16th Australasian Fluid Mechanics ConferenceCrown Plaza, Gold Coast, Australia2-7 December 2007
A Perspective on Computational Aerothermodynamics at NASA
Peter A. Gnoffo1
1NASA Langley Research CenterHampton, Virginia, 23681, USA
The evolving role of computational aerothermodynamics (CA)within NASA over the past 20 years is reviewed. The presenta-tion highlights contributions to understanding the Space Shuttlepitching moment anomaly observed in the first shuttle flight,prediction of a static instability for Mars Pathfinder, and theuse of CA for damage assessment in post-Columbia mission-support. In the view forward, several current challenges in com-putational fluid dynamics and aerothermodynamics for hyper-sonic vehicle applications are discussed. Example simulationsare presented to illustrate capabilities and limitations. Opportu-nities to advance the state-of-art in algorithms, grid generationand adaptation, and code validation are identified.
Computational fluid dynamics (CFD) is the numerical simula-tion of flowfields through the approximate solution of the gov-erning partial differential equations for mass, momentum, andenergy conservation coupled with the appropriate relations forthermodynamic and transport properties. Aerothermodynam-ics is the branch of fluid dynamics that focuses on the effectsof thermodynamic and transport models on aerodynamics andheating. It is especially focused on conditions of hypersonicvelocities where the energy content and exchange between ki-netic, internal, and chemical modes in the flow precludes theotherwise common use of calorically perfect gas assumptions.Computational aerothermodynamics is therefore defined in ex-actly the same manner as CFD, with the added emphasis thathigh temperature gas effects on pressure, skin friction, and heattransfer are included in the numerical simulation. The funda-mental role of computational aerothermodynamics is the simu-lation of aerodynamic forces and heating for external and inter-nal high speed flows. Reference  presents a review of recentapplications for access to space and planetary missions.
The evolving capabilities of computational aerothermodynamic(CA) simulation and its role in various NASA programs is re-viewed. The review provides context for discussion of new re-search and development in CA. Emphasis is placed on the morerecent role of finite-volume, (pseudo) time-dependent Navier-Stokes solvers using upwind discretizations to capture strongshocks. Within NASA, these schemes began displacing conven-tional discretizations (MacCormacks method or central differ-ence methods with second-order implicit and fourth-order ex-plicit smoothing and shock fitting) in the late 1980s. Space-marching, Parabolized Navier-Stokes methods were supplantedas well when more powerful computers became available suchthat the inherent restrictions to attached flow and spatial stepsize no longer had to be endured. It is interesting to note thatthe predominant algorithms of the 1970s for heating analyses(viscous-shock-layer (VSL) and boundary-layer equation solu-tions tied to inviscid surface pressure distributions are seeing aresurgence as engineering tools to provide quick running, mul-tiphysics simulations.
The first three sections of this paper highlight examples whereCA simulations offered unique insight to an important prob-
lem within NASA regarding hypersonic flight environment. Inthe first two examples - a shuttle orbiter body-flap anomaly onSTS-1 and a Mars Pathfinder static instability - CA simulationsdemonstrated that subtle changes in gas chemistry can havelarge effects on aerodynamics. Furthermore, the ability of CAto quantify the distribution of pressure and shear provided thebest understanding of the integrated effects measured in flightor ground-based tests. In the third example - Columbia acci-dent investigation - CA provided data early in the investigationto establish mass and energy flux into the wing as a functionof breach size and local boundary-layer thickness as well as theextreme heating environments on the downstream edge of thebreach. These breach simulations demonstrated that CA couldrespond in a timely manner to analyze damage to the outer-mold-line and evolved to inclusion of CA within the DamageAssessment Team that stands by for support in every missiontoday. All of these examples served to enhance the perceptionof CA as an ever stronger tool for understanding the environ-ment of a hypersonic vehicle.
