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Common Mistakes in Rotor–Stator Interface Setup

Gaurav

Correctly defining the rotor–stator interface is one of the most critical—and most frequently mishandled—steps in turbomachinery CFD. Even with good mesh and solver settings, small mistakes at the interface can lead to non‑physical losses, convergence problems, or completely misleading performance predictions.

What are Rotor-stator connections?

If two neighboring domains have different rotational speeds, a rotor-stator connection is required between them.

When a rotor-stator connection is present, the user has to:

  • Create a connection
  • Define the rotor-stator boundaries (automatic)
  • Select a rotor-stator treatment approach to simulate the interaction between the two domains

Using the Wrong Interface Type

Mistake
Applying an inappropriate interface model, such as:

  • Using Frozen Rotor when strong unsteady effects are present
  • Using the Stage / Mixing Plane when blade-to-blade interaction is important

Why It’s a Problem

  • Frozen Rotor preserves circumferential non-uniformity but assumes a steady relative position
  • Mixing Plane circumferentially averages the flow, eliminating wakes and potential fields
  • An incorrect choice can suppress or exaggerate losses, pressure rise, and unsteadiness

How to Avoid

Use a Full Non-Matching Frozen Rotor for:

Steady-state approximation

Initial convergence or design screening

Use a Full Non-Matching Mixing Plane for:

Overall performance maps

Steady

A pitchwise averaging of the flow solution is performed at the rotor-stator interface, and the exchange of information at the interface depends on the local direction of the flow.

Unsteady (Harmonic)

A pitchwise averaging of the flow solution is performed at the rotor-stator interface for the turbulence equations. For other equations, a reconstruction based on the spatial Fourier decomposition is used to ensure continuity at the rotor-stator interface, and a 1D non-reflecting treatment is applied. The exchange of information at the interface depends on the local direction of the flow.

  • Use Transient Sliding Mesh when:
    • Wake passing, blade vibration, or noise is important
    • Accurate unsteady forces are required

Are there any other useful tips or points that others could share for the benefit of fellow CFD users?

  • Gaurav

    Some other points that can affect the Rotor-Stator Interface setup are as follows:

     2. Inconsistent Rotational Speed or Axis Definition: A mistake in this regard can lead to errors in the setup.

     Specific mistakes to watch out for include:

    • The rotor speed is being defined incorrectly in terms of its sign, magnitude, or units.
    • The rotor axis is not aligned with the geometry.

     Why is this a problem?

    • It causes incorrect calculations of Coriolis and centrifugal forces.
    • This, in turn, results in incorrect predictions of swirl, pressure rise, and efficiency.
    • Often, these errors lead to subtle but severe performance issues.

     How to avoid these mistakes:

    • Double-check the following:

      o The magnitude and direction of the angular velocity.

      o The origin and orientation of the rotation axis.

    • Visually confirm the rotation direction by:

      o Examining velocity vectors.

      o Analyzing relative frame plots.

    3. Poor mesh quality at the interface includes:

     A significant cell size discrepancy across the interface.

    Highly skewed or non-orthogonal faces at the interface.

    Non-matching meshes with inadequate interpolation quality.

    Why is this a problem?

    Maintaining poor mesh quality at the interface can lead to:

    Increased numerical diffusion.

    Smearing of wakes and secondary flows.

    Unstable convergence or excessive residual values.

     To avoid these issues:

     Ensure consistent cell sizes on both sides of the interface.

    Verify the following:

    • Low skewness.
    • Smooth growth rate when approaching the interface.

    Refine the mesh in the following areas:

       Wake regions, Tip clearance zones near the interface.

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