Missions

Natural or laboratory turbulent flows have been observed to be prone to random bifurcations, in which the flow regime switches spontaneously from one type of behavior to another over a random time period. A paradigmatic example is the Von Karman flows, where large scale bifurcation has already been observed. However, a precise understanding of the origin of those bifurcations is still lacking, preventing the forecast of such events. One possibility is that the triggering occurs via small-scale extreme events that propagate to large scale.

A detailed check of this hypothesis thus requires multi scale tools and dedicated experimental setups as well as the development of an experimental database. In this context, one of the aims of the ALEAS project is to use two already existing experimental setups at SPEC and to instrument them accordingly: a small 10 cm radius Von Karam will be monitored with an ensemble of 1D pressure sensors and event cameras in a Particle Image Velocimetry (PIV) setting, to record the changes of large-scale velocity flow field. Event-based vision is quite new within the field of computer vision. The neuromorphic design allows for a much higher acquisition frequency but most and foremost much longer acquisition time spans. This makes it ideal to record bifurcations which occurring time is unknown. Finally, in the Giant Von Karman (GVK) at SPEC, high resolution 3D-PIV velocity measurements have and will be conducted in the interaction area between the boundary layer and the turbines, but also at the center. This should shed some new light on the dissipative structures and extreme events possibly responsible for the large scale evolution of the flow.

This project is included in a more ambitious project dedicated to the development of a strategy for identifying the precursor signals of rare events and/or bifurcations in turbulent flows. The first step is therefore to build up a database of these kind of events (both numerically and experimentally).

Activités

To experimentally investigate turbulent flows, the SPHYNX team at SPEC build over several decades and has extensive experience on Von Karman swirling flows with fluids ranging from water, to Superfluid. Measurement techniques have thus been developed over the years and research is being out using state of the art 3D optical diagnostic tools.

The activities would include experimental fluid dynamics, 3D PTV setting and laser handling, data post-processing, to improve the measurement and understanding of small scales.

Compétences

The candidate must hold a doctorate degree in fluid mechanics or related topics.

Skills required:
Experimental Fluid Dynamics, Physics and modeling of Turbulence, 3D-PTV experimental optimization for high density and high optical magnification, 3D-PTV software post treatment, laser and high speed cameras handling, Signal processing, Object-Oriented Programming

Language skills: Writing skills in English, ability to formulate/work on a scientific project

Ability to work in a large team and on a large experimental setup.

Applications must include a detailed CV

Contexte de travail

Joint Research Unit (UMR 3680) of the Centre National de la Recherche Scientifique (CNRS), and Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), SPEC carries out a wide range of research activities on condensed matter.

The recruited person will be assigned to the SPHYNX group. Its scientific topics cover a variety of fields: active matter, turbulence and applications to climate, glass and slow dynamics, magnetic and fracture properties of disordered solids, complex fluids, colloids, thermoelectricity & heat transfer, active matter and biological objects,… The common challenge, in all these systems, is to understand how a collective dynamics emerges at the macroscopic scale and can be described through a small number of parameters, without keeping track of all the microscopic degrees of freedom.

At Sphynx a dedicated team specializes in turbulent flows which dynamics remains an outstanding challenge of out-of equilibrium physics with important implications for industrial and natural settings. The research carried out at SPHYNX aims to characterize the energy pathways of such turbulent flows from the large injection scale to the tiny erratic dissipative structures. Focusing on flows driven by thermal convection, we have obtained an unambiguous observation of the 'ultimate regime' of thermal convection, at play in geophysical and astrophysical flows. We have developed a theory for the associated turbulent transport based on the study of idealized models of increasing complexity. Such large-scale properties of turbulent flows are intimately connected to the way the flow dissipates energy at small scale: turbulence induces intense dissipative structures at very small scale that remain out of reach to most experimental studies. We have thus designed a 'Giant Von Karman' laboratory experiment, which provides a unique opportunity for the detailed spatio-temporal characterization of the small-scale intermittent dissipative structures of the flow in connection with possible singularities of the Euler and Navier-Stokes equations. To access dissipative structures, we are developing two types of optical metrology methods : An innovative Diffusive-Wave Spectroscopy method that allows for the direct spatio-temporal measurement of the turbulent dissipation rate at a solid wall. New post-processing algorithms are also developed for Particle Image Velocimetry to measure extremely dense 3D time-resolved Lagrangian, Eulerian and pressure fields.

This activity is inherently multidisciplinary with strong collaborations with other scientific fields, as applied mathematics or statistical physics. Fluid mechanics is ubiquitous in geophysical and industrial applications. Better understanding of flows will help to address major challenges to deal with new energy and environmental constraints. Collaborations with experts in climate modeling, geosciences and in renewable energy development have been set-up to respond to these societal issues.

Contraintes et risques

Working in a Laser environment