1. Figure 19 shows the results of the modal decomposition at 48% and 60% speed. For each speed, two plots are shown, the left being a Joppa-style surface plot of the modal amplitudes as a function of mode number and frequency, and the right being a line plot of the individual mode amplitudes as a function of frequency. At low frequency, most of the acoustic energy is contained in the plane wave (mode order=0). The first circumferential mode (m=-1) is dominant over a frequency range from 500 to 1000 Hz. This mode is associated with the first resonant peak seen in the spectra. The -2,3, and -4 modes are associated with the remaining main peaks in the spectra. A double peak characteristic can be observed for modes -1 and -2, but a cause for this characteristic has not been determined. Note that the amplitudes of the positive and negative are nearly identical. This is contrast to modal measurements made in an APU combustor, where the positive and negative mode amplitudes peaked at different frequencies, presumably due to swirling flow in the combustor2. The spectral peaks associated with the combustor modes are not observed in the far field data (Figure 20). There is little evidence of these resonances in the far field. However, the water brake could not fully load the low pressure turbine for this case, so there may be other sources present due to the components operating off the operating line. There are only small differences in the combustor noise spectra between 60% and 48% corrected fan speeds. There are small reductions in amplitude, but little shiftinthe modal frequencies. 0 500 1000 1500 2000 2500 3000
2. 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000
3. Figure 21 shows a comparison of the component noise predictions, the total prediction, and the far field data from the no fan test at 48% corrected fan speed and at 120 deg and 150 deg from the inlet. Also shown are the baseline hard wall and fan acoustic treatment configurations for the engine. The prediction methods for combustion and turbine noise indicated that combustion noise was the apparent dominating source from 250 - 1000 Hz and that turbinenoise wasa dominating sourcefrom5000- 10000Hz. Notethatfrom1250- 4000Hz,the fan duct acoustic treatment had significant effect on the noise with the fan. In addition, the difference between the data with and without the fan was largest in that frequency range. It therefore appeared that the spectrum was fan noise dominated inthatregion. Similartrendswerealso seenat54% (Figure22)and60% (Figure23).
4. Baseline Noise Measurements from the Engine Validation of Noise and Emissions Reduction Technology Program