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Chapter4. DataAnalysis 58 As shown in Sect. 4.2, the flight stream influences the sound field at the lowest frequencies (e.g. TOB 250 Hz). It is visible at the far left end of the directivity matrix and depending on the frequency has a SPL of 85 − 95 dB, decreasing for increasing frequency. For lower frequencies, the flight stream noise overrides the jet sound events, resulting in unintelligible plots when the source position grid in Fig. 3.12 is used, see Fig. 4.13. Figure 4.13: Flight stream dominance over the sound field observable at low frequencies, TOB 125 Hz of DLR 325. Due to the dynamic range of the plot, the other mixing sources disappear. At higher frequencies,jet mixing noise dominates and so the plots look similar to the isolated cases. The overall SPL drops significantly compared to the iso- lated test setup. The difference amounts to up to 20 dB. This can be attributed to the fact, that the slow flight stream surrounds the fast core jets. According to Lighthill’s theory of sound (see Sect. 2.3.3) the source strength is proportional to a higher power of the velocity difference. It has to be considered that during real flight, the FS and the SPL reduction it entails are always present, resulting in a significantly different noise characteristic as during isolated, land-based testing. Another noticable effect is the refraction of sound waves caused by the FS. Sound waves are pressure fluctuations, and as such are always convected with a flow and refracted between fluids of different velocity. To a lesser extent this effect also appears between core and bypass jets, but it is more pronounced with FS ac- tivated as the diameter of the NTF FS nozzle is relatively greater than that of the aeroengine jets. The refraction causes sound events to appear farther downstream than their actual occurence. As this work does not use shear layer correction, the resulting offset in direction of flow is observable in all TOBs in Fig. 4.12, but can be further emphasized by inspecting the comparison plot of isolated (DLR 317) and flight stream (DLR 325) test points (Fig. 4.14). The dark blue area at the nozzle outlet equates to a lower SPL in the flight stream test.PDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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