Control-oriented linear and nonlinear modeling for jet aeroacousticzz – Cool Jazz

Wavepackets are investigated from three different perspectives: by extracting coherent structures from experimental and numerical data, by computing linear instability flow structures, and by characterising the deterministic properties of dynamical systems. Special expertise in these areas is provided by the respective consortium partners. Reduced-order models are then built on the basis of the identified wavepacket structures, in order to obtain dynamical predictions for the purpose of control design. Modelling and control success is validated against experimental and numerical data at every stage.

Extensive databases have been generated with near-field and far-field data from jet experiments at Ma=0.4 and Ma=0.9. It has been demonstrated that coherent structures extracted as eigenmodes of the cross-spectral density correspond to the linear flow response to optimal forcing, as obtained from instability analysis, in a statistical sense. Nonlinearity enters the dynamics in the form of a stochastic forcing of linear structures. It has further been shown that such wavepackets represent an efficient basis for flow modelling, and that these structures are highly controllable.

The project has opened exciting new perspectives for jet modelling and control. Our international consortium will continue to explore these avenues.

The project has led to a large number of journal publications and conference contributions. The generated data is publically available. An international colloquium on «Jet Noise Modelling and Control« is held at Ecole polytechnique in September 2016 as a result of the project.

The proposed research programme aims at identifying noise-generating mechanisms in subsonic turbulent jets, and at the development of closed-loop control laws for the reduction of jet noise through flow actuation. An interdisciplinary approach combines experiment, numerical simulation and theoretical modelling in a coordinated effort, between three partner institutions with complementary expertise. While optimal control laws can, in principle and at enormous computational cost, be devised on the empirical basis of numerical simulations, taking into account the entire turbulent spectrum, the present proposal focuses on the dominant noise component associated with large-scale coherent flow structures, that drive the low-angle sound field. Fundamental progress in the understanding of the dynamics of these coherent structures, as well as their sound generation, will provide guidance for novel strategies to actively control and reduce jet noise.

The programme addresses the following questions: Which mechanisms govern the formation of orderly structures in jet turbulence? Can these structures be accurately described as instability wavepackets forming on top of a steady mean flow, as has often been conjectured? To what extent do nonlinear phenomena determine the wavepacket structure and the resulting acoustic field? And how can knowledge of these mechanisms be leveraged for jet noise reduction? Control strategies will be devised, and these will be tested in a real experiment during the final stage of the project.

The proposal builds on ongoing research activities at the three partner institutions, which so far have been developed independently without formal collaboration. The synergy potential of these complementary activities is considerable, and the proposal precisely aims to provide a framework for a coordinated interaction with a common set of objectives. Operational tools and preliminary results exist for all the main stages of the proposed programme. These include ongoing experiments on jet dynamics and their acoustic signature at PPRIME; a validated LES code; numerical tools for jet instability analysis at LadHyX, that are currently used on model configurations and await application on real-life jet data; model-free control concepts, developed at LadHyX, ONERA and LIMSI, that have been successfully deployed to reduce sound emission from flow over cavities; and reduced-order modeling for flow control (ANR Chair of Excellence at Pprime). International collaborations on jet noise research, with Tim Colonius at the California Institute of Technology and with André Cavalieri at Instituto Tecnológico de Aeronáutica (Sao José dos Campos, Brazil), are already in place and will be further intensified during the course of the proposed programme.

The proposal seeks funding for (i) one PhD student (3 years) and four postdoc years; (ii) experimental equipment for particle image velocimetry in high-speed jets; (iii) travel expenses for conference participation and for the collaboration between partners, including the external collaborators at Caltech and at ITA.