## 11.2.1 Overview

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  10.1, 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  11.2.1).

Note that for flow situations where there is no interaction between stationary and moving parts (i.e., when there is only a rotor), the computational domain can be made stationary by using a rotating reference frame. (See Section  10.2 for details.) When transient rotor-stator interaction is desired (as in the examples in Figures  11.2.2 and 11.2.3), 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  10.3.1 and 10.3.2.

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 Section  6.3.15.) 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 "grid interface.'' The two cell zones will move relative to each other along the grid interface.

 Note that the grid interface must be positioned so that it has fluid cells on both sides. For example, the grid interface for the geometry shown in Figure  11.2.2 must lie in the fluid region between the rotor and stator; it cannot be on the edge of any part of the rotor or stator.

During the calculation, the cell zones slide (i.e., rotate or translate) relative to one another along the grid interface in discrete steps. Figures  11.2.4 and 11.2.5 show the initial position of two grids and their positions after some translation has occurred.

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

Grid Interface Shapes

The grid interface and the associated interface zones can be any shape, provided that the two interface boundaries are based on the same geometry. Figure  11.2.6 shows an example with a linear grid interface and Figure  11.2.7 shows a circular-arc grid interface. (In both figures, the grid interface is designated by a dashed line.)

If Figure  11.2.6 were extruded to 3D, the resulting sliding interface would be a planar rectangle; if Figure  11.2.7 were extruded to 3D, the resulting interface would be a cylinder. Figure  11.2.8 shows an example that would use a conical grid interface. (The slanted, dashed lines represent the intersection of the conical interface with a 2D plane.)

For an axial rotor/stator configuration, in which the rotating and stationary parts are aligned axially instead of being concentric (see Figure  11.2.9), 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.

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