure 1b. Gauges Q6-Q8 and ports P1 and P2 are located along the centerline. Calibration tests were performed with a copper transition piece ahead of the calibration plate.

B. TPTF Tests The 20-MW TPTF, which has a semi-elliptical nozzle with a 3.81-cm x 17-cm nozzle exit, was also used for laminar testing. The flat panel, either a calibration plate or test article, was mounted flush to the bottom surface of the nozzle exit at a 2o table angle. Calibration tests were executed using a water-cooled aluminum calibration plate that contains an array of twelve Gardon gauges and eight pressure ports,9 as illustrated in Figure 2b. In the TPTF, the region with the highest and most uniform heating on a panel is limited to a 10.16-cm x 10.16-cm test area, which corresponds to the location of the Avcoat sample, shown in Figure 2a; the rest of the test article panel was closed out with graphite side frames and a FiberForm back frame.

C. TFD Tests The 20-MW TFD, which has a rectangular nozzle with a 5.08-cm x 22.86-cm (2-inch x 9-inch) nozzle exit, was used for turbulent testing. The water-cooled wall that is flush against the 22.86-cm side of the nozzle exit is

a) b) Figure 1. a) Wedge holder with Avcoat test article, mounted vertically on sting arm, and b) wedge calibration plate schematic.

American Institute of Aeronautics and Astronautics

3

instrumented with up to eight Gardon gauges, seven of which were located on the calibration plate and one of which was located upstream of the test section, and up to twenty pressure ports, as shown in Figure 3b. The opposite-facing wall accommodates the test article, which consisted of a 20.32-cm x 25.40-cm full, flat panel of Avcoat that was instrumented with two Avcoat thermocouple plugs along the centerline. The thermocouple plugs, which were shorter than the panel thickness, were instrumented from base of the panel, leaving the top surface blank, as shown in Figure 3a.

a) b) Figure 2. a) TPTF Avcoat test article, and b) TPTF calibration plate schematic.

a) b) Figure 3. a) TFD Avcoat test article, and b) TFD calibration plate schematic.

III. CFD Simulations Computational fluid dynamics (CFD) analyses of the arc jet tests were performed through simulation of

nonequilibrium expanding flow in the arc jet nozzle, supersonic jet, and test box, and simulation of the flow around the test articles. For all CFD calculations, the DPLR code,10, 11 a code developed at NASA Ames, is used. For the present CFD simulations, the axisymmetric and 3-D Navier-Stokes equations, supplemented with the equations accounting for nonequilibrium kinetic processes, are used in the formulation. The present calculations employ a 6-

American Institute of Aeronautics and Astronautics

4

species air model (N2 O2, NO, N, O, Ar) for arc jet flow, and the thermal state of the gas is described by two temperatures: translational-rotational and vibrational-electronic, within the framework of Park's two-temperature model.12 Further details of the computational approach can be found in Refs. 9 and 13.

IV. Results Only one condition is described for each test facility in test series AHF 292, IHF 222, TPTF 137, and TFD 32.

The tests were designed such that AHF 292, TPTF 137, and TFD 32 would provide similar low heating rate distributions that were predicted for the Orion forebody, while IHF 222 would cover a higher heating rate on the forebody’s flight envelope. Observed and predicted results of test conditions and material response for each test series are also presented below.

A. Test Conditions Results for facility conditions and CFD simulations are summarized for laminar (in the AHF, IHF, and TPTF)

and turbulent (in the TFD) environments in Tables 1-3. Data from one calibration run are shown for the AHF, IHF, and TFD, while data from three calibration runs are shown for the TPTF. Select measured surface quantities are demonstrated as discrete values determined at specified gage locations, which are situated at the calibration plate’s centerline or correspond to the test sample area. Computed surface quantities are represented as ranges from the leading edge, where they are the highest, to the trailing edge of the calibration plate. Due to discrepancies in both heat flux and pressure measurements taken during the AHF calibration runs, these results are not listed at this time.

