Actualités

OpenFOAM development at Marche Polytechnic University

(by Valerio D’Alessandro v.dalessandro@univpm.it)

In the last years Thermofluids research group of the Marche Polytechnic University started an active development of CFD solvers and models adopting OpenFOAM.
The main areas of interest are the following:

  • development of low-dissipative solvers for incompressible flows based on high-order Runge-Kutta schemes;
  • transition models for RANS equations;
  • new hybrid RANS/LES models;

The previous work has been performed in collaboration with students, Ph.D. candidates, post-doc researches and professors of the Industrial Engineering and Mathematical Sciences Department of Marche Polytechnic University. Two Ph.D. thesis, involving significant OF work, were successfully defended.
Our work is constantly carried out two different small Linux clusters and on supercomputing facilities, likes those available at CINECA.

Development of low-dissipative solvers for incompressible flows

This research activity was started in order to obtain low-dissipative solvers for DNS/LES of incompressible flows involving heat transfer. The OpenFOAM community is focusing on reducing the numerical dissipation for both incompressible and compressible flows. Various efforts are in progress, especially relating to low-dissipation Runge-Kutta (RK) methods for compressible flows, so we focused here on incompressible flows, with an emphasis on the time integration approach.

More specifically we have implemented different solvers adopting both implicit and explicit approaches for time integration. In the first case singly diagonally implicit Runge-Kutta (SDIRK) schemes were employed. It is worth noting that an iterated PISO-like procedure for handling the pressure-velocity coupling in each RK stage was used. For the explicit approach a projected-type RK methods was considered.

We found that the RK-based solution techniques, implemented using only OpenFOAM standard classes, completely canceled the numerical diffusion induced by the time integration approach, but demanded more computational resources than standard OpenFOAM solvers, [1]. The solvers proved reliable, robust and accurate  on different computational setups, i.e. grids and spatial schemes, as well as in a large-eddy simulation. Implicit technique has proved computationally more efficient than explicit one. For this reason we have applied our implicit SDIRK solver within an ongoing research activity at Marche Polytechnic University. More in depth this research is aimed in the evaluation of dimples effect on low-Re number operating laminar airfoils. The NACA 64-014A airfoil has been considered. A row of dimples was placed at 55 % of chord length with one dimple diameter spacing in the span-wise direction. Our LES computations, validated with experimental tests performed at Environmental Wind Tunnel of our University, show that dimpled airfoil configuration experience a significant laminar separation bubble reduction, [2]. This result is particularly appealing in several applications, since dimples realization is objectively cheaper than other boundary layer control devices.


(a) Mesh representation                                       (b) Vortical structure representation
Figure 1: NACA 64014A airfoil LES, [2]

Transition models for RANS equations

In this research we have devised a transition approach for RANS based on the coupling between the γ-ReΘ,t approach and the Spalart-Allmaras (SA) turbulence model. The motivation behind this activity is to obtain a suitable transition model requiring a limited computational cost. Indeed, DNS approach is still inapplicable to the study of engineering interest problems due to the significant computational resources required. On the other hand, the applicability of the LES technique is very limited for the same reason. Moreover standard RANS models are not always reliable to transitional flow predictions because they assume a fully turbulent regime.

It should be noted that γ-ReΘ,t models were initially coupled with the SST k-ω turbulence model by its developers, but the γ-ReΘ,t model could be applied to other models too. In this work, we have presented a γ-ReΘ,t RANS model for transitional flows based on the SA equation. The main reasons behind this approach are as follows:

  • the SA model offers very reliable results for external flow applications; and
  • the SA model has lower computational costs than the SST k-ω equations.

For the analyzed cases, we thoroughly investigated the role of the different empirical correlations involved in the transition model. This is a crucial issue because a marked sensitivity of the γ-ReΘ,t model to its empirical correlations in the k-ω context was apparent from the literature. Analyzing all the results collected in [3], we surmise that the Malan et al. and Langtry and Menter approaches produce better results in a wide range of cases.

It is worth noting that we obtained a good feedback from our model’s implementation in predicting airfoil performance (see Fig. 2), as well as in the computation of the natural transition on a zero pressure gradient flat plate.


(a) Velocity magnitude contour plot                        (b) Pressure distribution along the airfoil wall
Figure 2: E387 at Re = 2 105 results, [3].

