The main aim of this proposal is to implement a synergistic approach leveraging innovative High-Performance Computing techniques and observations to advance fundamental knowledge on the dynamics underlying the emergence of large scale extreme events and local instabilities in stratified and rotating turbulent fluids and their feedback on mixing and transport properties of such flows. This project has the ambition to achieve an unprecedented statistical and phenomenological characterization of large scale extreme events and their feedback on the small scales in a novel paradigm in fluid turbulence: that of three dimensional rotating and stratified flows where the energy goes to both large and small scales with a dual constant flux cascade. The fundamental study proposed is therefore a synthesis of major research themes of the Axe 7.1 The capability to design state of the art high-resolution DNS of rotating and stratified turbulence able to capture all these phenomena in a parameter space compatible with the real flows, together with the expertise and means present within the scientific team of the EVENTFUL project to design experiments and field campaigns in the stratosphere and Mesosphere-Lower Thermosphere (MLT), will lead to a comprehensive assessment of the emergence and dynamics of large-scale powerful events in the active flow fields, by means of an – innovative and synergistic – combined use of observations, DNS and machine learning techniques, as detailed in the following. Objectives will be accomplished taking advantage of the extensive experience gained by the scientific team in implementing the proposed methodologies and in the investigation of anisotropic turbulent flows.

Turbulence, defined generically as an out-of-equilibrium state of systems with a large number of degrees of freedom, is not a concept restricted to fluid dynamics. An ensemble of dispersive waves in nonlinear interaction is indeed also said to be in a turbulent state, called Wave Turbulence, which is expected in a wide spectrum of systems from quantum mechanics to astrophysics. A theoretical framework for this “other kind” of turbulence has been developed starting with the work of Zakharov in the 1960’s focused on the weakly nonlinear limit. During the last two decades, this Weak Turbulence Theory (WTT) has been increasingly successful in describing the “turbulence of waves” in 2D mechanical systems such as surface water waves or bending waves in elastic plates. In fluid mechanics, the situation is more complex when bulk waves are present since the system is prone to entangle strong hydrodynamic turbulence and weak wave turbulence. Turbulence in stratified or rotating fluids, enabling the propagation of internal gravity and inertial waves respectively, are typical examples of this situation which is still poorly understood. Rotation and stratification are moreover central ingredients of Earth atmospheric and oceanic dynamics. In comparison with 2D systems, the assessment of WTT in these systems raises the difficulty of dealing with 3D anisotropic velocity fields. This makes this dawning research area technically more challenging. Independently, approaching wave turbulence regimes in these systems, i.e. a turbulence dominated by weakly nonlinear waves, is in itself a challenge, which has rarely been realized. The relevance of WTT for rotating and/or stratified turbulence remains consequently an open question. The primary goal of our project is to achieve wave turbulence regimes in experimental and numerical rotating and/or stratified turbulent flows. For this, we will set up original turbulence experiments and direct numerical simulations, promoting weak nonlinearity and injection of energy in waves. This strategy aims at exploring regimes significantly different from those of past studies in which energy was most often injected in eddy structures. The coordinated effort of the 4 partners, exploring different systems expected to develop similar behaviors, is a cornerstone of our project. Experiments in water using stratified fluid channels and/or precision rotating platforms will be designed at LPENSL and FAST. These setups will allow us to install sophisticated wave generators injecting energy in weakly nonlinear waves. Experiments will also be conducted in the cryostat dedicated to turbulence at Institut Néel settled on a rotating platform in 2016. Taking advantage of the low viscosity of liquid helium, this technically extremely challenging experiment, which allows image-based velocimetry, aims at reaching unprecedented regimes of weak nonlinearity with respect to rotation (keeping nonlinearity strong with respect to viscous effects). Besides, high-resolution long-term direct numerical simulations will be performed at LMFA. In order to access the high spatio-temporal resolution necessary to uncover the dynamics of the turbulent flows, we will use multiple camera velocimetry systems in experiments and high-performance computing facilities in numerics. The final objective is to implement a systematic spatio-temporal statistical analysis of the data gathered during the project, with the aim of disentangling waves from strongly nonlinear structures and, for the first time, to thoroughly test the relevance of WTT and its strongly nonlinear extensions in flows dominated by weakly nonlinear 3D waves. The DisET project aims at producing a breakthrough in the understanding of bulk wave turbulence which is a key ingredient of large-scale geophysical flows and therefore fundamental regarding weather and climate forecast.

