Numerical simulation of phase separation and a priori two-phase LES filtering (original) (raw)
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Towards large eddy simulation of isothermal two-phase flows: Governing equations and a priori tests
International Journal of Multiphase Flow, 2007
This article reports on the potential of application of LES in the calculation of turbulent two-phase flows, in the case where each phase is resolved and interfaces remain much larger than the mesh size. In comparison with single-phase flow, successful application of LES to resolve two-phase flow problems should account for the complex interaction between turbulence and interfaces. Non-linear transfers of turbulent energy across the interface have to be accurately modeled. The derivation of the complete filtered two-phase flow governing equations has been formulated to deal with turbulence at the interface in a comprehensive and practical way. Explicit filtering of 2D direct numerical simulations has been employed to evaluate the order of magnitude of the new subgrid contributions. A parametric study on the academic test case of two counter-rotative vortices and a more complex test case of phase inversion in a closed box have been utilized to perform an order of magnitude analysis of different transport mechanisms. Important features of turbulent energy transfer across the interface have been discussed. Analyses of the numerical results have been conducted to derive conclusions on the relative importance of the different subgrid scale contributions, and modeling issues and solutions are provided.
2007
This article reports on the potential of application of LES in the calculation of turbulent two-phase flows, in the case where each phase is resolved and interfaces remain much larger than the mesh size. In comparison with single-phase flow, successful application of LES to resolve two-phase flow problems should account for the complex interaction between turbulence and interfaces. Non-linear transfers
Governing equations and a priori tests for the LES of two-phase flows
This communication reports on the potential application of Large Eddy Simulation in the calculation of turbulent isothermal two-phase flows, in the case where the large scales of each phase are resolved and small interface structures can be lower than the mesh size. In comparison with single phase flows, application of LES to two-phase flow problems should account for the complex interaction between interface and turbulence. The complete filtered two-phase flow equations are formulated to deal with turbulence at the interface. Explicit filtering of 2D and 3D Direct Numerical Simulations of a phase separation problem has been employed to evaluate the order of magnitude of the specific subgrid contributions. Analyses of the numerical results have been conducted to derive conclusions on the relative importance of the different subgrid scale contributions. Modeling issues and turbulent energy transfer across the interface are discussed.
LES of turbulent non-isothermal two-phase flows within a multifield approach
Thermodynamique des interfaces et mécanique des fluides
Safety issues in nuclear power plant involve complex turbulent bubbly flows. To predict the behavior of these flows, the two-fluid approach is often used. Nevertheless, this model has been developed for the simulation of small spherical bubbles, considered as a dispersed field. To deal with bubbles with a large range of sizes, a multifield approach based on this two-fluid model has been proposed. A special treatment, called the Large Bubble Model (LBMo), has been implemented and coupled to the dispersed model. However, only laminar and isothermal flows were considered in previous papers. Thus, here, Large Eddy Simulations (LES) are investigated to model turbulence effects. For this purpose, the two-fluid model equations are filtered to highlight the specific subgrid terms. Then, an a priori LES study using filtered Direct Numerical Simulation (DNS) results is detailed. This analysis allows classifying these terms according to their relative weight and then concentrating the modelling efforts on the predominant ones. Five different turbulence models are compared. These results are finally used to perform true LES on a turbulent two-phase flow. Moreover, in order to tackle non-isothermal flows occurring in industrial studies, a new heat transfer model is implemented and validated to deal with phase change at large interfaces using the Large Bubble Model.
A large eddy simulation subgrid model for turbulent phase interface dynamics
In this paper we report on the outline of a Large Eddy Simulation subgrid model for liquid/gas phase interface dynamics. A key feature of the proposed model is to take the subgrid phase interface dynamics fully into account by employing a dual-scale approach. Instead of modeling the LES subgrid phase interface geometry, we fully resolve it on an auxillary grid using the Refined Level Set Grid approach . We then propose to model the LES subgrid velocity on the auxillary grid needed to move the fully resolved phase interface, by solving a dedicated PDE for its evolution near the phase interface. This PDE contains three different contributions. First, the subfilter turbulent eddies are taken into account by modeling the subfilter acceleration in lines of Oboukhovs log-normality conjecture on the stochastic field of ε. The second term, a velocity increment due to the relative motion between the two phases is modeled deriving renormalized velocity boundary condition at the phase interface. The final term, due to subfilter surface tension induced subfilter velocities is modeled following a Taylor analogy. Knowing the fully resolved phase interface geometry, all previously unclosed terms in the filtered Navier-Stokes equations can be directly closed using explicit filtering.