Measured cold wall fully catalytic (CWFC) heat flux and pressure are plotted for the IHF, TPTF, and TFD in Figure 4. Figures 5-8 show plots of the CFD results and calibration data, which illustrate the empirical and predicted variations of surface quantities that fall with distance from the leading edge. The CFD simulations and calibration measurements show that there is good agreement between computed and empirical results for a smooth surface. Measured and predicted results show very similar heat flux and pressure distributions in the TPTF and the TFD, while the predicted boundary layer thickness in the TFD, listed in Table 3, is almost twice as large as it is in the TPTF, which is shown in Table 2. Meanwhile, higher quantities and gradients of heat flux and pressure are seen in the IHF, however, predicted boundary layer thickness is thinnest in this facility. Although measurements for the AHF are not included, CFD simulations of heat flux show that predicted cold wall heat flux in the AHF is similar to measured and predicted heat flux in the TPTF and the TFD, and the predicted boundary layer thickness in the AHF, listed in Table 1, is much smaller than in the TPTF and TFD. At this time, the values for predicted boundary layer thickness in the TPTF and TFD fall within the range of boundary layer thickness values that are predicted for flight, while the predicted boundary layer thickness in the AHF and IHF falls below the range of values predicted for flight. Table 1. Summary of arc jet data and CFD predictions for the wedge test series.

American Institute of Aeronautics and Astronautics

5

Table 2. Summary of arc jet data and CFD predictions for the TPTF test.

Table 3. Summary of arc jet data and CFD predictions for the TFD test.

American Institute of Aeronautics and Astronautics

6

a) Heat flux b) Pressure Figure 4. Distributions of centerline measurements for each calibration run at each facility.

a) Heat flux b) Pressure Figure 5. Comparisons of computed centerline surface quantities in the AHF.

a) Heat flux b) Pressure Figure 6. Comparisons of computed and measured centerline surface quantities in the IHF.

American Institute of Aeronautics and Astronautics

7

a) Heat flux b) Pressure Figure 7. Comparisons of computed and measured centerline surface quantities in the TPTF.

a) Heat flux b) Pressure Figure 8. Comparisons of computed and measured centerline surface quantities in the TFD.

B. Surface Recession and Roughness A total of six representative test articles are presented in the following discussion to describe material response

results in terms of roughness in each facility. The representative test articles were tested for different durations: the AHF wedge test article, referred to as CT-46d-002, was tested for 216 seconds; the IHF wedge test article, referred to as AV-005-008, as well as one TPTP test article, referred to as AV-023-006, were tested for 60 seconds; another TPTF test article, referred to as AV-023-004, as well as one TFD test article, referred to as AV-020-002, were tested for 120 seconds; and another TFD test article, referred to as AV-020-004, was tested for 180 seconds.

The test articles from the AHF, IHF, and TPTF tests were laser scanned before and after testing, and the difference between pre-test and post-test measurements is the surface recession, which is shown as thick lines for locations along the centerline of each test article in Figure 9. The TFD test articles were scanned after testing; however, leading edge to trailing edge recession, represented as discrete data points in Figure 9, was determined at every centimeter along the centerline using pre-test and post-test measurements from a manual profilometer. The laser scanner and manual profilometer have been used extensively to measure pre-test and post-test profiles of previous Orion test articles. Both methods have an accuracy of ±0.5 mm, and there is good agreement between both methods.

As noted in previous studies of ablative material response in shear,6, 7 the recession behavior of articles tested in the IHF and TPTF is characterized by a rise near the leading edge followed by a decrease approaching the trailing

American Institute of Aeronautics and Astronautics

8

edge. The decline in recession downstream of the leading edge is consistent with surface heat flux and pressure data and predictions, which also fall with distance from the leading edge. Centerline recession from the AHF test article, which experienced the longest test duration, rises until the trailing edge is reached. For this test article, excessive recession occurred along the centerline from the middle of the test article to the trailing edge, which is illustrated in the laser-scan map in Figure 10. Centerline recession from the TFD test articles is negative near the leading edge due to swelling and accumulations of glass melt at the upstream portions of the test articles. With the exception of AV-005-008 and AV-023-006, which were tested for the shortest durations in the IHF and TPTF, respectively, the test articles experienced a substantial amount of recession.

