Acoustic simulation of coaxial hot air jets using cold helium—air mixture jets

This work examines the ability of small-scale helium–air mixture coaxial jets to simulate the acoustics of large-scale hot air jets representing the exhaust of separate-flow turbofan engines. Experiments employed a one-eighth-scale model of a separate-flow nozzle used in hot tests at NASA John H. Glenn Research Center. Comparisons were conducted for two set points using the following methods: matching velocity and density, and matching velocity and Mach number. For both methods, the helium–air data compare well with the hot data in all measures of noise: spectral shapes, spectral levels, and overall sound pressure levels. The method of matching velocity and Mach number gives slightly better agreement in the spectral shapes at angles close to the jet axis and in the overall sound pressure levels. The overall agreement between the hot air and helium–air mixture data is within 1.2 dB. Nomenclature a = speed of sound B = bypass ratio cHe = helium mass fraction D = nozzle diameter f = frequency L = potential core length M = Mach number Mc = convective Mach number p = pressure r = distance from nozzle exit U = velocity Uc = convective velocity = polar angle relative to jet axis = density Subscripts f = fan hot = heated air mix = cold helium–air mixture p = primary (core) stream s = secondary (bypass) stream 0 = total (stagnation) 1 = ambient I