This paper is declared a work of the US Government and is not subject to copyright protection in the United States.HYPERSONIC BOUNDARY LAYER STABILITY OVER A FLARED CONE IN A QUIET TUNNELJason T. Lachowicz* and Ndaona Chokani?North Carolina State University, Raleigh NC 27695andStephen P. Wilkinson?NASA Langley Research Center, Hampton VA 23681AbstractHypersonic boundary layer measurements wereconducted over a flared cone in a quiet wind tunnel. Theflared cone was tested at a freestream unit Reynoldsnumber of 2.82x106/ft in a Mach 6 flow. This Reynoldsnumber provided laminar-to-transitional flow over themodel in a low-disturbance environment. Pointmeasurements with a single hot wire using a novelconstant voltage anemometry system were used tomeasure the boundary layer disturbances. Surfacetemperature and schlieren measurements were alsoconducted to characterize the laminar-to-transitional stateof the boundary layer and to identify instability modes.Results suggest that the second mode disturbances werethe most unstable and scaled with the boundary layerthickness. The integrated growth rates of the second modecompared well with linear stability theory in the linearstability regime. The second mode is responsible fortransition onset despite the existence of a second modesub-harmonic. The sub-harmonic wavelength also scaleswith the boundary layer thickness. Furthermore, theexistence of higher harmonics of the fundamental suggeststhat non-linear disturbances are not associated with ?high?free stream disturbance levels.NomenclatureADisturbance rms amplitude (square of powerspectral density, arbitrary units).f Frequency (kHz)R (Res)1/2ResReynolds number based upon freestreamconditions and surface distance from the apex ofthe cone, SrbBase radius of coneSDistance along the surface of the model, measuredfrom the apex of the cone modelToTotal temperatureTwSurface static temperaturerU Mass fluxVsConstant voltage anemometer output voltageXCoordinate along the cone model axis ofsymmetry, measured from the apex of the coneYCoordinate perpendicular to the cone axis ofsymmetry, measured from the cone axis ofsymmetryYw Y-location of the wall surfaceGreek-aiNon-dimensional amplification rate, -ai = 12AdAdRdBoundary layer thicknesslDisturbance wavelengthhNon-dimensional Y-distance , h = (Y-Yw)RSSubscripts?Conditions in freestreamrms Root mean square of fluctuating componentSuperscripts ' Fluctuating component of a time dependentquantity( )Time average of a particular quantityIntroductionThe change from laminar to turbulent flow in thehypersonic boundary layer is accompanied by largechanges in both heat transfer and skin-friction drag. Thesechanges are important to the aerodynamic design ofhypersonic vehicles since the aerodynamic coefficients arevery sensitive to the large changes in heat transfer andskin-friction that accompany transition.1 Furthermore, thestability, control, and structural design of the vehicle areaffected due to the increased thermal and aerodynamicloading.The conical geometry is prevalent in many hypersonicaerodynamic applications, but only a few stabilityexperiments2of hypersonic cone boundary layers havebeen conducted. These studies have provided afundamental understanding of the hypersonic boundarylayer stability problem. However, these few stabilityexperiments have been conducted in conventionalhypersonic wind tunnels where relatively large freestreamdisturbances occur. The primary source of the free streamdisturbances is acoustic radiation from convecting eddiesgenerated by the turbulent boundary layer on the nozzlewall.3,4The frequency content of this incident noise fieldprovides a stimulus to excite disturbances in thehypersonic boundary layer which may lead to transition.Thus, some of the observed anomalies betweenexperiment and theory5,6 may be due to effects of the windtunnel noise.*Graduate Research Assistant, Department ofMechanical and Aerospace Engineering, Student MemberAIAA. Presently, NRC Research Associate, NASALangley.?Associate Professor, Department of Mechanical andAerospace Engineering, Member AIAA.?Group Leader, Quiet Tunnel & Transition Group, FlowModeling and Control Branch, Fluid Mechanics andAcoustics Division, Senior Member AIAA.