3The stagnation conditions for the present tests were atemperature of 810? R and a pressure of 130 psia. Themeasured freestream Mach number was 5.91. Theseconditions yielded a freestream unit Reynolds number of2.82x106/ft.Prior to the present tests, the characteristics of thefreestream flow were documented through a series ofpitot-probe and hot-wire measurements. The details of thefreestream measurements are presented in Refs. 8 & 12,but a few salient features are presented here. The pitot-probe surveys verified that the nozzle mean flow wasuniform within the flow test volume. Within this volumethe Mach number was 5.91 ? 0.08. Measurements incross-sectional planes indicated that the mean flow wasaxisymmetric and varied by no more than 1% in the testvolume which encompassed the model. The RMS datashowed that the low-level disturbance field wasaxisymmetric and was associated with sound-modegeneration of the nozzle wall turbulent boundary layer.MeasurementsThe measurements were conducted in two stages. First,model surface temperature and schlieren imagingmeasurements were conducted, and then hot-wireboundary layer surveys were conducted. The surfacetemperature measurements were used to verify that themodel was in thermal equilibrium, and to estimate thelocation of transition onset. The schlieren images wereused to verify the laminar-to-transitional state of theboundary layer, and to identify the character of theinstability modes.The hot wire boundary layer surveys were conducted at17 streamwise stations, spaced 0.5" apart over the rangeX=10.97" (R=1610) to X=18.97" (R=2120). At eachstreamwise station, the wire was traversed perpendicularto the cone axis of symmetry. The mean and rmsmeasurements were obtained at 13 points clustered nearthe boundary layer edge. At each measurement point, 7wire voltages were applied. The rms profiles were theninspected to determine the maximum energy (rms)location. Wave traces were subsequently measured at themaximum energy locations using the largest, practicablewire voltage. At this operating condition, the CVA is moresensitive to changes in the mass flux as opposed tochanges in the total temperature12. In addition, the CVAsignal-to-noise ratio is improved.ResultsSurface Temperature DataFig. 2 presents the experimental and computationalsurface temperatures. The temperatures are shown alongthe left ordinate and the flared-cone surface coordinatesalong the right ordinate. The experimental surfacetemperature error is ?2?R (?0.0025 To?). Thecomputational values14 represent laminar adiabatic walltemperatures. Over the range, R=690-1700, the flow islaminar and the experimental data compare well with thecomputational data. Further downstream, R=1800-2110,there is sharp temperature rise region. This rise isassociated with transition since a transitional boundarylayer is heated and some of this heat is convected to themodel surface via turbulent-like vortices. An estimate oftransition onset was determined as the intersection point oftwo straight lines passing through the laminar region andsharp temperature rise region. Based on this criterion, thetransition onset is estimated to be in the range R=1960-1990. The estimate compares well with linear stabilitytheory9 which predicts an N-factor for the most unstablefrequency of about 8 at R=1975. Downstream of R=2110,the temperature decreases due to the combined effect of arelatively cold model base and, possibly, the flow fieldtending to fully turbulent flow.Schlieren DataSchlieren data are presented in Fig. 3. Thesemeasurements were conducted over the aft 3.5" of themodel which was positioned downstream of the nozzleexit plane. A wavy structure can be identified near theedge of the boundary layer. The wavelength of thesewaves is measured to be approximately twice theboundary layer thickness. These waves occur in wavepackets and are associated with second modedisturbances2,6. The second mode disturbances are firstdetected at about R=2025 according to a closerexamination of the video records used to construct Fig. 3.This location is slightly downstream of transition onset asestimated from the surface temperature measurements.Boundary Layer Mean DataThe experimental and computational boundary layerthickness distributions are presented in Fig. 4. Note thatthe computational14 boundary layer thickness distributionwas curve fit using a second order polynomial, and theexperimental error is ? 2% of the plotted values. Exceptfor a few locations over the range, R=1610-1915, theexperimental d is slightly lower than the computational d.As discussed below this suggests a misalignment of themodel such that the boundary layer measurement ray is onthe windward side of the model. From R=1945 to R=2120,the experimental d becomes greater than thecomputational d, confirming the transitional nature of theboundary layer over this region. For 1610 ? R ? 1915, theclose agreement between the laminar flow computationalpredictions and the experimental data suggests that,experimentally, the mean flow is laminar over this region(i.e. no mean flow distortion). This laminar character isseen more clearly with the aid of Figs. 5 and 6 which arediscussed next.The experimental mean total temperature and mass fluxprofiles are presented in Figs. 5 and 6 at 4 streamwiselocations. Also, laminar total temperature and mass fluxprofiles, computed from the Navier-Stokes code of Ref.14, are presented as the solid lines in Figs. 5-6. AtR=1785, the experimental and computational datacompare well; no effect of model misalignment is evident.The good agreement with computational data at R=1785 istypical of all total temperature data over the range, 1610 ?R ? 1915. This is consistent with the boundary layerthickness, confirming the laminar flow region, R ? 1915.At R=1945, the transitional nature of the boundary layerbecomes evident due to the slight total temperaturedistortion from h=5.27 (0.706 d) to h=6.62 (0.887 d) andmass flux distortion from h=5.61 (0.751 d) to h=6.93