This project aims to quantify the uncertainties of the pollutant concentrations that are computed by an operational urban air quality model. The uncertainties refer to the range of values that the errors (i.e., the discrepancies between the model outputs and the true values) can take. These errors are usually modeled as a random vector, whose probability density function is the complete description of the uncertainties. Our strategy to approximate this probability density function is the generation of an ensemble of simulations that properly samples the errors. The application is air quality simulation across Clermont-Ferrand, using a dynamic traffic model to compute traffic emissions and using an atmospheric chemistry-transport model that explicitly represents the streets of the city. Based on the emission data, meteorological conditions and background pollutant concentrations, the air quality model computes every hour the concentration fields (across the whole city) of several air pollutants, especially dioxide nitrogen and particulate matter. As a result of the complexity of atmospheric phenomena and the limited observations, the simulations can show high uncertainties which need to be estimated. Our objective is to propose a tractable approach to provide uncertainty estimations along with any urban simulation. The approach should apply to short-term forecasts as well as long-term simulations (e.g., for impact studies). One major uncertainty source lies in the traffic emissions. We will carefully estimate the uncertainties of traffic assignments in the streets and of associated pollutant emissions. Using multiple simulations of a state-of-the-art dynamic traffic model, an ensemble of traffic assignments will be generated. The ensemble will be calibrated with traffic observations so that it should be representative of the uncertainties of the traffic model. The associated ensemble of pollutant emissions will provide inputs to the air quality model. An ensemble of air quality simulations will be generated, using the different traffic emissions, using perturbed input data (Monte~Carlo approach) and possibly a multimodel approach. This ensemble will also be calibrated using observations of pollutant concentrations in the air. The air quality model is a high-dimensional model with high computational cost. In order to generate an ensemble of simulations, it is necessary to reduce the computational costs. Consequently a part of the project deals with the reduction of the air quality model. This project is proposed in a context of increasing use of numerical air quality models at urban scale. The models are used for daily forecasts, for assessment of long-term exposure of populations to pollution, for the evaluation of the impact of new regulations, ... We will propose methods that can be applied in an operational context to the core modeling chain, from traffic assignment to atmospheric dispersion. The scientific results of the project will be integrated in an operational modeling system that is currently used for many cities in France and abroad.

The destabilization of a liquid jet into droplets by a fast gas stream is at the heart of many applications, in particular related to propulsion (turboreactors, rocket engines). However, existing models and numerical simulations used to dimension these systems have been validated far from the conditions of applications, and are unable to describe fragmentation mechanisms in fully turbulent conditions or for low surface tension fluids. This project aims at experimentally and numerically studying jet instabilities and droplet formation in cryogenic conditions, where relevant dimensionless numbers (namely Reynolds number Re for turbulence impact, and Weber number We for surface tension role) are at least two orders of magnitude larger than in the fragmentation literature and previous laboratory experiments. The experimental approach will be carried out at CEA SBT with liquid/gaseous helium, and at Institut Néel with liquid/gaseous nitrogen. CEA is already equipped with a suitable cryostat for the helium experiments, where few modifications are necessary. Significantly larger We, Re and dynamic pressure ratios can be reached in Helium for selected conditions. The second nitrogen cryostat will be built during the project at I. Néel, and will be more versatile, allowing for a large number of experiments at Weber and Reynolds numbers much larger than in the air water case. The nozzle geometry retained for the experiments will be defined in agreement with CNES, which has agreed to share its expertise on this project. Theses geometries will include a swirled liquid central jet, as is in LOX/CH4 spatial applications. Experiments will use a vapour overheating/liquid undercooling strategy, in order to avoid phase change issues encountered in previous works. The cryogenic spray will be characterized with advanced experimental methods: Phase Doppler anemometry, optical fiber probes and holography, methods that the present partners are familiar with. In parallel, we will use one of the best fluid mechanics codes existing for industrial applications including two phase flows, YALES2, which has already been used with success at LEGI. We will also use the Lattice Boltzmann Method LBM (LMFA), which is totally different. The complementarity of LBM resides in the fact that it introduces modelling at smaller scales than classical fluid mechanics approaches. This method has been recently tested with success on a liquid fragmentation configuration. Note that the lower density ratio of our experiments will be favorable compared to the large density ratio limit of the usual laboratory experiments in the air water case. These codes, once adapted and validated by our new experimental data, can then be used to reliably predict fragmentation in multiphase conditions relevant to propulsion systems. This will directly lead to optimization of atomization processes, but also to a significant decrease of the environmental impact caused by unburnt droplets (NOX emissions). Covering a much wider range of physical parameters will provide an opportunity to either unify or invalidate the various existing scaling laws proposed in models for jet instability and drop formation. In order to reach these objectives, we have constituted a consortium with experts in atomization and jet instabilities (PI at LMFA, but also LEGI), cryogenics (CEA and Institut Néel), optics (L. Mées at LMFA), experimental and numerical methods for two phase flows (LMFA and LEGI).

