The TWIN project proposes a study of the coupling of inertial particles with the aerodynamics of Ahmed bodies. Using a strategy that combines the complementarity of four institutions such as the PRISME, LMFL, LEGI and CSTB laboratories, the effect of inertial particles on wakes will be studied experimentally, including the study of near-wall effects. Our aim is therefore to gain knowledge of particle accumulation effects for turbulent near and far wakes, and a better understanding of the effect of turbulence on cluster formation under real environmental conditions. To this end, joint measurements will be carried out in the wind tunnels of the three laboratories to fully characterize the single-phase and two-phase flows involved. Finally, the complementary nature of the three partners' facilities will enable real-life tests to be carried out in a climatic wind tunnel at CSTB. The TWIN project is structured in three parts: the first is dedicated to the study of the interactions of inertial droplet-like particles on the solid surface of Ahmed's body, the second concerns the influence of roughness models representative of droplet impacts on near and far wakes, and the third part of the project will enable all the mechanisms studied to be apprehended under realistic conditions.

Dispersed two-phase flows occur in many industrial applications and natural phenomena. In the past decade, significant progress has been achieved in this field, due both to a revolution in experimental techniques (PIV, PLIF, PTV…) and to the extraordinary development of numerical simulations. Interestingly, this domain has been tackled by different scientific communities, in Fluid Mechanics, Physics, Meteorology, or Geophysics, with different focuses and different methods. The case of inertial particles is an emblematic example, with the development of efficient Eulerian or Lagrangian models in Engineering, associated with deep insights in the dynamics of particle-turbulence interaction by physicists studying turbulence, which helped to improve current models. The participants to this project were involved in this global effort and mixing of scientific cultures in a previous ANR program DSPET, devoted to inertial particles. The case of dispersed flows involving mass transfer, due to evaporation, condensation, or dissolution or coalescence is even more intricate, due to the dependence in time of the size of the particulates (drops, bubbles, and particles), which precludes some convenient approximations such as point-particles and one-way coupling. Among the many fundamental difficulties of the problem, the present project objective is to focus on the interaction between turbulence and phase change in particulate flows. Although the specific features of flows with either drops, bubbles or particles make it unlikely that all situations can be described by a single general framework, we believe that common physical principles can be identified in all cases, and that much can be understood with the newly developed experimental and numerical tools. Since the problem is clearly multiscale, the project involves three steps corresponding (i) to a local study at the scale of the particles (Bubble, drop or solid particle), (ii) to the intermediate level (mesoscale) of a swarm of interacting inclusions, basically through turbulent induced collisions, (iii) to the macroscopic level of mean field equations used in applications. Although several aspects of the problem have been investigated, other important aspects, such ascrossing trajectories effects and preferential concentration, or finite size effects on mass transfer and forces, have received little attention until recently. The goal of this proposal is to take advantage of recent advances in the investigation of this long-standing problem, by using the latest available techniques, and a strong coupling between experiments, simulations and modeling. The challenge is to determine both the evaporation/condensation rate along the trajectories, and the local characteristics of the continuous phase in the vicinity of the particle. Experimental techniques developed in the previous ANR program DSPET by the same group (high-speed PTV, high-speed holography, PIV+PLIF) appear to be well adapted for this purpose. Numerical methods accounting for severe evaporation conditions will also been developed. In a second methodological step, we shall consider situations involving many condensing/evaporating bubbles or drops. In such situations, phase changes can be greatly modified by two classes of mechanisms: collisions and coalescence processes, and collective effects. The first one involves essentially binary collisions between particulates, which are affected in a still incompletely understood way by turbulence. The four teams involved in the project, at the LMFA, the Physics Laboratory at ENS Lyon, the LEGI and the OCA, gather some unique competences in advanced measuring techniques and theoretical and numerical modeling. They have a strong experience of common projects, and use to share experimental equipments and numerical codes.

