Aeroacoustic Measurement of Transient Hot Nozzle Flows

Abstract

An assessment of a low-cost transient testing technique for obtaining acoustic measurements from hot, highspeed nozzle e ows is presented. A shock tunnel was used to produce short-duration (10‐20 ms) supersonic air jets from three sizes of American Society of Mechanical Engineers conic nozzles and a mixer-ejector nozzle typical of a high-speed civil transport (HSCT) aircraft application. A comparison between shock-tunnel transient noise data and steady-state data is presented. Theassessment establishes the uncertainty bounds on sound pressure level measurements over the range of frequency bands, nozzle pressure and total temperature ratios, and nozzles scales for which the facility can be used as a substitute and/or complementary mode of investigation for steady-state hot-e ow test facilities. Using the transient facility, far-e eld narrowband spectra were obtained at directivity angles from 65 to 145 deg, and the data were extrapolated to full-scale e ight conditions. The constraint of short test duration is alleviated through the use of multiple runs to reduce the uncertainty associated with transient acoustic measurements. Sound pressure level vs frequency trends with nozzle pressure ratio and directivity angle are comparablebetweenthesteady-stateandtransientdatafortheconicandmixer-ejectornozzles.Conicnozzleresults demonstrated that thetransient noise data replicatethe steady-state data to within §2-3 dB, and themagnitude of effectiveperceived noiselevel valuesagree to within 1 ‐3 dB depending on test condition and nozzlesize. The mixerejector model demonstrated agreement with the steady-state noise data of around 2 ‐5 dB on sound pressure level vs full-scale frequency over the range of nozzle pressure ratios and total temperature ratios relevant to the HSCT mixer-ejectordevelopmentprogram.Trendsinnoisevariationwithazimuthalangle,asmeasuredcircumferentially about the inlet axis, as well as variation with mixer area ratio were also captured using transient testing.