Segregated Flow Troubleshooting

Flow solution diverges during initial iterations

If the momentum and continuity residuals show solution divergence during the first several iterations and the residuals show no signs of decreasing, this indicates a problem.

• Make sure that the overall setup is correct. Some things to look for are:
  • Mesh extents for viscous flow
  • Mesh quality
  • Boundary conditions for both flow and turbulence, if turbulent flow is activated
• If the flow is incompressible, a slight reduction of the under-relaxation factors for pressure and velocity can stabilize the solution. It is also appropriate to try setting the model properties to use a first-order convection scheme.
• If the flow is compressible, reduce the under-relaxation factor for energy (say to 0.3). Also, the under-relaxation factor for pressure can be set to a small number (say 0.01), for just a few iterations before resuming iterating with the default value of 0.3.

Warnings occur of pressure corrections being limited for compressible flow

For compressible flow, the absolute pressure must be a positive definite quantity. Simcenter STAR-CCM+ tries to limit the pressure corrections so that large pressure changes do not drive the absolute pressure negative.

• It is acceptable for these warnings to occur during the startup for compressible flow, as long as they cease after several iterations. If they do not, it is necessary to reduce the under-relaxation factor for pressure. This factor can be set to a small number (say 0.01), for just a few iterations, before resuming iterating with the default value of 0.3.
• Ensure that the under-relaxation factor for energy is set to an appropriate value (say 0.3).

Warnings occur about reverse flow at a flow split boundary

In Simcenter STAR-CCM+, flow split outlet boundaries are not designed to permit recirculation.

• If warnings occur during initial convergence but cease after several iterations, there is no cause for concern.
• If warnings persist, there are two options:
  • Rebuild the mesh but extend the outlet region in such a way as to minimize the risk of recirculation.
  • Change the boundary type to a pressure outlet.

Warnings occur about reverse flow at a pressure outlet boundary

In Simcenter STAR-CCM+, pressure outlet boundaries are designed to permit recirculation, although a unique solution can not exist resulting in convergence problems.

• Recirculation causes a non-uniform pressure distribution at the boundary.
• If a uniform pressure distribution is desired, rebuild the mesh with the outlet region extended in such a way as to prevent recirculation.
• Convergence problems due to unsteady recirculation are indicated by the number of faces at which reversed flow occurs changing from one iteration to the next. (The number of faces is printed in the warning messages in the output window.) If unsteady recirculation occurs, extend the outlet region in such a way as to prevent it.

Energy solution diverges during initial iterations

If the energy residuals show solution divergence during the first several iterations and the residuals show no signs of decreasing, this scenario indicates a problem.

Energy solution converges slowly

The under-relaxation factor can be adjusted to control the convergence rate.

• The default Under-Relaxation Factor value of 0.9 is aggressive. It is not advisable to use a larger value than 0.9 for flow simulations.
• For compressible flow problems, the under-relaxation factor is normally kept in the range 0.1-0.5.