In the current context of global change, numerical models are key tools to explore the characteristics of the Earth climate and anticipate its evolution. Despite the increasing complexity of climate models (CMs), their representation of air-sea interactions remains a fundamental issue. This includes a wide range of aspects: parameterizations of atmospheric and oceanic boundary layers (BLs), estimation of air-sea fluxes, time-space numerical schemes, matching of different grids at the interface, coupling algorithms... The coupling effort in CMs, which started in the 90’s, primarily addressed modularity of model interfaces and conservation of energy and water. More recent studies led to additional improvements, but these were generally performed independently from each other. In this context, our project aims at revisiting the overall representation of air-sea interactions in CMs, by coherently considering physical, mathematical, numerical and algorithmic aspects. To achieve this goal, this project brings together expert scientists in those disciplines. Turbulent air-sea fluxes ensure a large part of the coupling between the ocean and the atmosphere. They are estimated in numerical models from values of physical quantities (wind and current, temperature, humidity) near the air-sea interface, and are heavily dependent on the parameterizations chosen for the oceanic and atmospheric BLs and for the flux computation (bulk formulae). In this project, we intend to improve the coherence of these parameterizations (generally chosen independently) by designing objective characterization criteria, and to study its influence on model results. We will also enrich these parameterizations by adding representations of high impact phenomena (gustiness, oceanic warm layers, SST front effects on boundary layer), while assessing the relevant level of complexity to be considered for global coupled ocean-atmosphere (OA) applications. The interest of these new capabilities will be assessed both from a deterministic (e.g. representation of specific events) and statistical (e.g. intra-seasonal and inter-annual variability) viewpoint. We will focus on the tropical regions, where latent heat transfers largely condition the atmospheric water content and where the most important biases are observed. In order to actually improve climate simulations with a better physical representation of coupling processes, it is also necessary to improve the mathematical and algorithmic aspects. Current CMs suffer from inconsistencies linked to the discretization in space and in time. In time, current asynchronous coupling algorithms do not allow for a correct phasing between the ocean and the atmosphere, and hence between their diurnal cycles, potentially inducing biases in the estimation of daily maxima. We will correct this defect by introducing an iterative coupling algorithm based on Schwarz methods, which additional cost will be limited by locally making use of reduced models. Considering the problem in space, physical inconsistencies result today from the difference in the numerical grids used by atmospheric and oceanic models. These inconsistencies will be mitigated by implementing intermediate exchange grids to compute the surface fluxes. The improvements described above will first be implemented and tested in a series of 1D test cases, involving a coupled 1D OA model and reference solutions from coupled large eddy simulations that will be specifically developed. Transfer into real CMs and evaluation of the impact will then be performed through 3D simulations in the two French CMs (IPSL-CM and CNRM-CM). A number of outcomes of this project are not tied to a specific CM but virtually apply to any coupled OA model. Moreover particular attention will be paid to document and distribute our new tools (1D coupled OA model, coupled LES simulations, specific test cases and associated metrics) to the scientific community.

The aim of this project is to develop a theoretical approach and associated computer code for Large-Eddy Simulation (LES) of liquid jet fragmentation and atomization in all thermodynamic conditions, from subcritical to supercritical pressure and temperature conditions. The transition between sub and supercritical conditions will be considered through a new and unified model, able to deal with capillary effects, phase transition between a liquid and its vapour as well as the dynamics of supercritical fluids. These different regimes of fuel jet dynamics are found in liquid rocket as well as Diesel engines depending on the combustion chamber temperature and pressure conditions. The situation is possibly already prevailing in current aircraft engines or will be reached in the future as the chamber pressure is being increased. For sufficiently low pressure and temperature with respect to the critical point, the fluid undergoes a classical break-up process. The interface separating liquid and gas phases is discontinuous and droplets appear as the final stage of fragmentation. As pressure is increased to supercritical conditions, the phase discontinuity is no longer present. The jet evolves in the presence of a continuous interface and mass transfer takes place through turbulent fluxes and the process is analogous to mixing of variable density jets. It is planned to deal with all these aspects in LES and DNS frameworks. Such a unified approach has never been considered in the author’s knowledge as phase transition modeling in arbitrary flow conditions is a true scientific challenge.

