Experimental Investigation of the Pressure Field of a Heated Supersonic Jet with a Centered Total Temperature Non-Uniformity

摘要

The impacts of a centered total temperature non-uniformity on the near and far pressure fields of a heated supersonic jet are presented. The total temperature at the exit plane of a heated Mach 1.5 jet was altered by introducing cooler air into the centerline of a heated jet plenum upstream of a converging-diverging round method of characteristics supersonic nozzle. This configuration was chosen for study with the hypothesis that the colder, lower velocity plume would remain largely intact until the end of the potential core, altering the mixing and convection velocity of large scale structures responsible for peak noise emissions. From total temperature measurements, the temperature of the centered cold plume in the non-uniform case was found to be 45% lower than the surrounding heated plume. The TTR of an equivalent uniformly mixed jet, calculated by mass weighting the cold and hot streams, was 1.94. The difference in far-field model spectra for the TTR=2 baseline and equivalently mixed jet was found to be 0.3 dB for low and peak noise frequencies. An analysis of the far-field acoustics revealed a decrease of up to 2 dB sound pressure level at a range of frequencies in the narrowband spectrum of the non-uniform case at 40° from the downstream jet axis. Further analysis revealed a 5% decrease in the measured near-field pressure convection velocity for the non-uniform case. The correlation ridge wavenumber revealed the non-uniform case shifted away from the baseline and towards the sonic wavenumber line. Crackle was also found to be present in both the uniform and non-uniform cases. A 5% reduction in the skewness of the time derivative of the far-field pressure time series was found for the non-uniform case. These results indicate that thermal non-uniformities may be an effective method for reducing jet noise, especially when considering the large temperature deltas that could be created in the after burning engines of tactical aircraft.