Even with these and other successes much advance to the state-of-art (SOA) is still required. A brief review of current SOA fol-lows in the next section. Too often, a new capability is inextri-cably hardwired throughout a single code or is demonstrated foran idealized test problem but is never matured for routine use bythe larger community. A reference to SOA herein refers to capa-bilities in codes accepted by project offices for vehicle design.A recent simulation of a flexible ballute - deployable aerobrake- is provided to show specific advances required in both numeri-cal and physical model capability. It provides context to say thisis what we can do now and this is what we still need to be ableto do. CA is a very broad field and because of the difficulty inobtaining data at hypervelocity conditions in ground-based fa-cilities it becomes an especially important tool for establishingaerothermodynamic environments. It is hoped that these exam-ples of CA evolution within NASA provide helpful perspectivefor researchers entering this field.
Pitching Moment Anomaly in STS-1 (Reference )
On the entry phase of its first flight in April 1981, desig-nated Space Transportation System (STS)-1, the Shuttle Or-biter exhibited hypersonic pitching moment characteristics sig-nificantly different from those derived from preflight predic-tions. The vehicles bodyflap had to be deflected to an angleover twice that predicted prior to the flight in order to main-tain trim. The aerodynamic performance characteristics of theOrbiter had been determined by extensive testing in ground-based facilities. Because real-gas effects could not be fully sim-ulated in ground-based facilities, some analytical assessmentswere made for real-gas effects on Orbiter aerodynamics. Theseanalytical assessments of real-gas effects were not viewed withmuch confidence (pre-1981), and thus they were applied to theuncertainty in the aerodynamics of the vehicle rather than toits expected aerodynamic performance. Because of the largeuncertainty assigned to these predicted aerodynamics in the hy-personic speed range, ample control power was built into thesystem to overcome the anomaly in the flight pitching moment.
Figure 1: Computed windward centerline pressure distribution.
Still, the large bodyflap deflection required to trim the vehicleraised concerns for the structural and thermal integrity of thebodyflap.
The inability to predict the hypersonic pitching-moment char-acteristics of the Orbiter, despite extensive wind-tunnel testing,was a fundamental concern. The pitching-moment anomalyhad been attributed to a number of phenomena including vis-cous effects, diminished bodyflap effectiveness, Mach numbereffects, and real-gas effects. Contemporary SOA analyses ei-ther did not include gas chemistry nor have the ability to pre-dict separated flow in front of a deflected body-flap  orwere forced to make approximations to vehicle geometry andgas chemistry due to limitations imposed by computer hard-ware . The shuttle aerodynamic databook was correctedbased on flight data. Still, a more definitive answer from CArequired increased sophistication and robustness of flow solversand a several-order-of-magnitude increase in computer poweravailable in the early 1990s in order to better define the Orbiterconfiguration and to utilize proper gas chemistry models.
CA simulations on a modified Orbiter configuration were usedto establish that high-temperature gas effects in a seven-speciesair model account for the different aerodynamic characteristicsof the Orbiter at wind tunnel and flight conditions. The aero-dynamics of the basic body and the bodyflap were investigatedindependently, and it was shown that most of the increment canbe attributed to the basic body. The bodyflap was shown to beas much as 1.5 times more effective in flight than in the windtunnel, which contradicts assertions that the Orbiter pitching-moment anomaly was caused by reduced bodyflap effective-ness in flight. In fact, had the bodyflap exhibited the same ef-fectiveness in flight as in the wind tunnel, the vehicle may nothave trimmed at all. All evidence from CA analyses led to theconclusion that the so-called pitching-moment anomaly thatoccurred on STS-1 was caused by the inability of perfect-gasground-based facilities to simulate the real-gas chemistry en-countered by the vehicle in hypersonic flight. The aerodynamicincrements between wind-tunnel and flight conditions in Figure1 is attributable to a relatively small pressure differential actingon an expansion surface at the aft end of the vehicle, which hasa very large surface area. The net effect of this difference onpitching moment is shown in Figure 2 in which the flight sim-ulation (closed circles) is in agreement with derived flight data(open squares).
Notable restrictions in the simulation from that period includeda post-processing approximation to include effects of elevongaps and near wake flow because the complexity of resolvingthese features strained available resources. As it was, the noseto trailing edge simulation was implemented in a block march-
Figure 2: Comparison of predicted, measured, and computedpitching moment coefficient over the hypersonic portion of theSTS-2 entry trajectory.