Lastly, the model briefly described here and a test case are freely downloadable on gitHub at the follwing address: https://github.com/vdalessa/gammaReThetatSA.

New hybrid RANS/LES models

The Detached-Eddy Simulation (DES) technique introduced by Spalart et al. [4], is a hybrid RANS/LES approach that operates like RANS in the near-wall regions and like LES in separated flow zones. DES is probably the most popular hybrid RANS/LES method becauseit is simple to apply to a wide range of existing RANS models. It is also particularly attractive because it produces good results in several conditions with fewer computational resources than standard LES. The original DES formulation is based on the Spalart-Allmaras (SA), [5], eddy-viscosity RANS model, for which LES behavior is achieved by modifying the length scale used in the turbulence model.

DES models have also since been introduced that are based on two-equation RANS. In particular, Travin et al. [6] developed a DES model based on the SST k-ω model, and other modifications were proposed in various papers too (for this specific model), that involved replacing the length scale in the transport equations for turbulence.

It is only very recently that DES approaches based on the v2-f RANS model have appeared in the literature, [7]. A first advantage of this model by comparison with other RANS methods lies in its ability to accurately predict the near-wall effects without specific treatments or expedients. DES approaches based on k-equation are very appealing because they use a length scale based on flow properties, not on grid size like the SA model. In LES mode, the v2-f DES models also have a transport equation for the Sub-Grid Scales (SGS) kinetic energy that is less empirical than the SGS modified turbulent viscosity, ν, used in the standard SA-DES. The main drawback of these models lies in that they use four additional equations for turbulence modeling, while other DES techniques are less costly from this point of view.

In this research we have exploited the open-source features of OpenFOAM to implement the v2-f DES model. We considered Dirichlet-type wall boundary condition for ε equation. From a theoretical point of view, this choice does not affect the quality of the results, while it provides a suitable equivalent boundary condition that is easier to be coded within OpenFOAM if compared with the Neumann condition proposed by Jee and Shariff.

The model was successfully validated computing the flow field past a cylinder at Re = 3900, Fig. 3. Ongoing work is devoted to the DDES development and to the application of this model to problems involving heat transfer.

 

Figure 3: Vortical structure, identified by the Q-criterion, isosurface colored by pressure.
Q = 0.5u2/D2 at T = 500D/u , [8].

Acknowledgements

We acknowledge the CINECA Award N. HP10CV8M72 YEAR 2015 under the ISCRA initiative, for providing high-performance computing resources and support.
We acknowledge the CINECA Award N. HP10CDBABZ YEAR 2016 under the ISCRA initiative, for providing high-performance computing resources and support.

References

[1] V. D’Alessandro, L. Binci, S. Montelpare, and R. Ricci. On the development of OpenFOAM solvers based on explicit and implicit highorder RungeKutta schemes for incompressible ows with heat transfer. Computer Physics Communications , 222:14 30, 2018.
[2] L. Binci, G. Clementi, V. D’Alessandro, S. Montelpare, and R. Ricci. Study of the ow eld past dimpled aerodynamic surfaces: numerical simulation and experimental verication. Journal of Physics: Conference Series , 923, 2017.
[3] Valerio D’Alessandro, Sergio Montelpare, Renato Ricci, and Andrea Zoppi. Numerical modeling of the ow over wind turbine airfoils by means of Spalart–Allmaras local correlation based transition model. Energy , 130:402  419, 2017.
[4] P.R. Spalart, W.H. Jou, M. Strelets, and S.R. Allmaras. Comments on the feasibility of LES for wings, and on a hybrid RANS/LES approach. In C. Liu and Z. Liu, editors, 1st AFOSR Int. Conf. on DNS/LES , Ruston, LA, August 48 1997.
[5] P.R. Spalart and S.R. Allmaras. A one-equation turbulent model for aerodynamic flows. La Recherche Aérospatiale , 1:521, 1994.
[6] A. Travin, M. Shur, M. Strelets, and P.R Spalart. Physical and numerical upgrades in the detached eddy simulation of complex turbulent ows. In Proceedings of EU-ROMECH Colloquium. , pages 239-254, 2002.
[7] K. Jee and K. Shari. Detachededdy simulation based on the v 2  f model. International Journal of Heat and Fluid Flow , 46:84101, 2014.
[8] V. D’Alessandro, S. Montelpare, and R. Ricci. Detachededdy simulations of the flow over a cylinder at Re = 3900 using OpenFOAM. Computers & Fluids , 136:152  169, 2016.

