Investigation of the noise emitting zones of a cold jet via causality correlations |
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Authors: | M Schaffar JP Hancy |
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Institution: | Institut Franco-Allemand de Recherches de Saint-Louis, 12, rue de l''Industrie, 68301 Saint-Louis, France |
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Abstract: | Within the framework of lighthill's acoustic analogy the causality method proposed by Ribner and Siddon is used to identify equivalent noise sources inside a cold jet. An exploration of a few cross-sections shows that a two-dimensional investigation suffices in a first approach for integrating the source function provided the upper frequency limit does not exceed a Strouhal number approximately equal to 0·5. Furthermore the transverse distribution of the source term shows the jet region located on the microphone side to be dominant; the effective diameter of the source region is comparable with that of the nozzle. It is shown that in a direction with an angle of 30° to the jet axis, the “shear noise” is dominant (about 70% of the noise measured in the same frequency range). The noise emanates essentially from the transition region and from layers located between and . This analysis is suitable for frequency range bounded above by St = 0·54. For the direction with an angle of 45° to the jet axis comparable results are obtained in a frequency range also limited at St = 0·54. However, this range contains only 40% of the total acoustic energy. The source region of the “shear noise” (near 70% of the total energy) and that of the “self-noise” remain always in the transition region located at 4–11 D. Radially the main part of the noise originates from the layers located on the microphone side between and 0·375. For the direction with an angle of 60° to the jet axis the “shear noise” is no longer measurable and the calculated “self-noise” represents only a few percent of the noise measured. For an acoustically excited jet (white noise filtered between St = 0·39 and St = 0·52, 320 Pa at the nozzle) another type of correlation appears which is believed to be related to a coherent structure travelling inside the jet at 0·75 vj. The study of the source term shows that this structure must be related to noise originating from the nozzle outlet. |
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