| 1. |
- Gullbrand, Jessica, et al.
(författare)
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A comparison of subgrid scale models for large eddy simulation of a co-annular swirling flow field
- 1999
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Ingår i: First Symposium on Turbulence and Shear Flow Phenomena. ; , s. 747-752
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Large Eddy Simulation (LES) of an incompressible, turbulent, co-annular swirling flow field have been performed in order to compare the influence of different subgrid scale (SGS) models. Two models were investigated, an implicit model and a stress similarity model. In the simulations using the implicit approach, no explicit SGS model was used. For the stress similarity model, similar behaviour between the resolved and unresolved stresses was assumed. The results from the simulations are compared with experimental data for mean velocities and turbulence intensities.
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| 2. |
- Revstedt, Johan, et al.
(författare)
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Large eddy simulations of turbulent flow in a stirred reactor
- 1998
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Ingår i: Chemical Engineering Science. - 0009-2509. ; 53:24, s. 4041-4053
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Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulations of a baffled reactor stirred with a single Rushton turbine were performed. The turbine was positioned at mid-height in the reactor. The unsteady spatially filtered incompressible Navier–Stokes equations have been solved numerically. These large eddy simulations (LES) provide details of the flow field that cannot be obtained with the so-called Reynolds averaged equations and corresponding models. The un-resolved scales have been modelled implicitly through the truncation errors of the discretisation. The motion of the blades was described as time-dependent momentum sources. The time-averaged flow properties, mean flow and turbulence, have been studied. Phase averaging was used to characterize the flow relative to the impeller blades. Power spectral density and correlations, both in time and space were investigated. The results have been compared with the experimental data from several sources and also with the data from other simulations. Comparing our data with measurements we see an overall good agreement both qualitatively and quantitatively. In the impeller stream we observe an acceleration of the flow in the radial direction due to the trailing vortex pair. This acceleration can also be deduced from published experimental and numerical data. There are however some discrepancies concerning velocity fluctuations in the impeller area. These discrepancies are probably caused by inaccuracy in the impeller description.
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| 3. |
- Revstedt, Johan
(författare)
-
On the modelling of turbulent flow and mixing in stirred reactors
- 1999
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the numerical simulations and modelling of the flow structure and scalar transport in stirred reactors. For this Large Eddy Simulations (LES) have been used. The flow situation in a stirred reactor includes several flow phenomena that are not well predicted by "standard" two-equation models, e.g. swirl, stagnation points and anisotropic turbulence. Also, using LES one can obtain data unavailable when using a Reynolds Averaged Navier-Stokes (RANS) approach, such as the spectral content of the solution. However, LES requires a high degree of resolution in time and space which, combined with long sampling times to obtain converged statistics, put higher demands on the computer hardware. One major difficulty when simulating flow in stirred reactors is how to incorporate the effects of the moving boundaries, i.e. the turbine blades. Several approaches have been proposed in the past spanning from deforming and/or sliding grids to stationary boundary conditions based on LDA measurements and the use of moving momentum source terms in the equations of motion. The numerical aspects of representing complex moving boundaries have been investigated and a model based on local velocity differences is presented. The results show that complex moving boundaries can be represented on a Cartesian grid in this way with reasonable numerical accuracy and efficiency. The influence of several Sub-Grid Scale (SGS) models, both for turbulence and mixing, on the flow and the transport of inert additives has been studied. However, for this the geometrically simpler case of a circular jet impinging on a flat plate is considered. The main reasons being that simulating the complex flow structure of stirred reactors is quite time consuming, in terms of CPU time. Also, the additional geometrical complexity may mask some physical factors/problems. The study of SGS model effects suggests that the choice of SGS model has a significant effect on the results and that a eddy viscosity based dynamic model could be the best choice.
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