Aeroacoustics of a Mach 0.9 Jet with Chevron-Microjet Combination

Abstract

The two noise reduction methods, chevrons and microjets, are used together such that the combination is more beneficial than each method alone. The acoustic field is mapped by sideline microphones and a microphone phased array. The near field flow characteristics are obtained for the first time by means of stereoscopic Particle Image Velocimetry (PIV). The jet noise measured with sideline microphones is lowered significantly below the previous levels observed from the best of either method. These results confirm that the OASPL benefits are nearly additive in all measured radiation directions. For instance, at a 150 nozzle inlet angle and at 100 D distance the isolated chevron and/or microjet reduction is only about 1 dB, whereas the microjet-chevron combination is 2 dB. The narrow band spectra at aft angles also show the better performance obtained by the combination at a large range of frequencies. Furthermore, no additional high frequency lift has been observed in any measured aft radiation angle with the application of microjets in combination compared to isolated chevron case. Noise source distributions obtained from the acoustic phase array also confirm these results and reveal that sources at high frequencies are located further upstream of the chevron-only case in contrast to low frequency sources that are downstream of the chevron-only case. Although previous detailed flow measurements of both isolated devices indicate that a potential synergy exists, this is the first study to report flow characteristics of the combination concept that confirms the explanation proposed by the author. The flow-field measurements show that the microjets create additional counter-rotating streamwise vortex pairs at the chevron tip locations besides the ones created by the chevrons. The eect of these additional pairs are more persistent in the flow due to their translation direction. When enabled, the microjet vortex pairs initially remain isolated from the chevron vortex pairs. In the initial (x/D<1) shear layer, the combination has a spreading rate that is about 20% higher than the chevron-only or microjet-only shear layer and 120% higher than the round (base) shear layer. Increased initial shear layer mixing and thickness reduces the large scale turbulence production at downstream. At x/D = 2, the peak turbulent kinetic energy for the combination is 35% lower than the base case and 10% less than the chevron-only case.