Congrès, Colloques ...
Du 27 au 28 juin
Salle de conférences du CORIA
Chairman : Prof. Luminita DANAILA,
Co-chairs : Michael GAUDING & Emilien VAREA.
Organiser : Christophe Letailleur
Secretary : Florence FRADET
Despite its large number of applications (climate, biological flows, chemical industry etc.), turbulence is still today, one of the less understood phenomena in physics. Though Navier—Stokes equations, describing the fluids motion, are relatively simple, their non-linear character does not allow for a general solution. The origin of this impossibility, associated with a high number of degrees of freedom and a very high sensibility to initial conditions, has been only recently understood via the dynamical systems theory. In the absence of a unifying theory, progress in understanding turbulent flows is primarily based on experimental measurements, numerical simulations and on our ability to interpret results using specific tools (wavelets, spectra etc.). Understanding turbulence is a major point for industrial and environmental applications : reducing noise and pollution, energy economy, optimisation of chemical reactors, better combustion efficiency etc. Because of their complexity, a reliable description and modelling of turbulent flows is based on a good understanding of turbulence, from a fundamental viewpoint.
Turbulent flows are known as containing a wide range of scales, each range of scales being characterized by different phenomena. For instance, the dissipation process is known as being a small-scale phenomenon. Therefore, in different industrial process, a role particularly important is played by the small scales. They are to be properly taken into account in the sub-grid scales (SGS) models. Another example deals with modelling micromixing (chemical industry, combustion), in which the small scales are the most important players.
In this context, one fundamental question is how to reconcile two different worlds : that of the fundamental viewpoint in turbulence (theories ‘à la Kolmogorov’ mainly developed for a single fluid, with constant viscosity) and that of turbulent flows of fluids with variable properties (density, viscosity).
Different studies were dedicated to variable-viscosity flows : PhD thesis of N. Taguelmimt, 2014 (Numerical study of the temporally evolving mixing layer) and the PhD of L. Voivenel, 2015, post-doc of M. Gauding in the context of the project VAVIDEN. The main results obtained in these projects will be presented and discussed. From analysis of conditional statistics in free shear flows with respect to the distance from the T/NT interface, we found a significant contribution of variable viscosity effects near the interface.
Another aspect to be discussed concerns the practical application of the fundamental description of small-scale turbulence. We particularly aim at the non-reacting mixing, as a first—level stage of the reacting mixing. This phase of “mixing preparation” is particularly important for reactive flows (chemical industry, combustion etc). As an example, as far as combustion is concerned, a fine and precise characterization of the air/gaseous fuel mixing, in connexion with a better turbulent combustion, is recently of a particular importance (expensive fuels, pollution reduction, energy economy etc.). In this direction, it is important to better understand the micromixing properties, i.e. of the small-scale mixing, where the chemical reactions that constitute combustion take place. Number of questions concerning reactive flows remain without clear answer. As an example, the alignment of the mixing fraction gradient with the velocity gradient eigenvectors controls the evolution of the scalar dissipation and thus plays an important role in the non-premixed turbulent flames. We note finally that a correct and quantitative diagnostic of small scales in a turbulent mixing are to be useful for the LES technique, in the context of sub-grid scales models.
In many different scientific domains, significant progress has been possible because of the techniques imported from other domains. This progress was possible only because researchers used knowledge a priori specific to different activity domains. The communities traditionally involved in turbulence study are numerous (classical fluid mechanics, physical mechanics, magneto-hydrodynamics, chemical industry, etc.…). However, if some of them have already developed regular contacts and discussions, some others are still isolated. In all the cases, turbulence is the common point. This is an important reason for which it seems necessary to bring together researchers originating from different communities, in a workshop which is aimed at establishing the most relevant results in variable density and viscosity turbulence, and to generate contacts and collaborations.