Tables 4 and 5 list tabulated mean and maximum roughness heights for each test article, and Figures 10-15 show post-test photographs, laser-scan maps of post-test surface heights, post-test surface profiles, and plots of surface roughness across regions of interest to illustrate the effects of the facility conditions on the test material’s surface.

Figure 9. Surface recession along the centerlines of six test articles. Test facility and duration are indicated within parentheses in the legend. Table 4. Mean and maximum roughness heights at three locations for laminar test articles.

Table 5. Mean and maximum roughness heights at three locations for turbulent test articles.

American Institute of Aeronautics and Astronautics

9

For the AHF, IHF, and TPTF test samples, post-test surface profiles and surface roughness were evaluated normal to the flow direction and across the centers of the honeycomb cells at 7.62 cm from the leading edge, since shape change is minimal at this location of the test sample area. Profiles were also taken at 1 and 2 honeycomb cell units away from the chosen 7.62-cm location—that is, at 8.57 cm and 9.53 cm from the leading edge. A typical honeycomb cell width is 0.95 cm. For the TFD test samples, scans were also taken away from the plate leading edge, where the effects of shape change are minimal. Scans of a 10.16-cm x 10.16-cm region, located at the sample’s center and shown in Figure 14 and Figure 15, were used for post-test surface analysis. Post-test profiles normal to the centerline were taken at 15.24 cm, 16.19 cm, and 17.15 cm away from the leading edge. The difference between the maximum and minimum surface heights was evaluated at an interval of about one honeycomb cell unit, every 1.02 cm, to determine peak-to-valley surface roughness for each test article. Surface roughness was assessed from the three selected profiles across a 6.10-cm region of interest that is centered about the centerline and highlighted in each corresponding set of post-test surface profiles in Figures 10-15.

The surface data of all samples demonstrate, to varying degrees, honeycomb cell walls receding at a slower rate than the ablator material within the walls as well as stream-wise streaks. The differential recession produces roughness elements that, depending on the thickness of the boundary layer, disturb the flow downstream, perhaps enough to develop vortices, as suggested by the stream-wise patterns on the post-test surfaces. In Figure 11, the laser-scan map of the article tested in the IHF, where test conditions were more severe, shows differential recession at the leading edge followed by grooves, which may be an indication that the resulting roughness elements near the leading edge shed many vortices in its wake. However, the surface shows that the grooves diminished downstream, leaving the region of interest with roughness heights of up to 1.3 mm, which is low compared to the remainder of the laminar test articles, which were tested at lower conditions.

The laser-scan map of the AHF test article in Figure 10 also reveals differential recession occurring between the honeycomb and the ablator near the leading edge followed by stream-wise streaks, one of which substantially influenced centerline recession in the downstream portion of the test article, as mentioned earlier. These surface features, which may affect flow mixing and trigger boundary layer instability, mark zones of augmented heating that appear as deep, stream-wise channels (with depths of up to 5.5 mm) that grow in diameter as they move downstream.

This transition from material-dominated differential recession to non-uniform-flow dominated recession is even clearer in the results from TPTF, where Avcoat was tested at heating rates and pressures similar to those in AHF. Since the TPTF samples were tested at two different durations, it is possible to notice the time dependence of the flow patterns. Figures 12 and 13 illustrate that the longer test duration allows for the development of more pronounced streaks in the wakes of many honeycomb cells at the leading edge. The post-test profiles and roughness plots show that AV-023-004 developed significantly deeper and wider channels across the surface compared to AV-023-006. Also, the laser-scan maps reveal that these prominent streaks across AV-023-004, which may have been caused by vortices, extend from the leading edge to the trailing edge. Meanwhile, the roughness heights near the trailing edge of AV-023-006 suggest less pronounced grooves. The most pronounced surface feature nearest the centerline of AV-023-006 has a maximum roughness height of 3.1 mm at 8.57 cm from the leading edge verses half the roughness height at 9.53 cm from the leading edge, which perhaps resulted from premature development of vortices.

