STAR-CCM+ Realizable k-ε Model

Start from the engineering decision, not the menu option

Use realizable k-ε as a robust RANS option for many industrial shear flows, while recognizing its wall and separation limitations. 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:

• wall treatment: confirm the value, definition, units and spatial location before using it to justify the setup.
• inlet k and epsilon: confirm the value, definition, units and spatial location before using it to justify the setup.
• recirculation strength: confirm the value, definition, units and spatial location before using it to justify the setup.
• strain/rotation dominated regions: 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+ Realizable k-ε Model. This prevents a software setting from becoming the objective itself.

2. Verify the physical inputs.

   Check wall treatment and inlet k and epsilon first. Then confirm recirculation strength and strain/rotation dominated regions 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 pressure drop, mixing rate and recirculation size. 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:

• pressure drop: compare the baseline with at least one targeted sensitivity or independent physical expectation.
• mixing rate: compare the baseline with at least one targeted sensitivity or independent physical expectation.
• recirculation size: 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

• using it for strong separation without sensitivity checks.
• inconsistent wall-function y+.
• default turbulence inputs unrelated to the upstream flow.

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