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.

The development of hydrogen as a reliable energy vector is strongly connected to the performance and level of safety of the components of the supply chain. In this respect, achieving an efficient storage is crucial to address transition markets and automotive markets. For near term, compressed hydrogen storage is currently the most promising technology. Compressed hydrogen for industrial applications is stored at 200 bar in metallic cylinders which have poor mass storage efficiency but present high impact resistance. To achieve required performance in terms of autonomy and weight efficiency, hydrogen must be stored at pressure up to 700 bar in carbon fibers composites cylinders. However the damage resulting from a shock, its evolution during service and thus the cylinder tolerance to damage are not well described. As a consequence, the design of the composites cylinders is conservative and even minor shock on cylinder results in the cylinder withdrawal from the supply chain, which affects the cost without an enhanced guaranty of safety. In the scope of hydrogen energy markets, the cylinders can be subjected to a broad range of impacts either usual or accidental (car accident, during handling and transportation of transportable cylinders) and can be in the hand of people with no experience of compressed gas handling. It is thus critical to assess impact resistance of the storage and to determine which impact causes a cylinder burst immediately or after some time in service. In addition, taking into account that some 2015 DOE performance targets are almost reached by composites cylinders and that there are on-going projects to improve manufacturing & materials, a study on damage tolerance of these structures (i.e. thick composites made by filament winding) is justified and would be complementary to current approach. The development of scientific knowledge on the behavior of carbon fiber composites cylinder subjected to impacts and of numeric tools to predict residual performance of a cylinder in service presenting damage from a shock are the main objective of the project TOLEDO, submitted to the French call “AAP ANR HPAC 2010. The project gathers an industrial partner Air Liquide as an end-user of composites cylinders with experience on cylinder supply chain and safety, CEA who has cylinder testing facilities and cylinder design experience and two academic partners with complementary competences in impact generation, damage characterization and composites structure durability that are acknowledged by the academic world (LAMEFIP from ENSAM Bordeaux and Institut P’ – ENSMA, Poitiers). In the framework of TOLEDO program, a significant number of high pressure composites cylinders will thus be subjected to drop and shock tests representative of normal and accidental situations in the Hydrogen Energy supply chain and during handling by the customer. Different techniques will be used to characterize the resulting damage on the composite structure. The criticality of the damage for the cylinder will be assessed by the study of residual performance of the cylinder after the impact and more importantly after further use (static and cyclic pressure load, effect of temperature). This part of the study will involve tests on specimens, a numeric study and a validation on cylinders and will provide knowledge on lifetime predictions. This approach will lead to recommendations for the industry and normative committees on the design of cylinders taking into account a quantitative analysis of damage tolerance and possible protections and for the control of cylinders in service by providing knowledge to define a withdrawal threshold.