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2In order to provide a more reliable test environment forthe experimentalist, the NASA Langley Research Centerhas developed a series of supersonic/hypersonic quiettunnels.7 In these facilities, the free stream noise ismaintained at low levels by treating the settling chamberflow and maintaining the nozzle wall boundary layer in alaminar state. Hence, transition on the nozzle wall isdelayed providing lower free stream disturbance levels inthe quiet tunnel relative to conventional tunnels.The primary objective of the present study is to obtainexperimental hypersonic boundary layer stability data overa conical body in a quiet tunnel. In a parallel paper, Ref. 8,the disturbance environment of the quiet tunnel used in thepresent work is documented. In this paper, the firsthypersonic boundary layer stability measurementsobtained in a quiet tunnel are presented.Experimental ApparatusTest FacilityAll tests were conducted in the NASA Langley NozzleTest Chamber Facility. This is an open-jet blow-downfacility, and was equipped for the present tests with aslow-expansion, axisymmetric, quiet Mach 6 nozzle. Thenozzle, which is more fully described in Ref. 9, had athroat diameter of 1.00", exit diameter of 7.49", and lengthfrom throat to exit of 39.76". The nozzle is equipped withan annular throat slot to remove the boundary layer thatdevelops upstream of the throat. As a result, the boundarylayer that forms on the nozzle wall remains in a laminarstate until far downstream. This provides low disturbancelevels in the nozzle test section. The nozzle may beoperated over a range of stagnation pressures from 80 to200 psia, and stagnation temperatures up to 400 ?F. Runtimes from minutes to several hours are possible.Test ModelThe model, used in this study, was a 20" long cone witha curved-flare afterbody, shown in Fig. 1. For sake ofbrevity, this model is referred to as the flared cone. Thestraight cone surface extended from X=0" to X=10", witha semivertex angle of 5?. The flare surface had a radius ofcurvature of 93.07", and extended from X=10" to X=20".The sharp model tip had a nominal radius of 0.0001". Themodel surface was instrumented with 29 pressure orificesand 51 thermocouple gages placed along diametricallyopposite rays as shown in the side view of Fig. 1. Hot-wireboundary layer surveys were conducted along a raylocated 90? from the surface measurement rays as shownin the top view of Fig. 1.The flared cone was used instead of a straight cone inorder to induce transition on the model within the quietflow capability of the tunnel. The model surface was ofhigh fidelity, and had a maximum rms radius error of lessthat 2.8% of the model boundary layer thickness.Hot-Wire ProbesThe hot-wire probes were constructed of 10% platinumplated tungsten wire of 100 min. diameter. The wire wassoldered onto 0.005" diameter stainless steel broacheswhich were attached to the main probe body. The nominallength-to-diameter ratio of the wire was 210. The wire wasslack to minimize the ?strain-gage effect?. An electricalcontact probe was located about 0.005" below thebroaches in order to determine the location of the modelsurface.Hot-Wire AnemometerThe hot-wire anemometer system used in the presentwork was a new, proprietary constant voltage anemometer(CVA). In this system, a steady DC voltage wasmaintained across the hot wire through the use of acomposite-amplifier-compensation circuit. The operatingprinciples of the CVA are described in detail in Ref. 11.Only the CVA, in contrast to attempts with constantcurrent and constant temperature anemometers, providedthe ability to obtain measurable signals in the freestreamof the quiet nozzle flow. The reason for the betterperformance of the CVA has not yet been investigated.The present CVA system had a bandwidth of about 350kHz with a 40 dB/decade roll-off.The CVA operation was computer controlled for alltests conducted. At each boundary layer measurementpoint, the wire voltage was automatically changed through7 levels; the voltage magnitudes were optimized for theindividual wires used. The constant wire voltage wasmonitored throughout testing using a 51/2 digit digitalmultimeter (DMM). The DMM was also used to measurethe mean CVA output signal. The rms and fluctuatingcomponents were measured from the AC-coupled CVAoutput signal. An analog true RMS voltmeter was used todetermine the CVA rms output voltage. Prior tomeasurement, the CVA output was high-pass filtered at 1kHz and low-pass filtered at 1 MHz. An 8-Bit digitaloscilloscope was used to obtain time traces of thefluctuating CVA output voltage. The high- and low-passfilter settings were 1 kHz and 630 kHz, respectively, andthe sampling rate was 2 MHz. Standard FFT proceduresemploying a Hanning window, data length of 512 points,and 78 averages were used to obtain the spectra.A calibration procedure for the CVA was developed,and is outlined in Ref. 12. This procedure enabled meanand rms mass flux and total temperatures in the boundarylayer to be obtained. The comparisons of the present datawith computational predictions verified the validity of theprocedure for mean flow quantities. However, theaccuracy of the quantitative rms data could not be verifiedfor the fixed-time-compensation CVA system.Nevertheless, the absence of quantitative fluctuation datadid not prevent an analysis of the spatial amplificationrates in the linear and weakly non-linear stability regimes.Inthe linear stability region, fluctuations are expected togrow (or decay) exponentially. This exponential growth isdescribed by normal mode decomposition5 which specifiesthat the amplification rate is the same for separatefluctuation components (pressure, velocity, etc.). Thus, anuncalibrated approach is valid in the linear stability regionas verified from both controlled13 and uncontrolled6stability experiments. In the weakly non-linear stabilityregime, analysis of the present data indicated that theexperimentally-derived amplification rates are mainly of amass-flux nature12.Experimental ApproachTest Conditions