3.2 Sliding Mesh Theory

When a time-accurate solution for rotor-stator interaction (rather than a time-averaged solution) is desired, you must use the sliding mesh model to compute the unsteady flow field. As mentioned in Section  2.1, the sliding mesh model is the most accurate method for simulating flows in multiple moving reference frames, but also the most computationally demanding.

Most often, the unsteady solution that is sought in a sliding mesh simulation is time-periodic. That is, the unsteady solution repeats with a period related to the speeds of the moving domains. However, you can model other types of transients, including translating sliding mesh zones (e.g., two cars or trains passing in a tunnel, as shown in Figure  3.2.1).

Figure 3.2.1: Two Passing Trains in a Tunnel

Note that for flow situations where there is no interaction between stationary and moving parts (i.e., when there is only a rotor), it is more efficient to use a rotating reference frame. (See Section  2.2 for details.) When transient rotor-stator interaction is desired (as in the examples in Figures  3.2.2 and 3.2.3), you must use sliding meshes. If you are interested in a steady approximation of the interaction, you may use the multiple reference frame model or the mixing plane model, as described in Sections  2.3.1 and 2.3.2.

Figure 3.2.2: Rotor-Stator Interaction (Stationary Guide Vanes with Rotating Blades)

Figure 3.2.3: Blower

The Sliding Mesh Technique

In the sliding mesh technique two or more cell zones are used. (If you generate the mesh in each zone independently, you will need to merge the mesh files prior to starting the calculation, as described in this section in the separate User's Guide.) Each cell zone is bounded by at least one "interface zone'' where it meets the opposing cell zone. The interface zones of adjacent cell zones are associated with one another to form a "mesh interface.'' The two cell zones will move relative to each other along the mesh interface.

During the calculation, the cell zones slide (i.e., rotate or translate) relative to one another along the mesh interface in discrete steps. Figures  3.2.4 and 3.2.5 show the initial position of two meshes and their positions after some translation has occurred.

As the rotation or translation takes place, node alignment along the mesh interface is not required. Since the flow is inherently unsteady, a time-dependent solution procedure is required.

Figure 3.2.4: Initial Position of the Meshes

Figure 3.2.5: Rotor Mesh Slides with Respect to the Stator

Mesh Interface Shapes

The mesh interface and the associated interface zones can be any shape, provided that the two interface boundaries are based on the same geometry. Figure  3.2.6 shows an example with a linear mesh interface and Figure  3.2.7 shows a circular-arc mesh interface. (In both figures, the mesh interface is designated by a dashed line.)

Figure 3.2.6: 2D Linear Mesh Interface

Figure 3.2.7: 2D Circular-Arc Mesh Interface

If Figure  3.2.6 was extruded to 3D, the resulting sliding interface would be a planar rectangle; if Figure  3.2.7 was extruded to 3D, the resulting interface would be a cylinder. Figure  3.2.8 shows an example that would use a conical mesh interface. (The slanted, dashed lines represent the intersection of the conical interface with a 2D plane.)

Figure 3.2.8: 3D Conical Mesh Interface

For an axial rotor/stator configuration, in which the rotating and stationary parts are aligned axially instead of being concentric (see Figure  3.2.9), the interface will be a planar sector. This planar sector is a cross-section of the domain perpendicular to the axis of rotation at a position along the axis between the rotor and the stator.

Figure 3.2.9: 3D Planar-Sector Mesh Interface