Periodic Reporting for period 1 - STALSUns (Short-time analysis of large-scale structures in unsteady flows)
Berichtszeitraum: 2023-09-01 bis 2026-02-28
Flow systems play a vital role in the operation and performance of modern aerospace applications. Understanding their physical behavior is crucial for developing new technologies that optimize the efficiency and safety of future generations of aircraft and reduce their environmental impact.
Most flow configurations encountered in the aerospace industry are unsteady and are characterized by turbulent motion, whose dynamics are not well understood. Examples include the separated flow in turbine blades, the noise generated by jet engines or the recirculation regions induced by flame holders in combustion chambers.
Stability analysis offers a physics-based theoretical framework for understanding the evolution of instabilities in a given flow field and to reveal actuation mechanisms that can be exploited to achieve flow optimisation and control. Almost all flow stability and sensitivity studies available up to date are based on time-invariant (either steady or time-averaged) flows. These approaches fail to provide the necessary framework for configurations in which the underlying flow is unsteady, therefore preventing the access to relevant dynamic information about the behaviour of such flows. Given this shortcoming, novel theoretical approaches are necessary to provide new insight into the behaviour of unsteady flow systems, and to develop models for their prediction and control.
The goal of the STALSUns project is to develop a new methodology capable of predicting the short-time stability characteristics of unsteady flows of practical interest, and apply it to relevant problems in aerospace engineering. The output of this goal consists of new physical understanding beyond the state of the art that can be exploited in the future to control such flows, leading to: (1) more efficient engines with lower pollutant emissions, (2) reduced engine noise levels and (3) safer transport, therefore reducing the environmental impact of future aircraft generations.
From the socioeconomic point of view, the outcomes of this research project are expected to contribute towards achieving the objectives set in Europe’s vision for aviation (Flightpath 2050).
The research undertaken during the project is also expected to have a scientific impact on the fields of dynamical systems analysis, renewable energy engineering and meteorology and climate sciences. The development of a new theoretical/computational methodology for performing finite-time stability analysis in general three-dimensional flow configurations will boost lines of research on the fundamental understanding of turbulent flows.
Most flow configurations encountered in the aerospace industry are unsteady and are characterized by turbulent motion, whose dynamics are not well understood. Examples include the separated flow in turbine blades, the noise generated by jet engines or the recirculation regions induced by flame holders in combustion chambers.
Stability analysis offers a physics-based theoretical framework for understanding the evolution of instabilities in a given flow field and to reveal actuation mechanisms that can be exploited to achieve flow optimisation and control. Almost all flow stability and sensitivity studies available up to date are based on time-invariant (either steady or time-averaged) flows. These approaches fail to provide the necessary framework for configurations in which the underlying flow is unsteady, therefore preventing the access to relevant dynamic information about the behaviour of such flows. Given this shortcoming, novel theoretical approaches are necessary to provide new insight into the behaviour of unsteady flow systems, and to develop models for their prediction and control.
The goal of the STALSUns project is to develop a new methodology capable of predicting the short-time stability characteristics of unsteady flows of practical interest, and apply it to relevant problems in aerospace engineering. The output of this goal consists of new physical understanding beyond the state of the art that can be exploited in the future to control such flows, leading to: (1) more efficient engines with lower pollutant emissions, (2) reduced engine noise levels and (3) safer transport, therefore reducing the environmental impact of future aircraft generations.
From the socioeconomic point of view, the outcomes of this research project are expected to contribute towards achieving the objectives set in Europe’s vision for aviation (Flightpath 2050).
The research undertaken during the project is also expected to have a scientific impact on the fields of dynamical systems analysis, renewable energy engineering and meteorology and climate sciences. The development of a new theoretical/computational methodology for performing finite-time stability analysis in general three-dimensional flow configurations will boost lines of research on the fundamental understanding of turbulent flows.
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.
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.
The main results obtained beyond the state of the art are:
1) The demonstration that the developed theoretical methodology is successful in characterizing the dynamics of non-axisymmetric jets. This has been achieved by validation of the wavepackets computed for twin jets with coherent structures extracted from experimental measurements. This result has a high potential impact since the methodology is therefore applicable to different engine configurations encountered in practice, such as elliptical and rectangular jets, and provides a way to model the instabilities in installed jets.
2) The characterization of the mechanisms by which the axisymmetry of wavepackets is lost in twin-jet configurations, as well as the sensitivity of this effect to parameters such as jet spacing. This outcome has a potential impact for engine design, as it can provide guidance on the limits of the parameter space at which three-dimensional effects become important.
3) The demonstration that the developed data-driven methodology is able to extract the low-dimensional dynamical information for the flame stabilizer configuration. This finding is relevant for the scientific community in propulsion, as the use of such approach in this context was unprecedented.
1) The demonstration that the developed theoretical methodology is successful in characterizing the dynamics of non-axisymmetric jets. This has been achieved by validation of the wavepackets computed for twin jets with coherent structures extracted from experimental measurements. This result has a high potential impact since the methodology is therefore applicable to different engine configurations encountered in practice, such as elliptical and rectangular jets, and provides a way to model the instabilities in installed jets.
2) The characterization of the mechanisms by which the axisymmetry of wavepackets is lost in twin-jet configurations, as well as the sensitivity of this effect to parameters such as jet spacing. This outcome has a potential impact for engine design, as it can provide guidance on the limits of the parameter space at which three-dimensional effects become important.
3) The demonstration that the developed data-driven methodology is able to extract the low-dimensional dynamical information for the flame stabilizer configuration. This finding is relevant for the scientific community in propulsion, as the use of such approach in this context was unprecedented.