
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 pressurebased and densitybased 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 pressurebased solver traditionally has been used for incompressible and mildly compressible flows. The densitybased approach, on the other hand, was originally designed for highspeed compressible flows. Both approaches are now applicable to a broad range of flows (from incompressible to highly compressible), but the origins of the densitybased formulation may give it an accuracy (i.e. shock resolution) advantage over the pressurebased solver for highspeed compressible flows.
Two formulations exist under the densitybased solver: implicit and explicit. The densitybased explicit and implicit formulations solve the equations for additional scalars (e.g., turbulence or radiation quantities) sequentially. The implicit and explicit densitybased 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 steadystate 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 pressurebased 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 pressurebased 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:

Note that the pressurebased solver provides several physical models or features that are not available with the densitybased solver:

The following features are available with the densitybased solver, but not with the pressurebased solver:
User Inputs for Solver Selection
To choose one of the solver formulations, you will use the Solver panel (Figure 25.7.1).
Define Models Solver...
To use the pressurebased solver, retain the default selection of Pressure Based under Solver.
To use the densitybased implicit formulation, select Density Based under Solver and Implicit (the default) under Formulation.
To use the densitybased 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:
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.