Congrès, Colloques ...

Workshop VAVIDEN

Du 27 au 28 juin 2019

Salle de conférences du CORIA

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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).

The fundamental viewpoint

The theory proposed by Kolmogorov (1941), or K41, developed for homogeneous fluids (uniform viscosity and density), postulated that for sufficiently large Reynolds numbers, the small-scale motion is statistically independent of the large scales. Similar statements hold for the scalar field, describing mixing. Realistically, in order to predict turbulent mixing we need to improve our understanding of the physics underlying this phenomenon and to formally account for the effect of the large-scales (often anisotropic) on small scales, without ignoring the finite-Reynolds-number effects.
On the other hand, most of traditional turbulence studies concern fluids with constant properties, or, at most, variable-density fluids. The effects of viscosity are thought to be present only at the level of small-scale motion. However, if viscosity was entirely absent, no vorticity would be created at non-slip surfaces or at the external interface of free shear flows.
There also would be no viscous dissipation of turbulent energy. Therefore, since viscosity represents the most important property of real flows, its variation also requires special attention. Real properties of the fluid are also reflected by variable density.

The practical viewpoint

Round jets (for example) are among the most important mixing devices in many engineering applications. Practical examples of round jets often involve different densities and viscosities of the jet fluid and the surrounding fluid. These properties strongly influence the penetration of the jet and thereby the mixing of the two fluids. While the effect of different densities has been extensively studied in the past, this proposal will first focus on jets with different viscosities. Variable density effects will be considered in a second time. We illustrate the mixing for three ratios (1, 20 and 40) of the viscosity of the jet *fluid to that of the surroundings in Fig. 1 (from Chhabra et al. (2005)) in a jet of water where the surrounding fluid had an artificially enhanced viscosity. While turbulent mixing is evident for equal viscosities in the left image, it is almost not realisable in the middle and right images where the surrounding fluid is 20 and 40 times more viscous, respectively.

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.


Interest of the topic and aims of the Workshop

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.

Scientific aims of the workshop

In this context, the workshop is aimed at bringing together researchers involved in different aspects of this research : fundamental aspects of turbulent flows, variable density and viscosity mixing, applications in flows with melting boundaries, combustion etc.
The aim is to bring together people from different scientific communities, that have a common point, the turbulence of fluid with variable thermo-physical properties. This would allow for clearly identifying different manners to treat turbulent flows. The final aim of this workshop is to point out open questions, eventually answers of them, contacts and collaborations among people.

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MàJ ··· 19 novembre 2019

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