practical reasons, it is seldom possible to make the mesh twice as fine in each direction. Instead, some critical regions can be selected and the mesh refined there.

The Choice of Solver and Solver Settings

The solvers and settings for the fluid flow interfaces are automatically selected for this purpose. They have been optimized for a large variety of fluid-flow conditions and applications.

Yet, adjustments may sometimes be required. Like the previously discussed parameters, start simply and increase the complexity. Testing the solver and settings is done primarily by simplifying a lot of the previous parameters, such as the number of physics to solve for, or the size of the material property values.

Once you are confident with a solver and its settings for a simplified description of the model, increase complexity and adjust the solver settings accordingly. Always, if you can, compare with known results from similar systems.

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T h e C F D M o d u l e P h y s i c s I n t e r f a c e s

The table below shows the fluid-flow physics interfaces available with the CFD Module. The various types of momentum transport include laminar and turbulent flow, Newtonian and non-Newtonian flow, isothermal and non-isothermal flow, multiphase flow, and flow in porous media.

The Conjugate Heat Transfer Laminar Flow (nitf) and Turbulent Flow (nitf) interfaces found under the Heat Transfer branch are identical to the

Non-Isothermal Flow interfaces. The only difference is that Heat transfer

T H E C F D M O D U L E P H Y S I C S I N T E R F A C E S | 37

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T H E C F D M O D U L E P H Y S I C S I N T E R F A C E S | 39

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T H E C F D M O D U L E P H Y S I C S I N T E R F A C E S | 41

* An enhanced interface is one that is included with the base COMSOL package but has added functionality for this Module.

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3

C h e m i c a l S p e c i e s T r a n s p o r t B r a n c h

The physics interfaces in the Chemical Species Transport branch () in the Model Wizard accommodate all types of material transport that can occur through diffusion, convection and migration due to an electric field—either alone or in combination with one another. Also available is The Transport of Diluted Species Interface, which is described in the COMSOL Multiphysics User’s Guide. The Mechanisms for Chemical Species Transport helps you choose the best one to start with.

In this chapter:

•The Transport of Concentrated Species Interface

•The Reacting Flow, Concentrated Species Interface

•The Reacting Flow, Diluted Species Interface

•Theory for the Transport of Concentrated Species Interface

•Theory for the Reacting Flow, Concentrated Species Interface

•Theory for the Reacting Flow, Diluted Species Interface

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T h e M e c h a n i s m s f o r C h e m i c a l S p e c i e s T r a n s p o r t

The behavior of chemical reactions in real environments is often not adequately described by the assumptions of perfectly mixed or controlled environments. This means that the transport of material through both time and space need to be considered. Physics interfaces in the Chemical Species Transport branch accommodate all types of material transport that can occur through diffusion, convection and migration due to an electric field—either alone or in combination with one another. The branch includes interfaces solving for diluted as well as concentrated mixtures, where the species propagation may occur in solids, free flowing fluids, or through porous media.

The Transport of Diluted Species Interface () (described in the COMSOL Multiphysics User’s Guide) is applicable for solutions (either fluid or solid) where the transported species have concentrations at least one order of magnitude less than their solvent. The settings for this physics interface can be chosen so as to simulate chemical species transport through diffusion (Fick’s law), convection (when coupled to fluid flow), and migration (when coupled to an electric field—electrokinetic flow). Further information is also in Theory for the Transport of Diluted Species Interface in the

COMSOL Multiphysics User’s Guide.

The Transport of Concentrated Species Interface () is used for modeling transport within mixtures where a no one component is clearly dominant. Often the concentrations of the participating species are of the same order of magnitude, and the molecular effects of respective species on each other needs to be considered. This interface supports transport through Fickian diffusion, a mixture average diffusion model, and as described by the Maxwell-Stefan equations.

The Reacting Flow, Concentrated Species Interface () combines the Transport of Concentrated Species and the Free and Porous Media Flow interfaces. This means that mass and momentum transport can be modeled from a single physics interface, with the couplings between velocity field and mixture density set up automatically. Also, the effective transport coefficients in a porous matrix domain are derived based on the corresponding values in for a non-porous domain. This interface is applicable for fluid flow in the laminar regime.

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