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.

Above a critical flow velocity, a flexible plate becomes unstable to flutter and enters large-amplitude self-sustained flapping oscillations. This instability and the resulting oscillations can develop over a large range of flow velocity, making this mechanism attractive to harvest energy from a geophysical flow (wind, tidal or river currents,...): a fraction of the energy associated with the spontaneous solid deformation can be converted into electrical form through piezoelectric patches placed on the plate's surface. Such a system is particularly promising in the domain of small-power generation, when the connection to the electrical grid is technically or economically prohibitive. The present research project will study theoretically, numerically and experimentally the dynamics of such a piezoelectric plate in a steady flow to evaluate the energy harvesting potential of this technology. A critical element of this project's success will be the assessment of the system's efficiency through the development of a fully-coupled fluid-solid-electrical model. The proper understanding and representation of this strong coupling between the fluid-solid dynamics and the output electrical circuit is an original element of the present proposal, that thereby distinguishes itself from existing studies in fluid-structure interactions (with a realistic representation of the harvesting mechanism and output circuit) and from the traditional approach in power electronics (with a realistic model for the fluid-solid dynamics taking into account threshold effects, non-linearities and spatial variations associated with the flag dynamics). The objectives of this project are: (i) to evaluate the system's efficiency, (ii) to optimize the output electrical circuit to maximize the harvested energy and (iii) to understand the potential couplings between multiple neighboring harvesting units. Hence, the project will be organized around three main tasks. The first task will study numerically and experimentally the dynamics of such a piezoelectric flag with a purely dissipative output circuit. A coupled model for the fluid-solid-electric dynamics will be proposed, taking into account the piezoelectric coupling and the non-linear fluid-solid dynamics associated with the self-sustained plate's oscillations. The efficiency of the system and its dependence with its main characteristics (inertia, stiffness, coupling, etc...) will also be determined. The second task will focus on the optimization of the output circuit to maximize the harvested energy. Linear propagative and/or resonant circuits will be considered, but the bulk of this task will be dedicated to the application of state-of-the-art non-linear active techniques recently developed. A new approach will also be proposed, that is better suited to the non-linear dynamics of the fluttering plate. The third task will study potential electro- and hydrodynamic interactions between two or more neighboring units, by identifying the constructive or destructive nature of these interactions in terms of harvested energy. This project is characterized by its pluri-disciplinarity, including three young researchers working at three different research institutions and with complementary backgrounds and sensitivities (fluid-mechanics and fluid-solid interactions on one hand, and electrical engineering/power electronics on the other hand).