The reduction of pollutant emissions in aircraft engines and power plants has become a major issue for gas turbine manufacturers as a result of more stringent environmental regulations and increased environmental concern. An efficient solution to reduce pollutant formation is to maintain a relatively low temperature in the combustor primary zone, by decreasing for instance the mixture equivalence ratio. The issue is that a low flame temperature induces slower chemical reaction rates often resulting in an increase of flame instabilities and extinctions. An emerging solution to enable flame stabilization in leaner regimes, suitable to a wide range of combustion applications, is to generate electrical discharges at the flame basis. Among these various types of discharges, the Nanosecond Repetitively Pulsed discharges have shown to beparticularly efficient. Despite this proven efficiency, the fundamental mechanisms of plasma-assisted combustion are not well understood. Also, the numerical tools needed by engineers to assess the performance of NRP discharge in practical configurations and optimize their design do not exist. The objective of this project is to elaborate and validate against experimental data a modeling route suitable to perform simulations of realistic turbulent combustion systems accounting for plasma-flame interactions. The discharge properties will be first characterized numerically and experimentally in a mixture representative of a combustor, which may contain fuel and recirculating burnt gases. These temporal and spatial distributions of species and temperature within the discharge will serve to calibrate a semi-empirical plasma-assisted combustion model recently developed. This model will be formulated in a LES context and implemented in an unstructured CFD solver. To build the validation database, experiments of plasma-assisted combustion will be conducted on two configurations: - The first one is a bluff body flame on which it has been already shown that the NRP discharge enables the flame stabilization over very lean regimes. The NRP discharge is efficient when located in small gases recirculation zone. We will perform complementary measurements on this configuration to characterize the composition and flow properties within the recirculation zone. - The second configuration is a swirled combustor representative of aeronautical combustion chambers. It is composed of a two-stage swirled injector and a rectangular combustion chamber with optical access ports. We will perform measurements to characterize the effect of the plasma on the flame stabilization process. It will be interesting to show that the plasma impacts significantly the stabilization mechanisms. The LES plasma-assisted combustion model will be applied to both bluff-body flames and swirled combustor configuration. Finally, we will perform the LES of a practical Lean Premixed Turbomeca combustor stabilized by NRP discharges as a project final big challenge.

The project FLOCCON is dedicated to the development and validation of an innovative strategy to accelerate solvers for fluid mechanics. The project focuses on incompressible solvers, containing two parts: (1) a linear Poisson equation, and (2) a non-linear advection equation. The key idea of this project is to use deep learning to train neural networks based on solutions of these two equations. To go further, the project will examine learning methods which can guarantee a target accuracy. To do so, physical-based and long-term 'loss functions' will be introduced, in order to ensure a limited error accumulation in time. Moreover, an hybrid strategy will be proposed to obtain a robust solver. finally, an optimisation of this new network-based solver will be carried out on CPUs/GPUs. In addition to classical validation cases, a target application will be simulated on the pollutant dispersion in a large city, which is a challenging case for classical HPC solvers.

NextSim partners, as fundamental European players in Aeronautics and Simulation, recognize that there is a need to increase the capabilities of current Computational Fluid Dynamics tools for aeronautical design by re-engineering them for extreme-scale parallel computing platforms. The backbone of NextSim is centered on the fact that, today, the capabilities of leading-edge emerging HPC architectures are not fully exploited by industrial simulation tools. Current state-of-the-art industrial solvers do not take sufficient advantage of the immense capabilities of new hardware architectures, such as streaming processors or many-core platforms. A combined research effort focusing on algorithms and HPC is the only way to make possible to develop and advance simulation tools to meet the needs of the European aeronautical industry. NextSim will focus on the development of the numerical flow solver CODA (Finite Volume and high-order discontinuous Galerkin schemes), that will be the new reference solver for aerodynamic applications inside AIRBUS group, having a significant impact in the aeronautical market. To demonstrate NextSim market impact, AIRBUS has defined a series of market relevant problems. The numerical simulation of those problems is still a challenge for the aeronautical industry and their solution, at a required accuracy and an affordable computational cost, is still not possible with the current industrial solvers. Following this idea, three additional working areas are proposed in NextSim: algorithms for numerical efficiency, algorithms for data management and the efficiency implementation of those algorithms in the most advanced HPC platforms. Finally, NextSim will provide access to project results through the “mini-apps” concept, small pieces of software, seeking synergies with open source components, which demonstrate the use of the novel mathematical methods and algorithms developed in CODA but that will be freely distributed to the scientific community.