ON THE SCALING OF SMALL, HEAT SIMULATED JET NOISE MEASUREMENTS TO MODERATE SIZE EXHAUST JETS

Modern military aircraft jet engines are designed with variable geometry nozzles to provide optimum thrust in different operating conditions, depending on the flight envelope. However, the acoustic measurements for such nozzles are scarce, due to the cost involved in making full scale measurements and the lack of details about the exact geometry of these nozzles. Thus the present effort at The Pennsylvania State University and the NASA Glenn Research Center- in partnership with GE Aviation is aiming to study and characterize the acoustic field produced by supersonic jets issuing from converging-diverging military style nozzles. An equally important objective is to validate methodology for using data obtained from small and moderate scale experiments to reliably predict the most important components of full scale engine noise. The experimental results presented show reasonable agreement between small scale and moderate scale jet acoustic data, as well as between heated jets and heat-simulated ones. Unresolved issues however are identified that are currently receiving our attention, in particular the effect of the small bypass ratio airflow. Future activities will identify and test promising noise reduction techniques in an effort to predict how well such concepts will work with full scale engines in flight conditions. Nomenclature a = ambient speed of sound AR = exit-to-throat area ratio D = diameter of nozzle at exit plane Dj = diameter of the jet plume f = frequency f, = characteristic frequency U /DJ M „ = acoustic Mach number U, /a, Md = design Mach number of the nozzle M = average Mach number of the frilly expanded jet NPR = nozzle pressure ratio OASPL = overall sound pressure level R = physical distance of the microphones from the jet exit Ali = gas constan