STAR-CCM+ Stopping Criteria and Convergence

Start from the engineering decision, not the menu option

Stop a simulation when the engineering outputs, conservation and iterative errors are demonstrably small enough for the decision—not after an arbitrary iteration count. In STAR-CCM+, the relevant setting only becomes meaningful when it is tied to a measurable output, a physical scale and a stated modelling assumption.

This guide deliberately avoids a release-specific click sequence. Interface names and solver options can change between releases, while the engineering checks remain stable. Use the Siemens documentation for your installed release to confirm exact menu names after the physical decision is clear.

Inputs and definitions to verify first

Before changing solver controls, confirm the quantities that actually define this problem. The most important checks for this topic are:

• residual trend: confirm the value, definition, units and spatial location before using it to justify the setup.
• key reports/monitors: confirm the value, definition, units and spatial location before using it to justify the setup.
• mass and energy balance: confirm the value, definition, units and spatial location before using it to justify the setup.
• statistical averaging for unsteady flow: confirm the value, definition, units and spatial location before using it to justify the setup.

Recommended STAR-CCM+ workflow

1. Define the engineering output.

   Write down the quantity that will determine success before changing STAR-CCM+ Stopping Criteria and Convergence. This prevents a software setting from becoming the objective itself.

2. Verify the physical inputs.

   Check residual trend and key reports/monitors first. Then confirm mass and energy balance and statistical averaging for unsteady flow are consistent with the real operating condition.

3. Create a documented baseline.

   Run one traceable baseline with the model, mesh, boundary conditions and reference values recorded before tuning secondary options.

4. Monitor solution evidence.

   Track monitor drift, normalized imbalance and repeatability over additional iterations. A stable residual history alone is not enough if the engineering evidence is still drifting.

5. Run one targeted sensitivity.

   Change the parameter that most directly controls the uncertainty and confirm that the engineering conclusion does not depend on one arbitrary setting.

What evidence should support the final setup?

The setup is credible when the engineering outputs are stable for the right reason—not merely because the solver stopped changing quickly. Build the evidence around:

• monitor drift: compare the baseline with at least one targeted sensitivity or independent physical expectation.
• normalized imbalance: compare the baseline with at least one targeted sensitivity or independent physical expectation.
• repeatability over additional iterations: compare the baseline with at least one targeted sensitivity or independent physical expectation.

Where possible, compare these signals with a hand calculation, correlation, test value, conservation balance or a deliberately simplified CFD case. Independent checks are especially useful before increasing model complexity.

Common failure modes

• stopping because residuals hit a round number.
• ignoring slowly drifting heat/force reports.
• using identical criteria for steady and transient cases.

If one of these appears, return to the physical definition before tuning relaxation, discretization or convergence controls. Numerical tuning should not compensate for an inconsistent model.

Turn the guide into an engineering check

Related STAR-CCM+ guides