This project is concerned with the unsteady aerodynamics and associated sound production mechanisms which result from flows around bluff bodies. Such systems comprise regions of fully separated turbulent flow and strong fluid-structure interaction. From an applied perspective, the motivation for studying such flows derives from clear societal needs (safety, chemical and noise pollution) and strong industrial competition, while from a fundamental point of view such flows present a real challenge to scientists working in the fields of aerodynamics and aeroacoustics: a comprehensive understanding of these kinds of flow is hampered by the difficulty of quantitatively analysing the unsteady flow field and the mechanisms by which it drives sound fields (both internal and external). Experimentally, quantitative analysis approaches suffer from the difficulty of accessing the full space-time structure of the flow, and the fact that much of the essential aeroacoustic dynamic is below the noise floor of the measurement device. Numerical approaches on the other hand, while capable of providing a more complete spatiotemporal picture, struggle to resolve the finer details of the flow in near-wall regions, and are not well suited to supplying the fully converged statistics which are required for implementation of analysis tools which can help better understand the dynamics of the flow. The principal objective of the project is thus to develop integral analysis methodologies for study of the flows and source mechanisms evoked above. The strategy which we propose to follow in order to achieve this, and which constitutes an important originality of the project, involves the association of experts from different fields (aerodynamic, aeroacoustic, numerical, experimental, theoretical). Such a multi-disciplinary initiative is necessary to obtain analysis tools adapted to the very large data bases generated by experiments and computations and is central to an understanding of the more subtle aspects of these flows. Three complementary model problems will be studied: (i) a massive two-dimensional separation generated by a thick plate [LEA-C1], (ii) a strongly three dimensional cavity flow [LIMSI-C2], (iii) a more complex three-dimensional separation involving a conical vortex interacting with a solid surface, which is of interest on account of the particular instabilities which it supports, and its capacity to act as a wave-guide for intermediate-scale perturbations [LEA – C3]. The three configurations will also be simulated by means of a number of complementary methods: Large Eddy Simulation (or DNS in C1) [LIMSI C1 + C2; PSA C3] and hybrid RANS/LES [LEA C1+C3]. Databases corresponding to C1 and C2 will be available from the project outset. The project will comprise two workpackages. The first will be dedicated to a direct analysis of the unsteady flows generated by the three configurations, and the developement of specific quantitative analysis tools. Further simulations and experiments will be performed during the course of the project, in order to complement those which currently exist, and to aid in the development of novel analysis tools. These will include Quantitative Topological Analysis, Lagrangian Coherent Structure tracking, Linear and Quadratic Stochastic Estimation, Extended Proper Orthogonal Decomposition, and Causality Correlation Analysis; and they will be largely based on synchronous sampling of pressure (in-flow, surface and farfield; experimentally obtained via arrays of unsteady pressure probes), and velocity via full-field and temporally resolved optical measurement tehniques (Stereo PIV and 3C LDV respectively). The objective will be to develop integral analysis methodologies for the extraction and tracking of flow events, important either in terms of their energy or their unsteady wall pressure signatures. The second workpackage will deal with the question of how the unsteady flow dynamic couples both with the model body and with the acoustic farfield. Our principle objective will be to understand how to pose the problem such that the source terms we generate experimentally and numerically are both amenable to physical understanding (for the wall region and the farfield), and robust enough to provide an accurate description of the most important flow/`source' events where the vehicle body and the acoustic farfield are concerned. The experimental and numerical databases generated for C1, C2 & C3 will serve to help us understand how the flow skeletons identified in workpackage 1 drive the near and farfield pressures. This ambitious project promises to be rich in fundamental and applied developments, thanks to the synergy of recent numerical, experimental and analysis techniques, and the association of experts in aerodynamics and aeroacoustics. Such a multidisciplinary fusion will ensure a dynamic research environment, necessary for and conducive to the generation of new scientific knowledge.

This project is devoted to the net shape manufacturing of two intermetallic materials by means of an emerging powder compaction technique called SPS (Spark Plasma Sintering). Application of this technology to high performance intermetallics that are difficult to manufacture or to form using conventional techniques will allow their cost-effective introduction in gas turbines or in car engines. These systems are niobium silicide-based materials and titanium aluminides. These low density materials have great potential to increase thrust-to-weight ratios and improve fuel efficiency of gas turbine aero-engines, thus reducing drastically the amount of emissions and contributing to a cleaner environment. These materials are then qualified to meet a range of applications identified by the aero-engine manufacturers, for temperatures ranging from 700°C to 1000°C. Furthermore, using TiAl alloys to manufacture car engine exhaust valves that are withstanding temperatures of 600 to 800°C induces a reduction in fuel consumption (reduced mass and inertia, higher performance) with respect to current high density alloys (steels, superalloys). The components selected to validate the SPS technology will be of increased complexity throughout the project: exhaust valve (axisymmetric 2D) and seal segment (similar to a curved plate, nearly 2D), then turbine blades (3D). As a preliminary stage on small-scale samples, full potentialities of the SPS technique will be delimited by varying the main process parameters for both intermetallic alloy systems, associated with a thermo-metallurgical and physical modelling. This model will be an essential tool to determine the processing conditions and to design the tools needed for complex shape component manufacture. Attempts to achieve an appropriate balance of low and high temperature mechanical properties, or even oxidation resistance, will be made through the utilisation of ultra-refined and/or homogeneous microstructures that can be tailored by SPS. Following these structural optimisations, industrial up-scaling and complex net shaping by SPS will become the major challenges in view of the fabrication of full-scale aeroengine components. This up-scaling will be achieved in two steps, first on large scale two-dimensional components of simple geometry, then on large scale three-dimensional complex shaped components, but each time with the help of numerical simulations. Each step will be validated on microstructural criteria and through basic mechanical characterisations. The use of SPS to manufacture bi-material components or to join identical materials will also be evaluated. The goal will be to manufacture, ideally in a single SPS step, a structural component using either two powders, or a bulk material associated to a powder. The joining quality will be characterised through microstructural analyses and mechanical tests.