You can modify the convergence test value, which is set to 0.001 by default. The convergence test is based on relative error. For each type of field, the modification at every node between two successive iterations is compared to the maximum value of the field at the current iteration. The global test is based on the highest relative variation for most fields (the exceptions including the pressure, the geometrical variables, and all quantities appearing inside parentheses in convergence messages). The default value of the convergence test is appropriate for most problems, except for integral viscoelastic simulations.

To modify the convergence test value, select Modify numerical parameters for iterations in the Numerical parameters menu, and then select Modify the convergence test.

As it can typically take longer for transported species to converge than for the main fields, there is a specific convergence test for transported species. By default, the convergence test is the same for species as for the other fields. By softening the convergence criterion for species transport, you can save a couple solution iterations, which reduces total solution time—particularly for 3D cases.

To modify the value of the convergence criterion for transported species, select Modify numerical parameters for iterations in the Numerical parameters menu, and select Modify the convergence test for species. Note that the convergence test for species must not be more stringent than the main convergence test (its value should be greater than or equal to that specified for the main convergence test).

Not all problems converge. When divergence occurs, the error rapidly increases with each iteration. Since there is usually no hope of convergence when the relative error has become too great, Ansys Polyflow stops the iteration (but not the evolution or time-marching scheme) when the error reaches a preset maximum value (the default is 1000).

To modify the divergence test value, select Modify numerical parameters for iterations in the Numerical parameters menu, and then select Modify the divergence test.

Most flow problems result in a system of nonlinear algebraic equations. The Newton-Raphson method is available for solving these systems. The quadratic convergence of the Newton-Raphson method means that only a small number of iterations is required, provided the starting solution is not too far from the converged solution. If a solution cannot be achieved, it is recommended that an evolution procedure be tried. See Evolution for information on evolution procedures.

During the simulation, if the calculation is converging, but cannot meet the convergence criterion within the specified maximum number of iterations, Ansys Polyflow will automatically perform additional iterations (up to 30% of the specified maximum number) to try to reach the convergence criterion.