The laser-scan maps and the post-test profiles of the TFD samples are shown in Figures 14 and 15. Although limited to representing the center portion of each test article, they show differential recession between the honeycomb and the ablator throughout the surface of each test article. However, the differential recession in these turbulent test articles do not result in the well-defined stream-wise features seen in its laminar counterparts; and maximum roughness heights, in most locations, are lower than maximum roughness heights from the AHF and TPTF test articles. Since test duration was also varied between test articles in the TFD, it is possible to distinguish the effect of a longer exposure time while keeping conditions constant. And though the longer test duration did result in a rougher test surface, AV-020-004 still lacked the streaky flow patterns seen in the laminar test articles. This difference may be attributed to turbulent flow providing more efficient mixing than laminar flow. Also, recall that the computed boundary layer thickness in the TFD is almost five times as thick as that in the AHF and almost twice as thick that in the TPTF, and any effects resulting from upstream protuberances would be more pronounced in a thinner boundary layer. Because the boundary layer thickness is relatively large in the TFD, the perturbations resulting from differential recession of the honeycomb ablator did not produce any distinct flow patterns as they did in the laminar environments in the AHF or TPTF, where the boundary layer was so thin that disturbances upstream, such as the slowly receding honeycomb, plausibly resulted in vortices seen downstream.

American Institute of Aeronautics and Astronautics

10

Figure 10. a) Post-test photograph, b) laser-scan map, c) profiles, and d) roughness plots of CT-46d-002, which was tested in the AHF for 216 seconds.

Figure 11. a) Post-test photograph, b) laser-scan map, c) profiles, and d) roughness plots of AV-005-008, which was tested in the IHF for 60 seconds.

American Institute of Aeronautics and Astronautics

11

Figure 12. a) Post-test photograph, b) laser-scan map, c) profiles, and d) roughness plots of AV-023-006, which was tested in the TPTF for 60 seconds.

Figure 13. a) Post-test photograph, b) laser-scan map, c) profiles, and d) roughness plots of AV-023-004, which was tested in the TPTF for 120 seconds.

American Institute of Aeronautics and Astronautics

12

Figure 14. a) Post-test photograph, b) laser-scan map, c) profiles, and d) roughness plots of AV-020-002, which was tested in the TFD for 120 seconds.

Figure 15. a) Post-test photograph, b) laser-scan map, c) profiles, and d) roughness plots of AV-020-004, which was tested in the TFD for 180 seconds.

American Institute of Aeronautics and Astronautics

13

V. Conclusions This review of laminar and turbulent arc jet test results portrays preliminary steps taken to address the challenge

presented by roughness induced by laminar flow transitioning to turbulence. The current shear testing capabilities at four different Ames facilities—the AHF, IHF, TPTF, and TFD—are discussed. For the IHF, TPTF and TFD, aerothermal environment predictions from CFD matched measured heat flux and pressure, and predictions showed similar conditions across the AHF, TPTF, and TFD. Flat panels of Avcoat were tested in each facility, and a method for analyzing laser scans of the honeycomb ablator was developed to compare post-test surfaces across facilities.

The analysis demonstrates that Avcoat roughness was observed in these shear tests, and Avcoat roughness is caused by the material’s intrinsic composition and is exacerbated in low as well as severe aerothermal environments. Surface analysis of post-test Avcoat revealed that the test condition with the higher heat transfer rate, achieved in the IHF, results in differential recession at the leading edge followed by stream-wise streaks that were likely induced by vortices. These vortices, however, appeared to attenuate downstream, leaving the surface with the lowest roughness heights of all test articles. Meanwhile, the articles that were tested at the similar low condition demonstrate that the turbulent tests in the TFD cause inherent material roughness and the efficient mixing and the thick boundary layer provided by the turbulent flow results in lower roughness heights than those encountered in the laminar tests. Differential recession between the honeycomb walls and the ablator is the dominant surface feature of the TFD test articles, while the laminar tests in the AHF and TPTF, which generate relatively thin boundary layers, capture this plus dominant downstream effects of roughness, represented as marked stream-wise channels that indicate the possible presence of vortices. Test durations were varied in the TPTF, and the results suggest that the flow patterns intensify with time and leave the surface with even higher roughness heights.

