A tensor-diffusity subgrid model for large eddy simulation

Subgrid-scale models for large-eddy simulation that are based on exact series expansions for filtered products are considered. In particular, if the first two terms are retained, the result is a diffusive subgrid term with a tensor diffusivity. This tensor is proportional to the rate-of-strain tensor of the large-scale velocity field. This leads to negative diffusion in the stretching directions. Implications of this result are considered for the filtered scalar advection-diffusion equation and for the momentum equation for incompressible fluid flow. When coupled with a dynamic Smagorinsky term to form a mixed model, very encouraging results are shown for turbulent, isotropic decay and for turbulent channel flow. In addition, it is shown that the model, mixed or not, transforms appropriately when differing frames of reference are considered. Modifications to the model are suggested for the case in which the unfiltered field(s) has discontinuities

A tensor-diffusivity subgrid model for large eddy simulation

Subgrid-scale models for large-eddy simulation that are based on exact series expansions for filtered products are considered. In particular, if the first two terms are retained, the result is a diffusive subgrid term with a tensor diffusivity.Thistensor is proportional to the rate-of-strain tensor of the large-scale velocity field. This leads to negative diffusion in the stretching directions. Implications of this result are considered for the filtered scalar advection-diffusion equation and for the momentum equation for incompressible fluid flow. When coupled with a dynamic Smagorinsky term to form a mixed model, very encouraging results are shown for turbulent, isotropic decay and for turbulentchannel flow. In addition, it is shown that the model, mixed or not, transforms appropriately when differing frames of reference are considered. Modifications to the model are suggested for the case in which the unfiltered field(s) has discontinuities. 1. Introduction In most formulations ..

Investigation of various wall modeling approaches for LES in the presence of mild pressure gradients

This work focuses on the investigation of various wall modeling strategies for the large-eddy simulation (LES) of high Reynolds number wall-bounded turbu- lent flows with mild pressure gradients. A half channel flow setup at Reτ = 2000 is considered. A periodic blowing and suction at the upper boundary is added to obtain a fluctuating pressure gradient. A wall-resolved LES is first performed to provide a reference solution and insight into the flow topology. Wall-modeled LES is then considered. Two strategies are investigated and compared. First, algebraic models are considered, without and with pressure gradient correction to the law of the wall used by the model. We also investigate various spatial and temporal averaging techniques when using the model. Second, RANS sub-layer models are considered, without and with pressure gradient corrections to the turbulent eddy viscosity model

Weiterentwicklung eines numerischen Programms zur Modellierung der Stroemung und Verbrennung in Feuerungen

During the period covered by the report the software development works on the programme packages KIVA II and on a completely new version of a modular programme package were carried on. The first technically relevant examples of application were calculated in the year covered by the report, especially for partly pre-mixed diffusion flames (gas firing), two phases flow (oil combustion and coal firing) as well as coal dust combustion with radiant heat exchange. Details can be read in the reports of each project. During the seminar in Aachen on 14th July an overview of the project was presented to the public. The aims and results which shall be achieved during the period of support/incentive can be described as follows. The basis forms the KIVA II programme which can be obtained as public-domain software and was developed by the Los Alamos Laboratories. During the first phase it is tested by some colleagues and applied to simple test experiments. According to our knowledge about the efficiency and the possibilities which the present programme package offered we divided the projects as follows: - heat exchange by radiation; - the Flamelet concept for diffusion flames and partly per-mixed flames; - injection and vaporisation of liquid fuel; - turbulent particle charged flows with chemical reactions (especially coal combustion). (orig./GL)Im Berichtszeitraum wurden die Softwareentwicklungsarbeiten an den Programmpaketen KIVA II und einer vollstaendig umstrukturierten Fassung fuer ein modulares Programmpaket weitergefuehrt. Erste technisch relevante Anwendungsbeispiele sind im Berichtsjahr, insbesondere zu teilweise vorgemischten Diffusionsflammen (Gasfeuerungen), Zweiphasenstroemung, (Oelbrenner und Kohlefeuerung) sowie Kohlestaubverbrennung mit Strahlungswaermeaustausch berechnet worden. Einzelheiten finden sich in den Berichten der Teilprojekte. Beim Kolloquium am 14. Juli in Aachen wurde eine Uebersicht ueber die Arbeiten der Oeffentlichkeit vorgestellt. Insgesamt lassen sich die im Foerderungszeitraum angestrebten Ziele und Ergebnisse folgendermassen darstellen: Den Ausgangspunkt bildete das als Public-Domain-Software erhaeltliche, von den Los Alamos Laboratories entwickelte KIVA II Programm, das in einer ersten Phase von den Mitarbeitern bewertet und auf einfache Testfaelle angewandt wurde. Die Erkenntnisse ueber die Leistungsfaehigkeit und Moeglichkeiten, die das vorgefundene Programmpaket bereitstellte, liess uns die Arbeiten folgendermassen aufteilen: - der Waermeaustausch durch Strahlung, - das Flamelet-Konzept fuer Diffusionsflammen und teilweise vorgemischte Flammen, - Einspritzung und Verdampfung von fluessigem Brennstoff, - turbulente partikelbeladene Stroemungen mit chemischen Reaktionen (insbesondere Kohlefeuerungen). (orig./GL)SIGLEAvailable from TIB Hannover: F94B1544+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

A new multiscale model with proper behaviour in both vortex flows and wall bounded flows

A new subgrid-scale (SGS) model which has an adequate behaviour in both vortical flows and wall-bounded flows is proposed. In wall-bounded flows computed using wall-resolved LES, the theory predicts that the SGS dissipation should vanish with a y +3 behaviour near the wall. In the case of vortex flows, one needs to have models which do not dissipate energy in the strongly vortical and essentially laminar part of the flow, e.g. in vortex core regions. The model presented here aims at combining the strengths of two models as it is a regularized variational multiscale (RVM) model (as in Jeanmart and Winckelmans [3]), thus acting on the high pass filtered LES field, and for which the subgrid-scale viscosity is evaluated using the WALE (wall adapting local eddy viscosity) scaling of Nicoud and Ducros [5], itself computed using the high pass filtered LES field. Thus this model is only active when there is a significant high wavenumber content in the flow and it has a natural near wall damping behaviour. The ability of this model to simulate vortex and wall-bounded flows is demonstrated on three test cases. The first case is the turbulent channel flow. The second case concerns a four-vortex system. The third case concerns a two-vortex system in ground effect. It is shown that this new model allows to perform successfully LES of these flows with the proper dissipative behaviour in both near wall and vortical regions. The LES are performed using a parallel fourth order code based on finite differences