How Do I Choose Between Coupled and Segregated?

The segregated algorithm uses less memory than the coupled.

The coupled algorithm yields more robust and accurate solutions in compressible flow, particularly in the presence of shocks.

The coupled algorithm is more robust for high-Rayleigh number natural convection.

The number of iterations required by the coupled algorithm to solve a given flow problem is independent of mesh size, while the number of iterations required by the segregated algorithm increases with mesh size.

In some situations the coupled algorithm, combined with the implicit solver, will permit very large CFL numbers. This would be analogous to an under-relaxation factor of 1 for all variables in a segregated algorithm. In contrast, the segregated algorithm needs significant under-relaxation for both velocity and pressure and, in compressible flows, energy.

With this in mind, it is suggested that the algorithm selection proceed as follows:

Choose the Coupled Flow and Coupled Energy models for compressible flows, natural convection problems and flows with large body forces or energy sources.

If computational resources are not an issue, choose the Coupled Flow model for incompressible and/or isothermal flows.

Choose the Segregated Flow model for incompressible or mildly compressible flows.