Validation of a novel very large eddy simulation method for simulation of turbulent separated flow
International Journal for Numerical Methods in Fluids, 2013
The paper describes the validation of a newly developed very LES (VLES) method for the simulation of turbulent separated flow. The new VLES method is a unified simulation approach that can change seamlessly from Reynolds-averaged Navier-Stokes to DNS depending on the numerical resolution. Four complex test cases are selected to validate the performance of the new method, that is, the flow past a square cylinder at Re D 3000 confined in a channel (with a blockage ratio of 20%), the turbulent flow over a circular cylinder at Re D 3900 as well as Re D 140, 000, and a turbulent backward-facing step flow with a thick incoming boundary layer at Re D 40, 000. The simulation results are compared with available experimental, LES, and detached eddy simulation-type results. The new VLES model performs well overall, and the predictions are satisfactory compared with previous experimental and numerical results. It is observed that the new VLES method is quite efficient for the turbulent flow simulations; that is, good predictions can be obtained using a quite coarse mesh compared with the previous LES method. Discussions of the implementation of the present VLES modeling are also conducted on the basis of the simulations of turbulent channel flow up to high Reynolds number of Re D 4000. The efficiency of the present VLES modeling is also observed in the channel flow simulation. From a practical point of view, this new method has considerable potential for more complex turbulent flow simulations at relative high Reynolds numbers.
Parametric study of LES subgrid terms in a turbulent phase separation flow
International Journal of Heat and Fluid Flow, 2010
The present work provides an a priori estimate of the specific LES subgrid terms occurring in the filtered 1-fluid Navier-Stokes equations that model the dynamics of turbulent two-phase flows involving separated phases. Three-dimensional simulations of a water/oil phase separation in a cubic cavity are carried out without explicit turbulent modeling. A parametric study is investigated for various surface tension coefficients, density and dynamic viscosity ratios. The numerical results obtained with these parametric simulations are then explicitly filtered to quantify the relative magnitude of each LES subgrid terms. A classification of these subgrid terms is finally provided for the phase inversion configuration.
About Multi-resolution Techniques for Large Eddy Simulation of Reactive Multi-phase Flows
Energy Procedia, 2015
A numerical technique for mesh refinement in the HeaRT (Heat Release and Transfer) numerical code is presented. In the CFD framework, Large Eddy Simulation (LES) approach is gaining in importance as a tool for simulating turbulent combustion processes, also if this approach has an high computational cost due to the complexity of the turbulent modeling and the high number of grid points necessary to obtain a good numerical solution. In particular, when a numerical simulation of a big domain is performed with a structured grid, the number of grid points can increase so much that the simulation becomes impossible: this problem can be overcomed with a mesh refinement technique. Mesh refinement technique developed for HeaRT numerical code (a staggered finite difference code) is based on an high order reconstruction of the variables at the grid interfaces by means of a least square quasi-eno interpolation: numerical code is written in modern Fortran (2003 standard of newer) and is parallelized using domain decomposition and message passing interface (MPI) standard.
Large Eddy Simulation of Separating Flows with a New Mixed-Time-Scale SGS Model
A new subgrid-scale (SGS) model for practical large eddy simulation (LES) is proposed. The model is constructed with the concept of mixed (or hybrid) time scale, which makes it possible to use consistent model parameters and to dispense with the distance from the wall. The model performance is tested in plane channel flows, and the results show that this model is able to account for near-wall turbulence without an explicit damping function as in the dynamic Smagorinsky model. The model is also evaluated in a backwardfacing step flow and in a flow around a circular cylinder. The calculated results show good agreement with experimental data, while the results obtained using the dynamic Smagorinsky model show less accuracy and less computational stability. The model is also applied to the three-dimensional complex flow around an Ahmed's car-shaped body. The agreement between the calculated results and the experimental data is quite satisfactory. These results suggest that the present model is a refined SGS model suited for practical LES to compute flows in a complicated geometry. Besides, by comparing these results with those obtained by a quasi-direct simulation, which has been widely used in engineering applications so far, the effectiveness of practical LES is confirmed.