Dynamics of free surface flows, sensitivity, inverse problems

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Presentation

This research theme focuses on the physical analysis and modeling of surface runoff in order to meet the expectations in the field of environment and sustainable development. The theoretical and numerical modeling of the physics of free surface flows covers both natural and experimental cases, and several measurement techniques, some of them being developed in the laboratory. With models and experimental/satellite data of first hand, our research aims to further enrich these models with the integration of sensitivity analysis and inverse problems. On an applicative perspective, this theme is related to the study of flooding, sediment transport in rivers, and conversion of hydropower.


Flooding Pilote

Flooding pilot

The flood pilot is an experimental device 5x5 m² built in 2012 [8-Arau12] and representing an urban suburb on a scale of 1/200. This platform, which is unique in the world in terms of size and complexity, can be used to simulate flooding in urban areas in order to study the water flows in the streets or crossroads (steady state) or a wave displacement between buildings. It is equipped with devices for automated acquisition of water height and velocity field distributions at any point of flow. The experimental data allows to understand the physical phenomena associated with the flood propagation in the city, the water-flow relationship in a pipe [2-IDBF14, 2-IDFV14], the processes of trapping suspended matter [2 -SDVF14, 3-SDVF13], and the phenomena of deposition / suspension [8-Schm13]. Our numerical models (Navier-Stokes and Saint-Venant equations) benefit from the presence of the pilot: the comparison between experimental results and numerical simulation allows to validate the mathematical models [1-AFLF14, 2-SDVF13, 2-AFABxx] and to propose adapted numerical schemes [2-AFGM12].


Flow at a Junction

Comprehension of sediment transport mechanics

In order to provide a better understanding of the mechanisms of sediment transport, weakly intrusive techniques for measuring velocities and granulometric characterization of solid transport in fluids are developed. The acoustic response of different types of particles (particle size and/or composition) is studied in the laboratory , rivers and sewer systems to improve observing devices and analytical techniques [2-PFFP15]. Our work has recently focused on the ultrasonic metrology of two-phase flows (movement of air bubbles injected under the hull of ships to reduce the drag phenomena) in partnership with the Laboratory of Flow Control at Hokkaido University and on a precise method for measuring the flow rate of hydrocarbons in pipes under load using pulsed ultrasonic waves [8-Bigo13]. The metrology techniques developped for these different purposes benefit from the experiments carried out on the flood pilot, and vice versa.

Dynamics and prediction of flows in river systems

The study of the dynamics and the prediction of flows in river systems is another key element of our research as it aims to improve the understanding and the modeling of the water cycle at different temporal and spatial scales. One of the major scientific locks is the identifiability of thehydraulic parameters in various observational contexts. This theme requires powerful models, satellite and in situ data sets, and data assimilation techniques. Models of staggered complexities are studied in the team for more flexibility, accuracy or speediness of the modeling of a given problem, whether in simulation or in data inversion problems. The recent arrival of two researchers confirms this last point. Our models usually involve a large number of parameters, initial conditions and boundary conditions, and often produce results of large spatial and/or temporal dimensions. Since these models are only a representation of the real and the physical parameters are often uncertainly known, modeling alone does not generally quantify precisely the impact of a perturbation of a given parameter on the studied system. The use of sensitivity analysis [2-CD15,2-Charxx, 7-GBMCM16], numerical optimization and data assimilation makes it possible to better understand the models, to compare them with the data [2 -CCCMVxx] and to bridge the gap between modeling and real phenomena. At the scale of a watershed, the resolution of inverse problems in hydrology involves the use of remotely sensed observations with the satellites currently in service. Our contributions in this field also concern the scientific support of the future SWOT (Space Waters Ocean Topography CNES / NASA) satellite mission dedicated to a high-resolution global observation of fluid enveloppes on continental surfaces, oceans and coastlines [ 2-PPSMxx, 2-GM15].


Amazon River Bassin

Conversion of hydraulic energy into electrical energy by micro power stations

Currently underutilized, the conversion of hydroelectricity into electrical energy through micro-power plants has the potential to generate significant hydroelectricity. New in the team, this research topic is approached experimentally and numerically to refine the energy performance of an Archimedes screw and proposes a prototype of an oscillating cylinder. The study of the Archimedes screw used as turbine has led to the emergence of several new models: a theoretical model linking the performance of the screw to its geometry and flow parameters [2-DTGGxx, 8-Dell15] , an equivalent model of the leakage rates, and a model of the downstream control induced by a screw on a stream. The oscillating cylinder prototype, energy recovery based on vortex-induced vibrations, is under development [5-DFML15].


Archimedes Screw Modelisation