The main work performed has focused on the development of a theoretical/computational methodology to model instabilities in three-dimensional flows and the development of a data-driven methodology to extract coherent structures from experimental datasets and large-eddy simulations.
For the major part of the project, the practical configuration analysed has been the turbulent shear flow generated by non-axisymmetric jets. The large-scale instabilities developing in the core and mixing layers of turbulent jets, also known as wavepackets, are mainly responsible for the generation of jet-engine noise.
First, a linear stability analysis tool for three-dimensional flows has been developed based on the parabolized stability equations, and has been applied to compute wavepackets in a twin-jet flow configuration.
Second, a data-driven methodology based on spectral proper orthogonal decomposition has been developed to extract coherent structures from instantaneous flow-field images.
Third, experimental measurements of the twin-jet system have been performed, consisting of high-speed schlieren visualizations, particle image velocimetry measurements, and microphone measurements.
The obtained instability models have enabled a characterization of the linear dynamics of twin jets and the mechanisms by which the axisymmetry of wavepackets is lost in this configuration, as well as the sensitivity of this effect to parameters such as jet spacing.
The developed data-driven tool has been applied to experimental schlieren images of the twin-jet system, which has enabled the extraction of empirical wavepackets that can be compared with the modelled instabilities. Thanks to this research methodology, a successful validation of modelled wavepackets with experimentally-educed coherent structures has been achieved.
Two additional configurations of industrial relevance have been studied with a more limited level of detail, namely, the recirculating flow behind a bluff body flame stabilizer and the separated flow induced by a bump in a transonic diffuser passage, which mimics flow separation in a low-pressure turbine blade.
The data-driven tool developed has been applied to large-eddy simulations of the bluff-body flame holder, which has revealed physical insights on the competing oscillating mechanisms in the flame and their origin.
A preliminary characterization of the separated flow induced by the bump geometry has also been achieved employing unsteady RANS calculations and empirical estimations of mixing layer and shedding oscillation frequencies in the recirculating region. These results constitute a starting point for the recently-started ERC project TRANSDIFFUSE.
