The goal of the PALAGRAM project is to investigate the connections between the local processes and the global dynamics of Particle-Laden Gravity Currents (PLGCs) encountered in numerous geophysical flows, from the interaction of particles near the bed or in the suspension to the scale of topographic features like mountains, lakes or continental shelves.
The main ambition is to provide models for the equivalent flow dynamics based on physical processes at the particle scale, including the slope influence. A major difference between compositional currents and PLGCs is the existence of typical length and time scales associated with the particles dynamics leading to complex interactions between turbulence and particles denoted as turbulence-particle interactions. Among the relevant mechanisms, turbulence damping and preferential concentration are expected to play key roles. Accordingly, the ambition is to provide models for the equivalent flow dynamics based on physical processes at the particle scale and accounting for these turbulence-particle interaction mechanisms. To reach these objectives by covering a wide range of flow regimes, one of the main originality/challenge of the project is to focus on the influences of the slope a and of the concentration of particles ? over a large range of values, as a?[0°,30°] and ??[0.1%,20%] respectively, using 4 experimental devices in 4 laboratories. <br />The scientific objectives will be addressed using a multi-scale experimental, numerical and theoretical approach, focussing on the following scientific questions: <br />SQ1: How to measure accurately velocity and particle concentration profiles in PLGCs over the full range of concentration ??[0.1%,20%]? <br />SQ2: What is the role of turbulence-particle interactions on erosion-deposition processes, dispersion of particles and mixing with the ambient fluid? <br />SQ3: What is the influence of the PLGC rheology (non-Newtonian) on the PLGC dynamics?
One of the key objectives of the first phase of the project is the development of adapted experimental methodologies devoted to the extraction of relevant data in complex fluid-particle flows as PLGCs. Associated difficulties are (i) the heterogeneity of the flow (time and spatially dependent for both the dynamics and particle concentration), and (ii) the opacity of the flow due to the presence of the particles. Three different techniques have been identified and developed in three laboratories of the consortium (see figure):
1. the light attenuation technique (LAT) at LEMTA will give access to concentration maps of the particles in the PLGCs in nearly two-dimensional setups,
2. ultrasonic velocity and concentration profiles (UVCP) are obtained at IMFT using commercial acoustic profilers with a specific calibration and inversion process, giving access to instantaneous turbulent profiles in all configurations,
3. refractive-index-matching (RIM) particles created at LEGI/INRAE will give access to the turbulence in the fluid phase using optical measurements (particle image velocimetry).
The experimental observations that will results from these metrologies are at the base of an adequate modeling of equivalent flow, which will also be tested using a two-phase flow numerical model sedFOAM, developed at LEGI.
The phase of development of metrological tools in the different labs of the consortium nears completion. First encouraging results are shown in the attached figure.
Gravity currents involving suspended particles are common features encountered in natural and industrial flows. The PALAGRAM project aims at characterizing the dynamics of particle laden gravity currents (PLGCs) from the local processes (erosion/deposition, turbulent dispersion) towards the macroscopic scale (rheology, turbulence modeling) through laboratory experiments and numerical modeling.
Our project focuses on the influence of the concentration of particles over a large range of values, with a volume fraction (Phi) varying from 0.1% to 20%. We plan to provide a unified vision of PLGCs from dilute (Phi<1%) to dense (Phi>10%) systems corresponding to situations such as snow avalanches and submarine landslides respectively. The evolution of Phi over such a large range, affects local interaction processes, i.e. particle-particle and turbulence-particle interactions, which in turns modify dispersion processes in the current and mixing of the PLGC with the ambient fluid, and therefore the global dynamics of PLGCs. More specifically, the project will address two main scientific questions: what is the role of turbulence-particle interactions on erosion-deposition processes, dispersion of particles and mixing with the ambient fluid? What is the influence of the PLGC rheology (non-Newtonian) on the PLGC dynamics?
A major technical barrier to achieve these goals is the great difficulty to measure velocities and particle concentration in dense suspensions given the very limited optical access. In order to overcome this bottleneck, we propose to develop advanced acoustic based and optical based (refractive-index matching with PIV/PTV, light-attenuation technique) measuring techniques during the first year.
The rest of the project is constructed around a benchmark dataset of experimental observations collected from three complementary setups based at LEGI/IRSTEA, LEMTA and IMFT. Experimental setups are different since they focus on various flow regimes for PLGCs (Tasks 1,2,3). The scenarios considered in the project cover local processes in steady-state and unsteady PLGCs, and global observations in unsteady PLGCs in different dynamical configurations obtained from lock-exchange experiments, in two-dimensional and three-dimensional setups. This requires experimental expertise and metrologies which are available and complementary in the teams. The experimental results from all teams will contribute to the modelling of the rheology of PLGCs associated with the suspension of particles, of the contribution of particles to turbulent dispersion, and of boundary effects (friction, mass exchange, momentum flux). The generic modelling aspects obtained will be implemented in a numerical tool developed at LEGI, in order to run numerical simulations that can be tested against the experimental results (Task 4).
The consortium of the PALGRAM project allows to gather various skills on fluid-particle flows and their modeling, dedicated to a similar situation in order to improve our knowledge and the impact of these complex flows.
Monsieur Laurent Lacaze (INSTITUT DE MECANIQUE DES FLUIDES DE TOULOUSE)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ETNA EROSION TORRENTIELLE, NEIGE ET AVALANCHES
LEGI Laboratoire des Ecoulements Géophysiques et Industriels
LEMTA Laboratoire d'énergétique et de mécanique théorique et appliquée
IMFT INSTITUT DE MECANIQUE DES FLUIDES DE TOULOUSE
Help of the ANR 647,728 euros
Beginning and duration of the scientific project: May 2020 - 48 Months