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Zurich, Switzerland

This paper introduces a novel modelling procedure for unresolved surface tension in interface tracking-based Large Eddy and Interface Simulation (LEIS) of interfacial flows. The approach involves modelling of the subgrid-scale (SGS) surface tension appearing in the filtered momentum equation, and explicitly the dominant SGS curvature part of that term. The procedure for modelling the SGS curvature is based on the analysis of variations in curvature resulting from using different filter widths, as well from using alternative discretization-based and direct-filtering techniques for generating coarse-filter curvature estimates. Unresolved surface tension modelling can act to smooth spatial curvature variations and dampen parasitic modes, and can alternatively restore interface wrinkling when super-grid scale resolution underpredicts local curvature. The paper outlines the principles involved in identifying local regions of unresolved surface tension, and in constructing a model that is robust across the range of interface topologies. The current implementation is based on the height-function curvature discretization scheme in Volume-of-Fluid (VOF) interfacial flow simulations. The procedure represents a SGS model in the true sense of the term - it is not a discretization scheme. The model should be used for Large Interface Simulation (LIS) of laminar interfacial flow modelling, too, complementing discretization schemes employed for resolved surface tension. Understandably the contribution of the model diminishes with mesh resolution as laminar flow does not continually introduce filter-scale interfacial deformations. The model may be more useful though for LEIS of turbulent interfacial flows, supporting discretization schemes for better capturing interface wrinkling, atomization and large bubble breakup. © 2012 Elsevier Ltd. Source


Lakehal D.,ASCOMP GmbH
Journal of Applied Fluid Mechanics | Year: 2011

The paper reports on the progress made in predicting large- and small-scale single and two-phase flows with heat transfer using the CMFD code TransAT. In the multi-phase context, the code uses the Level Set approach as the "Interface Tracking Method" of reference. The solver incorporates phase-change capabilities, surface tension and triple-line dynamics models, Marangoni effects, electric and magnetic fields, and a wall micro-film sub-grid scale model for lubrication. Complex 3D examples shown here were treated using a fully automatized version of the code, using the Immersed Surfaces Technique (IST) to map complex components into a simple rectangular Cartesian grid. It is shown that real coupled two-phase heat transfer (conjugate) problems are within reach of modern CMFD code using interface tracking, with relatively fast response times: 3D coupled two-phase flow heat transfer can run on a simple Linux PC cluster within 24 H time. Source


LAKEHAL D.,ASCOMP GmbH | LIOVIC P.,CSIRO
Journal of Fluid Mechanics | Year: 2011

Large-eddy and interface simulation using an interface tracking-based multi-fluid flow solver is conducted to investigate the breaking of steep water waves on a beach of constant bed slope. The present investigation focuses mainly on the 'weak plunger' breaking wave type and provides a detailed analysis of the two-way interaction between the mean fluid flow and the sub-modal motions, encompassing wave dynamics and turbulence. The flow is analysed from two points of views: mean to sub-modal exchange, and wave to turbulence interaction within the sub-modal range. Wave growth and propagation are due to energy transfer from the mean flow to the waves, and transport of mean momentum by these waves. The vigorous downwelling-upwelling patterns developing at the head and tail of each breaker are shown to generate both negative- and positive-signed energy exchange contributions in the thin sublayer underneath the water surface. The details of these exchange mechanisms are thoroughly discussed in this paper, together with the interplay between three-dimensional small-scale breaking associated with turbulence and the dominant two-dimensional wave motion. A conditional zonal analysis is proposed for the first time to understand the transient mechanisms of turbulent kinetic energy production, decay, diffusion and transport and their dependence and/or impact on surface wrinkling over the entire breaking process. The simulations provide a thorough picture of air-liquid coherent structures that develop over the breaking process, and link them to the transient mechanisms responsible for their local incidence. Source


Lakehal D.,ASCOMP GmbH
Nuclear Engineering and Design | Year: 2010

The paper centres on the use of the so-defined LEIS approach (Large-Eddy & Interface Simulation) for turbulent multifluid flows present in thermal-hydraulics applications. Interfacial flows involving deformable, sheared fronts separating immiscible fluids are shown to be within reach of this new approach, featuring direct resolution of turbulence and sheared interface deformations within the interface tracking (ITM) framework, such as level sets and VOF. In this technique supergrid turbulence and interfacial scales are directly solved whereas the sub-grid (SGS) parts are modelled, at least the turbulence part of it. First results are shown (feasibility), and difficulties and open issues are discussed. The connection between these two particular scales will also be discussed, and potential modelling routes evoked, including combining two-fluid and ITM, local grid refinement, or combing particle tracking and ITM for sub-grid inclusions smaller than the grid size. © 2009 Elsevier B.V. All rights reserved. Source


Grant
Agency: Cordis | Branch: H2020 | Program: RIA | Phase: NFRP-01-2014 | Award Amount: 6.64M | Year: 2015

The thermal-hydraulics Simulations and Experiments for the Safety Assessment of Metal cooled reactor (SESAME) project supports the development of European liquid metal cooled reactors (ASTRID, ALFRED, MYRRHA, SEALER). The project focusses on pre-normative, fundamental, safety-related, challenges for these reactors with the following objectives: Development and validation of advanced numerical approaches for the design and safety evaluation of advanced reactors; Achievement of a new or extended validation base by creation of new reference data; Establishment of best practice guidelines, Verification & Validation methodologies, and uncertainty quantification methods for liquid metal fast reactor thermal hydraulics. The SESAME project will improve the safety of liquid metal fast reactors by making available new safety related experimental results and improved numerical approaches. These will allow system designers to improve the safety relevant equipment leading to enhanced safety standards and culture. Due to the fundamental and generic nature of SESAME, developments will be of relevance also for the safety assessment of contemporary light water reactors. By extending the knowledge basis, SESAME will allow the EU member states to develop robust safety policies. At the same time, SESAME will maintain and further develop the European experimental facilities and numerical tools. The consortium of 25 partners provides American-European-wide scientific and technological excellence in liquid metal thermal hydraulics, as well as full alignment with ESNII and with NUGENIA where of interest. A close interaction with the European liquid metal cooled reactor design teams is foreseen involving them in the Senior Advisory Committee. They will actively advise on the content of the project and will be the prime end-users, ensuring their innovative reactor designs will reach highest safety standards using frontier scientific developments.

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