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Chapter4. DataAnalysis 61 Moreover, the low-frequency plots show a noticable SPL increase upstream of the nozzle. As the flight stream velocity is the same for installed and isolated cases, this increase has to come from wing installation effects alone. However, it is un- certain as to why additional noise sources occur along the nacelle when the wing is installed, as it would need some sort of upstream influence on the turbulence of the flow to achieve this effect. Installed approach and cutback ICAO reference points with flight stream Figure 4.16 shows the comparison plots for installed and isolated approach and cutback TPs with flight stream enabled. The large dynamic range of the approach and cutback comparison plot at TOB 1250 Hz is because of the relative SPL difference between installed and isolated TPs of up to 75 dB. There are areas within the original sound field which exhibit sound pressure values that result in negative decibel values, i.e. below the hear- ing range. For reference, the non-comparison plots with full dynamic range are shown in Fig. 4.17. It can be seen that the appearance of the comparison plots can be traced back to the areas of very low sound pressure, which render the com- parison plot unintelligible. It has to be noted, that due to the logarithmic nature of hearing, plots using a full dynamic range do not possess much physical valid- ity; generally speaking, sound events that are more than 25 dB below the main sound event are indistinguishable to the human ear and as such are not relevant when assessing the sound field data. All plots in this thesis using such a scale are therefore for demonstration purposes only. The question remains, as to why the uninstalled approach and cutback TPs show areas with a SPL of approximately 0 dB, whereas the same behaviour is not ob- servable for the sideline TPs, though there is no difference between SL and CB conditions besides the core and bypass flow velocity and the day of testing (one day apart). It is likely that this error occured during SODIX calculation, i.e. that SODIX continuously and erroneously emphasized a distinct region during the 200 iteration steps run on this simulation. However, reducing the amount of iter- ation steps or changing the value of σ1 is no solution to this problem, as this does not change the source distribution, but merely the dynamic range (see Sect. 2.3.4 and 3.2.2). The high-frequency plots are similar to the SL TP, though the defective microphone makes the difference appear more significant than it is.PDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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