The shear-tested surfaces of Avcoat exhibit shape change of the test articles and surface roughness. Consequently, the resulting surfaces presented in this study demanded a careful approach to interpret post-test results. The technique that was created for analyzing Avcoat tested in shear is recommended for evaluating materials that undergo differential recession.

Acknowledgments The authors gratefully acknowledge the support provided by the Orion TPS Insight and Oversight Project and

NASA Ames Research Center through their contract to the ELORET Corporation (NNA04BC25C) and ERC Incorporated (NNA10DE12C). Special thanks to our colleagues Jose Chavez-Garcia and Jerome Ridge, who supported these test series in various ways, including test article design, preparation, and characterization. We also acknowledge the arc jet test facility team at Ames and NASA-SCAP for their critical financial support of the arc jet operational capability at Ames.

References 1 Edquist, K.T., Dyakonov, A.A., Wright, M.J., and Tang, C.Y., “Aerothermodynamic Design of the Mars Science Laboratory Heatshield,” AIAA Paper 2009-4075, June 2009. 2 Reda, D.C., Ketter, F.C., Jr., and Fan, C., “Compressible Turbulent Skin Friction on Rough and Rough/Wavy Walls in Adiabatic Flow,” AIAA Paper 74-574, June 1974. 3 Wright, M.J., et al, “Sizing and Margins Assessment of the Mars Science Laboratory Aeroshell Thermal Protection System,” AIAA Paper 2009-4231, June 2009. 4 Bose, D., Skokova, K., Wright, M.J., Reuther, J., “Ground-to-Flight Traceability Analysis of Arcjet Testing for the Crew Exploration Vehicle,” AIAA Paper 2009-3845, June 2009. 5 Terrazas-Salinas, I., and Cornelison, C., “Test Planning Guide for NASA Ames Research Center Arc Jet Complex and Range Complex,” Thermophysics Facilities Branch, Space Technology Division, NASA Ames Research Center, April 2009. 6 Gökçen, T., Chen, Y.K., Skokova, K., and Milos, F., “Computational Analysis of Arc-Jet Wedge Tests Including Ablation and Shape Change,” AIAA Paper 2010-4644, July 2010.

American Institute of Aeronautics and Astronautics

14

7 Driver, D.M., et al, “Arc Jet Testing in a Shear Environment for Mars Science Laboratory Thermal Protection System,” AIAA Paper 2009-4230, June 2009. 8 Smith, M.D., Moody, H., Wanstall, C., Terrazas-Salinas, I., “The Design and Use of Calorimeters for Characterization of High-Enthalpy Flows in Arc-Heated Test Facilities,” AIAA Paper 2002-5236, Sept. 2002. 9 Balboni, J., Gökçen, T., Hui, F., Taunk, J., Noyes, E., and Schickele, D., “Calibration of the Truncated Panel Test Arc-Jet Facility,” AIAA Paper 2009-4090, June 2009. 10 Wright, M. J., Candler, G. V., and Bose, D., “Data-Parallel Line Relaxation Method for the Navier-Stokes Equations,” AIAA Journal, Vol. 36, No. 9, 1998, pp. 1603-1609. 11 Wright, M. J., “Data-Parallel Line Relaxation Code, DPLR Version 4.02,” Private Communication, April 2010. 12 Park, C., Nonequilibrium Hypersonic Aerothermodynamics, John Wiley & Sons, Inc., New York, 1990, Chap. 4. 13 Gökçen, T., Skokova, K., Balboni, J. A., Terrazas-Salinas, I., and Bose, D., “Computational Analysis of Arc-Jet Wedge Calibration Tests in IHF 6-Inch Conical Nozzle, ” AIAA Paper 2009-1348, Jan. 2009.