3D flow organization and dynamics in subsonic jets: Aeroacoustic source analysis by tomographic PIV

Abstract

To meet the increasingly stringent noise regulation, aircraft manufacturers are searching for solutions to jet noise. This, which constitutes a significant amount of the total noise emitted by civil aircrafts, is generated by the mixing processes between the exhaust flow leaving the engine and the atmosphere. A detailed understanding of such mixing process is of paramount importance to identify mechanisms responsible for noise production, i.e. the aeroacoustic source, and, ultimately, to develop noise control strategies. This thesis proposes an unprecedented experimental-based approach to visualize and measure the aeroacoustic sources in jet flows. Time-resolved tomographic particle image velocimetry (TR-TOMO PIV) is employed to obtain time-dependent three-dimensional (3D) measurements of the turbulent flow patterns, while the instantaneous aeroacoustic source is explored using Powell’s aeroacoustic analogy. TR-TOMO PIV experiments are conducted in a tailored jet facility. Measurements are performed both on jets issued through circular and 6-chevron nozzles, with the latter configuration that is nowadays used in jet engines to reduce acoustic emissions. The attention is placed upon the 3D organization and dynamics of flow transition, where coherent flow structures play a role in the generation of noise. The full 3D approach enables unambiguous descriptions of the vortex topology, while the temporal resolution allows addressing the growth and development of the coherent flow structures along with their mutual interaction. In the circular jet, the characteristic pulsatile motion of vortex ring shedding and pairing is accompanied by the growth of azimuthal instabilities and the formation of streamwise vortices leading to the breakdown of the vortex rings. In the chevron jet, instead, the axisymmetric ring-like coherence is replaced by streamwise flow structures whose decay is accompanied by the formation of C-shaped structures. The relation between coherent structures and the instantaneous acoustic source is investigated recalling Powell’s aeroacoustic analogy, with the acoustic source that is identified by the second time derivative of the Lamb vector. The spatio-temporal evolution of the source is mapped and is compared to that of the vortices, to detect flow events involved in the acoustic generation. In the circular jet, most pronounced source activity is observed during the vortex-ring breakdown, whereas, in the chevron configuration, is associated with the process of streamwise vortex decay and C structure formation. Performing unbiased acoustic predictions of the jet noise with TR-TOMO PIV measurements is a challenging task due to the constraints on the extent of the instantaneous measurement domain and on the required spatial and temporal resolutions. To meet this challenge, the thesis finally proposes a strategy to perform far-field acoustic predictions by direct evaluation of Powell’s analogy using TR-TOMO PIV data.