The project deals with optical properties of droplets and particles of “complex” shapes, and whose sizes are beyond a few visible light wavelengths. In this domain, rigorous theories and existing numerical methods cannot be applied to accurately describe light scattering phenomena. We propose a new methodology to calculate the interaction of light with such objects, in parallel to a series of experimental tests for validation. The project includes novel applications in optical metrology, and in optical trapping and manipulation of non spherical particles. Complex-shaped particles (CSP) are everywhere present in fluid mechanics problems (multiphase flows, sprays, aerosols..), chemical engineering and life science. Attempts to characterize particles in flows exploit their far-field light scattering properties (laser diffractometers, phase Doppler interferometers and particle imaging techniques are standard). Light scattering is also the source of the radiation pressure acting on a particle in a laser beam. The involved forces and torques make possible laser trapping (optical tweezers) and contact-less manipulation, a technique of ever growing importance in biophysics and micro technologies. Modeling the interaction of light with particles is essential. Many theories and models (scattering, absorption, radiation pressure) have been developed accordingly. Rigorous methods are limited (for theoretical or numerical reasons) to simple shape particles, i.e. spheres and cylinders, and then cannot deal with CSPs. Different numerical methods such as T-matrix, DDA , MoM and FDTD allow calculating the scattering properties of arbitrarily shaped particles, but their applicability is limited to sizes not more than a few tens of wavelengths, even with supercomputers. Thus, there is currently no accurate method to predict the light scattering properties of CSPs of sizes larger than a few tens of microns! This is the crux of AMO-COPS project: developing a novel model for large CSPs. Ray tracing, or geometrical optics, is flexible in terms of particle shapes. However ray models completely or partially neglect wave effects in general and contributions of high order rays. Recently, Partner 1 has successfully introduced wave properties in the ray model and developed a mathematical formalism that allows describing wave front curvatures and phase shifts due to focal lines. This approach, called “Vectorial Complex Ray Model” (VCRM), permits to compute precisely the scattering of a wave by large CSPs of smooth surfaces. VCRM has been applied to 2D scattering of ellipsoidal particles and elliptical cylinders. As a further contribution, the promoters of this project have proposed methods to include forward diffraction by Heisenberg’s uncertainty principle and near-critical-angle scattering effects in the model. The central goal of the project is to offer a generalized version of these works, to be cast into a general “Ray Theory of Wave” (RTW). Objectives of the project are: (1) Extension of VCRM to 3D CSPs and for various shaped beams; (2) Modeling of wave effects; (3) Prediction of radiation pressure forces and torques for CFPs; (4) Theoretical and experimental validation of RTW; (5) Application to optical characterization instruments and experimental tests on sprays and bubbly flows; (6) Manipulation and trapping of non spherical particles. The deliverables of AMO-COPS project will be computation software for prediction of optical forces, scattering properties of CFPs, and simulation of experimental characterization tools. We also anticipate providing various original theoretical and experimental results. PhD students will be trained to research throughout the project. Special attention will be paid to valorization through publications, software licenses and patents. Looking forward to the future, we expect potential applications of RTW well beyond the particular systems to be investigated in this 4-year program.