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Chapter2. Theory 16 sound waves propagate into the acoustic farfield. This velocity difference results in a different ratio of approximately I ∼ c3 [26, 33]. The directivity of jet noise is similar over all TOB. There is a peak in the center of the aft quadrant, from whence there is a gently decline towards the forward quadrant. Downstream, the noise rapidly diminishes, before terminating in a zone of almost no sound between 15◦ and 10◦, called the cone of silence. The cone results from refraction effects between high-speed jet and atmosphere. Studies referenced by Smith [33] showed, that jet noise increases for increasing tempera- ture, whereas the opposite holds true for decreasing temperature. Smith attibutes this phenomenon to the variation in the local speed of sound, coupled with the hot-cold fluid boundary. However, this effect can be considered second-order, because the differences in temperature profiles for aeroengines do not differ con- siderably between operating points. Figure 2.6 shows the general directivity of a low and high bypass engine, respectively. Figure 2.6: Sound directivity of a low (left) and high (right) bypass engine [33]. Two stream jet noise As explained in Sect. 2.1, turbofan engines with a high BPR are most commonly used in modern civil aviation. The accompanying reduction of flow velocity re- sults in a significant sound level reduction, since jet noise is generally assumed to be proportional to the eighth power of the speed of sound. During the last 35 years, noise levels of turbofan engines have been reduced by up to 24 dB [29, 33]. Despite this decrease, turbofan jet noise alleviation still faces manifold challenges.PDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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