PhD position : numerical modeling of an aeronautical injector !

Context and objectives

In order to improve the efficiency of their turbine, the Safran group develop their own fuel injector dedicated to their combustion chamber. This allows a decrease of fuel consumption during all the operative range, and also respect the norms which are more and more restrictive. The complexity of this kind of injection system and the physical mechanism involved (film instability, atomization …) require depth studies on the injector. Experiment studies permit to establish empirical correlation between the injector used and provide, for example, the probability density function of the number of droplets obtained. However, this kind of experiment cannot cover all the operative range of the injector.
This thesis has for main objective the improvement of the design of the injector and in the end, to improve the injection system. For this purpose, different numerical approaches will be developed and compared with experimental results.
In order to perform this study, a collaboration between the CORIA laboratory (Rouen), the CMAP of the Ecole Polytechnique (Paris) and the SAFRAN group have emerged.

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Interview de Jérôme Hélie au GENCI

Notre vice-président est le Dr Jérôme HELIE mais il est aussi  Senior Expert, Responsable d’équipe Nozzle & Spray, Advanced System Engineering pour Continental Automotive SAS – Engine System… ouf.

Suite à la 7ème édition des semaines de l’industrie, Il a donné une interview au GENCI que vous pouvez retrouver dans son intégralité en suivant ce lien. Vous trouverez ici quelques extraits.

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Epurer du biogaz avec OpenFOAM

Contexte technico-économique

La méthanisation est une des voies de valorisation des déchets organiques. Son principe est assez simple, il s’agit de faire digérer ces déchets par un consortium microbien en absence d’oxygène (digestion anaérobie). Dans de telles conditions, ces micro-organismes produisent ce que l’on appelle du biogaz : un mélange de CO2 et de CH4, principalement, mais dans des proportions variables et bien loin des besoins des applications industrielles. Pour utiliser ce biogaz, il faut donc le purifier, pour atteindre une teneur en méthane d’au moins 97 %. Pour cela, différentes technologies existent : PSA, lavage à l’eau, … Cependant, de telles installations coûtent cher et ne sont donc rentables que pour de grosses installations de méthanisation.

De par son modèle agricole, la France possède de nombreuses petites exploitations (50 % des fermes comportent moins de 200 UGB). Ces petites exploitations représentent  un gisement important pour la production de biogaz, surtout lorsqu’elles sont situées à proximité du réseau de gaz. Mais malheureusement, leur petite taille les empêche de mettre en place une unité de méthanisation rentable dans le contexte technique actuel.

Dans ce contexte, la Chaire de Biotechnologie de CentraleSupélec a étudié la possibilité d’utiliser une technologie innovante pour séparer le CO2 du CH4 et ainsi produire un biométhane de qualité réseau. Le procédé proposé est basé sur une technologie compacte, modulaire et surtout accessible financièrement : le contacteur à membrane. Grâce à son lit de fibres microporeuses hydrophobes, le contacteur à membrane permet de mettre en contact le biogaz avec de l’eau (Fig. 1). Le CO2 étant plus soluble que le CH4 dans l’eau, il migre de la phase gazeuse vers la phase, appauvrissant ainsi la phase gazeuse en CO2. Sur le plan du principe, cela est facile. Cependant, la réalisation expérimentale soulève de nombreuses questions : volume utile du contacteur, envahissement des pores par de l’eau, conditions optimales de fonctionnement …

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Ecole d’Automne CFD OpenFOAM

Après le succès de l’édition 2016, CentraleSupélec et le Centre de Calcul de Champagne-Ardenne ROMEO proposent de nouveau une formation dédiée à la CFD avec OpenFOAM. Cette formation durent trois jours et couvre les principaux aspects de l’utilisation d’OpenFOAM. Durant la formation, les participants apprendront des choses allant de la simple création de maillages jusqu’au lancement d’un code parallèle sur supercalculateur.

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Livre OpenFOAM®

Mise à jour du livre – sous format électronique – de Tobias Holzmann – qui est disponible au téléchargement sur Research gate.
« MATHEMATICS, NUMERICS, DERIVATIONS AND OPENFOAM® » Pour tout ceux qui veulent faire le lien entre les équations physiques des écoulements et leur mise en application dans OpenFOAM®.