Liquid jets impacting a pool entrain air beneath the free surface when their velocity is large enough. Large-scale plunging jets encountered in hydroelectric infrastructures play an important role on the performance and safety of dams. Furthermore, they play a relevant role on the ecosystem balance in rivers and basins, through generation of huge bubble clouds that can change the chemical balance on it. Despite the industrial and environmental relevance of these flows, our modelling capacities are so weak that we cannot answer even very fundamental questions. For instance, predicting the flowrate and bubble sizes of air entrained in a pool by a plunging jet is still an unresolved problem. A thorough understanding of the evolution during free fall, impact against a liquid surface and consequences of plunging jets (i.e. bubbly flow generated within the reservoir) is then key to increase our capabilities of modelling. It would also enable efficient, safe and environmentally friendly energy production. We propose a joint numerical-experimental investigation on the air entrainment and impact dynamics of large-scale jets under the influence of gravity. A combination of small and large-scale experiments and numerical simulations, will allow us to explore a vast range of the parameter space, that includes the jet fall height, its initial velocity, the nozzle size and geometry, among others. It involves two institutions with experience in the experimental study and modelling of these flows (LEGI and LMFA). LEGI will also have access to a large-scale facility, unique in the world for fundamental studies of impact jets: the ‘JetHigh’ experimental platform. It allows to generate plunging jets with injected velocities from 2 to 35 m/s with an injection diameter between 26 mm to 213 mm, along a 10-meter free fall height impacting a 23 m meter depth plunge pool. We will therefore be able to test and adapt our models and numerical simulations to realistic conditions. The team is completed by CORIA, that will use state-of-the-art numerical tools. For instance, they have developed the ARCHER code, that allows to quantify both the properties of the water-air interface and the evolution of the jet even at large scales. EDF will be part of both the experimental and numerical analysis, as it will contribute with its expertise on large-scale experiments and their expertise and capabilities for numerical simulations. As stated, the key measured parameters will be the jet aeration (entrained flow rate and void fraction), surface corrugation sizes, jet dynamics, as well as the bubble cloud size and recirculation dynamics. Other important parameters are the size distribution of the bubbles generated and the spatial distribution of local quantities of interest (such as concentrations, phase velocities, flux, etc.). In particular, their local evolution with depth is crucial. These parameters will be first measured on a common round jet configuration (benchmark case) by all partners, then at the large scale on JetHigh (LEGI) and at the lab scale in other geometries, e.g. 2D sheet, array of jets (LMFA). A practical objective of our project is also to develop new numerical tools and sensors at lab scales which will subsequently be tested in JetHigh and subsequently in hydroelectric facilities. The strength of our project resides in the wide range of scales we can cover with our setups and simulations; the fact that experimental partners are recognised specialists of fragmentation, two-phase flows , turbulence and more generally flow stability ; complementarity with CORIA which is a renowned specialist on two-phase flow simulations ; and finally the partnership with EDF, which will help us remain relevant to the conditions of applications, and which will bring a complementary expertise in hydraulic engineering.

Oceanic convection remains poorly understood even though it is one of the main driver of the oceanic dynamics. Convection can be penetrative (entrain water from below the mixed layer) or non-penetrative. While it is reasonably straightforward to formulate conceptual parameterizations of non penetrative convection in idealized settings, it remains challenging to extend the formalism to realistic settings of penetrative convection even for state of the art ocean models. In fact the most advanced parameterization schemes for oceanic convection are still calibrated based on atmospheric data. Moreover these parameterizations do not take into account the rotation of the earth which can substantially impact the individual and collective behavior of convective plumes. The first objective of this proposal is to build an observational database of convective events on the Coriolis Platform (the largest rotating tank of the world). We will complement this dataset with numerical simulations to explore many types of surface forcing and initial conditions. We will then combine these observations and model outputs with a robust theoretical framework to build a consistent parameterization of oceanic convection. Then, we will overcome the constraints of the mathematical framework of existing parameterizations and propose a data-driven approach to formulate a more generic parameterization. Last, we will test these parameterizations in coarse resolution ocean models. We will perform a sensitivity analysis of the oceanic heat uptake as a function of the free parameters to asses how and where our parameterizations can reduce uncertainty of climate projections.

Efficient energy storage of liquid energy vectors over long periods of time is at the heart of todays’ energy saving and of the coming energy transition (ground cryogenic H2 storage, cryogenic LNG tankers). It is also one of the main challenges for deep space missions (H2 tanks). Nevertheless, the behaviour of mass and energy transfer on large interfaces in cryogenic fluids is largely unmastered, owing to the complex physics (heat transfer, phase change, wetting, multiscale, turbulence, …) and to the inherent difficulties of performing experiments without altering the cryogenic system. This brings to energy losses on ground applications and obliging to expensive (in energy and capital) overestimations of fuel needs in space launches. The project aims at bringing numerical simulations closer to predict the evolution of macroscopic systems through upscaling, bridging the existing gap between predictive simulations at the bubble scale and macroscopic approaches relying on case-specific adjustable parameters. The project brings together specialists of numerical simulations of complex flows with complementary expertise, from highly resolved simulations of boiling at the bubble-scale, to large eddy simulations (LES) of turbulent multiphase flows in complex geometries. The proposed strategy is based on the development of scalable analytical models to ensure local conservation of mass, energy and momentum, and numerical stability. The consortium plans to develop models both at the bubble length scale and for wider interfaces, in order to proceed to an upscaling based on first principles. Validation test cases are proposed, also based on recent microgravity experiments.