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25.7.1 Overview

In FLUENT, two solver technologies are available:

Both solvers can be used for a broad range of flows, but in some cases one formulation may perform better (i.e., yield a solution more quickly or resolve certain flow features better) than the other. The pressure-based and density-based approaches differ in the way that the continuity, momentum, and (where appropriate) energy and species equations are solved, as described in Section  25.1.

The pressure-based solver traditionally has been used for incompressible and mildly compressible flows. The density-based approach, on the other hand, was originally designed for high-speed compressible flows. Both approaches are now applicable to a broad range of flows (from incompressible to highly compressible), but the origins of the density-based formulation may give it an accuracy (i.e. shock resolution) advantage over the pressure-based solver for high-speed compressible flows.

Two formulations exist under the density-based solver: implicit and explicit. The density-based explicit and implicit formulations solve the equations for additional scalars (e.g., turbulence or radiation quantities) sequentially. The implicit and explicit density-based formulations differ in the way that they linearize the coupled equations.
See Section  25.1 for more details about the solver formulations.

Due to broader stability characteristics of the implicit formulation, a converged steady-state solution can be obtained much faster using the implicit formulation rather than the explicit formulation. However, the implicit formulation requires more memory than the explicit formulation.

Two algorithms also exist under the pressure-based solver in FLUENT: a segregated algorithm and a coupled algorithm. In the segregated algorithm the governing equations are solved sequentially, segregated from one another, while in the coupled algorithm the momentum equations and the pressure-based continuity equation are solved in a coupled manner. In general, the coupled algorithm significantly improves the convergence speed over the segregated algorithm, however, the memory requirement for the coupled algorithm is more than the segregated algorithm.

When selecting a solver and an algorithm you must consider the following issues:

The following two lists highlight the model availability for each solver:

figure   

Note that the pressure-based solver provides several physical models or features that are not available with the density-based solver:

The following features are available with the density-based solver, but not with the pressure-based solver:

User Inputs for Solver Selection

To choose one of the solver formulations, you will use the Solver panel (Figure  25.7.1).

Define $\rightarrow$ Models $\rightarrow$ Solver...

Figure 25.7.1: The Solver Panel
figure

To use the pressure-based solver, retain the default selection of Pressure Based under Solver.

To use the density-based implicit formulation, select Density Based under Solver and Implicit (the default) under Formulation.

To use the density-based explicit formulation, select Density Based under Solver and Explicit under Formulation.

After you have defined your model and specified which solver you want to use, you are ready to run the solver. The following steps outline a general procedure you can follow:

1.   Choose the discretization scheme and, for the pressure-based solver, the pressure interpolation scheme (see Section  25.8).

2.   (pressure-based solver only) Select the pressure-velocity coupling method (see Section  25.9.1).

3.   (pressure-based solver only) Select the porous media velocity method (see
Section  7.19).

4.   Select how you want the derivatives to be evaluated by choosing a gradient option (see Section  25.3.3).

5.   Set the under-relaxation factors (see Section  25.9.2).

6.   (density-based explicit formulation only) Turn on FAS multigrid (see Section  25.10.3).

7.   Make any additional modifications to the solver settings that are suggested in the chapters or sections that describe the models you are using.

8.   Initialize the solution (see Section  25.14).

9.   Enable the appropriate solution monitors (see Section  25.18).

10.   Start calculating (see Section  25.16 for steady-state calculations, or Section  25.17 for time-dependent calculations).

11.   If you have convergence trouble, try one of the methods discussed in
Section  25.22.

The default settings for the first three items listed above are suitable for most problems and need not be changed. The following sections outline how these and other solution parameters can be changed, and when you may wish to change them.


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