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Acoustic characteristics of Mach 0.8 jets were experimentally investigated with a special interest to understand propagation of noise from near-field to far-field. Three different nozzle exit configurations consist of a conical base nozzle of 30mm diameter and other two by retrofitting two lip geometries attached to the base nozzle were examined in the semi-anechoic environment. Chevron nozzles are considered a better trade off between noise reduction and drag penalty among other passive noise control techniques. One of the configurations tested in the present work is nozzle with triangular chevrons and another is plain strips of 2mm width having similar characteristic dimensions (tip angle, length, & penetration) of the chevrons. Simultaneous measurements were taken in the near-field along the jet edge at an interval of 2 diameter of nozzle (D) up to 14D and in the far-field at a distance of 90D and polar angles of 30⁰ to 90⁰ with increment of 10⁰. OASPL in near-field suggests shift in prominent noise source location towards nozzle and in far-field shows reduction in magnitude in downstream direction for both the modified lip geometries, however noise reduction is more observed in chevron nozzle than strip configuration in far-field. Cross correlations between near-field measurements and far-field measurements are calculated to examine the variation in directivity pattern. Coefficient of cross correlation is lower than base nozzle in other two configurations suggesting distortion of the coherent structures due to streamwise vortices produced by chevrons and strips. Maximum coefficient of cross correlation was found to be at 8D in base nozzle and at 6D in others with 30⁰ in the far-field, which complies with shift in noise source location to upstream. Further analysis was carried out after segregating acoustic signals in different frequency bands using band-pass filters and then near-field and far-field measurements were correlated. This provides complete map of noise directivity pattern for a particular frequency band and helps in understanding the noise propagation of different frequency from near-field to far-field. Base nozzle showing very high correlation between most downstream near-field and shallow angle far-field locations at low frequencies of 1-2kHz compared to modified nozzles. It shows low frequency noises in chevron nozzle are not travelling in downstream direction as efficiently as of base nozzle while strip nozzle lies in between. Coefficient of cross-correlation between near-field location of 4D and far-field location of 50° for chevron nozzle is 1.5 times the base nozzle for frequency band of 6-10kHz. Efficiency of high frequency noise seems to be improved and shifted towards higher polar angle from upstream positions of near-field in chevron nozzle. Among these two configurations chevron nozzle appears to be more effective in noise control than strip nozzle despite having similar characteristic parameters. Only reason for this could be the delta wing like shape of the chevron itself which creates strong stream-wise vortices. Most of the existing literature reports penetration as the key parameter and do not discuss the effect of shape of the chevron. From these results it is very clear that not only the penetration but the shape of the chevron is also an important feature in noise reduction of the jet.
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