1. Foryearsexperimentershavegatheredtransition data and, with little knowledge ot the phenomena responsiblefortransition,haveplottedtransitionReynolds numberas afunctionof some parameter. Theseresults have proved useful for making transition location estimates for situations where the geometry, flow conditions, and environment in question are similar to those of the transition data base, but such empirical correlationshave an unknownuncertaintywhen applied to new situations. Although a considerable amount of transition data exists, these data have suppliedonly a limited understanding of the transition problem. High Mach number transitiondatawere correlatedalong with subsonic and low Supersonic data as if the instability phenomenawere the same at all Machnumbers. This approachwas considered logical because, prior to the mid-1960's. aerodynamicists had never hkard of the higher instability modes. The first author was one of these participants and the wall cooling results of Reference 1 illustrate this point. It was found that decreasingthe temperature ratio (TdTAD)reducedthe transition Reynolds number. These were shock tunnel resultsandthesurfacetemperatureremainedconstant at room temperatureandthe temperatureratiowas varied by vafying thetotaltemperature of thb tunnel. Thus, the problem of icecrystallizationonthesurface,as reported insomewindtunnelexperiments', didnot existforthese experiments Since the authors of Reference 1had no knowledge of secondmode disturbancesat the time of the study,theirdatawerecomparedwithsupersonicdata which reported a similar trend for surface temperature effectson transition, the so-called "transition reversal" data"'. In retrospect,it appearslikely that these Mach 5.5shocktunneltransitiondataweretheresult ofsecondmode-dominated-transition. Since second mode disturbances become more unstable asTInjTAD decreases, it would be expected that the.transition Reynoldsnumberwoulddecreaseas T d T A decreased. Thus, such results should then not be considered a transition reversal in the sense of a reversal from the expectedtheoreticaltrend.
2. The need for stability experimentation in high Machnumberboundary-layers isgreatsincetheunique characteristics of secondmodedisturbancesneedto be investgatedandthe credibilityof availabletheories and numericalmethodsneedstobeevaluated. Unfortunately, experimental stabiliiy results are sparse at all speed ranges, and particularly so at hypersonic conditions. There are only three sets of hypersonic stability experiments;those of Kendall",'2 inthe Jet Propulsion Laboratorysupersonic wind tunnel (M,= 4.5and 5.6) and the hypersonic wind tunnel (M, = 8.5), those of Demetriades'"`6 inthe GALCIT Leg 1 hypersonicwind tunnel (M,= 5.8) and the AEDC Tunnel B (M,= 8), and those of Stetson, et in the AEDC Tunnel B (M-= 6 and 8). Demetriade'*'eportedthe results of the first hypersonic stability experiment in 1958. However, this was before the "discovety"of the Mack instabilitymdesandhisinvestigationdidnotIdentifythe secondmode insiabiliiy. ItwasKendall's'`v`Z pioneering stabilityexperimentsthat providedthefirstconfirmation of the existence of second mode disturbances and documentedthat theywerethedominantdisturbancesin ahypersonicboundary-layer. In 1967Kendall" reported results which documentedthe existence of the second modeinstability. Inauniqueexperiment,heoperatedthe Jet PropulsionLaboratorysupersonic wind tunnel at low pressures to maintaina laminar boundary layer on the tunnel walls, thus avoiding the "noisy" freestream conditionsthat resultfromtunnelswithturbulentsidewall boundary layers. He then introduced attificial disturbances (by means of a glow discharge) into the Mach 4.5 boundary layer of a flat plate at frequencies correspondingto both first and second mode unstable frequencies. Using a hot-wire anemometer, he then measured their streamwise growth. Kendall's experimentalresultswereingoodagreementwithMack 3 numericalcalculationsandprovidedthefirstexperimental confirmationof the secondmode instability. Kendall's`2 Mach 8.5data documentedthat the secondmode was the dominant instability and provideddetails of second mode disturbance frequencies and growth rates. Subsequently,Dernetriade'*' performed his hot-wire experimentsinthe AEDC tunnel€3 at a Machnumberof 8. Demetriades obtained additional characteristics of hypersonicinstabilityphenomenaand verifiedthat wall coolingincreasedthe amplificationratesofsecondmode disturbances. Inspired by the results of Kendall and Demetriades,andutilizingtheinstrumentationtechnology developed by Demetriades, the present experimental programwas initiatedin 1979. A modest program has continued,to the extent that fundingpermitted, andthe results are documented in nine publication^"^^^. The objective of this paper is to review and summarize the resultsof this program.
3. The cooledcone hadan uncoolednose section of 5.659 inches that was fabricated of 13-8 stainless steel. The tip radius was 0.002 inches. A thermal insulatormadeof MicartaRseparatedtheuncoolednose tip from the cooledfrustum. The insulatorconstituted 0.062 inches of the model axial length. The frustum sectionwas labricatedof aluminum alloy 6061-Teand hadan axial lengthof 34.28 inches. The frustumwas corvposedoftwo concentricconicalshellswithprovision lor circulating cooling water between the shells. The outer shell had a uniformthickness of 0.125 inches to withinzppmximately 0.2 inchesofthefotwardendofthe frustum. Thefrustumsurfacetemperaturewasconstant at a nominal temperature of 544°R(T,/T0=.42). The cone frustum was instrumented with four pressure orifices, eight Schmidt-Boelterheat-fluxgages,andfour flush-mountedhot-filmanemometer gages. Fig. 2 is a photographofthecooledconemodelinstalledinthetest section of Tunnel B.
4. Procedures used in the interpretationof hot-wire anemometermeasurementsinaflowfieldforthepurpose of inferring(1)whichflowfluctuationsaresignificantand (2) the magnitudes of the fluctuations, are generally referredtoas"modalanalysis"procedures.Insupersonic flow the hot wire respondsto fluctuations of mass flux andtotaltemperature(thesensingvariables)whereasthe fluctuationswhichcharacterizeanunsteadyflowfieldare those of vorticity, entropy,and sound (the independent mode variables). Fluctuationsin the sensingvariables and the mode variables are inferredfrom hot-wire a-c voltage response(rms)measurementsusingthe"mode" concept and associated techniques of "fluctuation diagrams" introducedby Kovasznay'' andexpandedby MorkovinZ9.