Methods for Prediction of High-Speed Reacting Flows in Aerospace Propulsion

ESEARCH to develop high-speed airbreathing aerospacepropulsion systems was underway in the late 1950s. A majorpart of the effort involved the supersonic combustion ramjet, orscramjet, engine. Work had also begun to develop computationaltechniques for solving the equations governing the flow through ascramjet engine. However, scramjet technology and the computa-tional methods to assist in its evolution would remain apart foranother decade. The principal barrier was that the computationalmethods needed for engine evolution lacked the computertechnology required for solving the discrete equations resultingfromthenumericalmethods.Eventoday,computerresourcesremainamajorpacingitem inovercomingthisbarrier.Significantadvanceshave been made over the past 35 years, however, in modeling thesupersonic chemicallyreacting flowin ascramjet combustor. Toseehow scramjet development and the required computational toolsfinally merged, we briefly trace the evolution of the technology inboth areas.We begin with a review of the history of efforts to model thescramjet environment and thenconcentrate onmore recent activitiesthat lead to today’s computational capabilities. The NationalAeroSpace Plane (NASP) technology program provided strongmotivation for advancing the computational capabilities of thecountryinboththegovernmentandprivatesectors.RequiredgroundtestfacilitieswithsufficienttesttimeswerelimitedtoaroundMach8,and higher Mach numbers, achievable in pulse facilities, could onlybe maintained for the order of milliseconds. In addition, the numberof facility cycles available to parameterize a given engine flow-path were limited, and the facilities were expensive to operate.Computationalcapabilitieswereneededtofilleachoftheseareasthatexistedingroundtestfacilities.AlthoughtheNASPprogramwasnotsuccessful in developing a vehicle, it did spawn the development ofnewcomputational algorithms.TheHyper-XProgram,beginningin1995, revived high-speed computational research and development.A flight program is the catalyst that drives technology developmentandsynthesizesalloftheeffortsintoaunifiedtoolfordevelopmentofthe ultimate experiment, the flight of a hypersonic vehicle. Thegenesisofmostofthecurrentdaystate-of-the-artcomputationaltoolsfor scramjet research and development began with the Hyper-Xprogram. This paper attempts to cover this story from NASP andHyper-Xtothepresentday.Webeginwithabriefhistoryofscramjetdevelopment leading up to the NASP Program. Although this paperwill use the history of scramjet development as a roadmap for theevolution of computational tools, the reader interested in a moregenerallookatthehistoryshouldconsultthepapersbyBillig[1]andCurran[2]ontechnologyanditsissuesandHallion[3]onhypersonicsystems.FollowingpioneeringeffortsofFerri[4],Dugger[5],andWebberand MacKay [6] in the 1950s, a significant increase in research todevelopscramjetengineconceptsoccurredinthe1960s.In1965,theNASA Langley Research Center initiated the Hypersonic ResearchEngine (HRE) project to develop a high-speed air breathingtechnology for hypersonic cruise vehicles [7]. The goal of the HREproject was to flight test a regeneratively cooled, hydrogen-fueled pylon-mounted scramjet on the X-15 research airplane anddemonstrate design performance levels. The HRE did not reach theflight demonstration stage due to cancellation of the X-15 program,but the ground-based program did continue and resulted in thedevelopment and construction of two variable geometry enginemodels.Workwiththesemodelssignificantlyincreasedthescramjettechnology database to be applied in more advanced configurations.Following completion of the HRE project, attention moved topropulsion concepts that would provide high performance wheninstalled on a vehicle. The original concept, a pylon-mounted HRE,would have resulted in excessive levels of external drag, and so thepylon was removed, and work began to highly integrate the engine