Understanding of the physics of jet noise generation mechanisms is necessary for effective development of noise reduction concepts. Azimuthal decomposition of far-field jet noise, at small-scale models, made it possible to gain insight into the nature of the structure of jet noise sources. In TsAGI, Azimuthal Decomposition Technique (ADT) is realized in anechoic chamber conditions as a standard in-house tool for the investigation of aeroacoustic noise sources. By means of ADT, it was shown that only 3 low-order azimuthal modes contribute to the far-field noise for low and medium frequencies (including spectral maximum), each mode possessing individual, and very nontrivial, directivity. All features of azimuthal modes of jet noise (modal directivities, spectra, cross-correlations) were modeled in our previous works with encouraging accuracy by a special version of correlation approach model based on quadrupole-type sources, for jet velocities from 120 m/s up to 240 m/s. For higher jet velocities, small-scale quadrupoles seems to underpredict jet noise levels, pointing out that another mechanism starts to play a role. One of the candidates for this mechanism is the noise radiation by instability waves evolving in the jet shear layer. Instability wave concept of jet noise was successfully validated for supersonic jet also on basis of ADT data analysis. Thus, application of ADT to small-scale jets demonstrated its ability to deliver rich information for understanding of noise source features, and its application to full-scale jets of real turbofans is of high interest - it was one of the main task for the Laboratory of Noise Generation Mechanisms and Modal Analisys under the Government Decree #220 (Contract No.14.Z50.31.0032). But implementation of ADT at open test rig faces several challenges: non-free field conditions (due to the presence of the ground) and large geometric dimensions.
In the present work, the results of application of this method to the analysis of large-scale jet noise are presented. It is shown that jet noise measurements by only 3 microphones in each cross-section allow reconstruction of 3 azimuthal modes directivities (axisymmetric, 1st and 2nd) for low and moderate frequency bands, while 2-microphone measurements make it possible to reconstruct axisymmetric mode directivity and sum directivity of 1st and 2nd modes, the microphones being properly located. The latter method is shown to be suitable for utilizing on an outdoor test bench. At first stage, the modified methodology is validated on the small-scale laboratory database, and then it is applied to the jet issuing from the real engine in the ground tests.
Comparison between the results for model scale and large scale jet showed qualitatively similar azimuthal content of the two. Normalized directivity patterns for large-scale and small-scale jets are shown to be in good qualitative conformity in terms of variation of total signal power with Strouhal number, increasing contribution of the axisymmetric mode for downstream observation angles, increasing contribution of 1st and 2nd modes for sideline direction (see figure, where solid lined correspond to the small-sacle jet, and dashed lines - to the full-scale jet). The observed discrepancies may be related to differences in jets structure and operating conditions, simplified data reduction procedure, measurement errors for large-scale-tests or other factors. It is also shown how the proposed method can be extended to allow approximate identification of all 3 azimuthal modes at open test rig.
The results obtained may help in the analysis of the physics of jet noise generation mechanisms for real turbofan engines.