10.3.1 The Multiple Reference Frame Model

Overview

The MRF model [ 46] is, perhaps, the simplest of the two approaches for multiple zones. It is a steady-state approximation in which individual cell zones can be assigned different rotational and/or translational speeds. The flow in each moving cell zone is solved using the moving reference frame equations (see Section  10.2). If the zone is stationary ( $\omega = 0$), the equations reduce to their stationary forms. At the interfaces between cell zones, a local reference frame transformation is performed to enable flow variables in one zone to be used to calculate fluxes at the boundary of the adjacent zone. For more information about the MRF interface formulation, see this section in the separate Theory Guide.

It should be noted that the MRF approach does not account for the relative motion of a moving zone with respect to adjacent zones (which may be moving or stationary); the mesh remains fixed for the computation. This is analogous to freezing the motion of the moving part in a specific position and observing the instantaneous flowfield with the rotor in that position. Hence, the MRF is often referred to as the "frozen rotor approach."

While the MRF approach is clearly an approximation, it can provide a reasonable model of the flow for many applications. For example, the MRF model can be used for turbomachinery applications in which rotor-stator interaction is relatively weak, and the flow is relatively uncomplicated at the interface between the moving and stationary zones. In mixing tanks, for example, since the impeller-baffle interactions are relatively weak, large-scale transient effects are not present and the MRF model can be used.

Another potential use of the MRF model is to compute a flow field that can be used as an initial condition for a transient sliding mesh calculation. This eliminates the need for a startup calculation. The multiple reference frame model should not be used, however, if it is necessary to actually simulate the transients that may occur in strong rotor-stator interactions, the sliding mesh model alone should be used (see this section in the separate Theory Guide).

For more information about and examples of multiple rotating reference frames, see this section in the separate Theory Guide.

Limitations

The following limitations exist when using the MRF approach:

• The interfaces separating a moving region from adjacent regions must be oriented such that the component of the frame velocity normal to the boundary is zero. That is, the interfaces must be surfaces of revolution about the axis of rotation defined for the fluid zone. For the example shown this figure (in the separate Theory Guide), this requires the dashed boundary to be circular (not square or any other shape). For a translationally moving frame, the moving zone's boundaries must be parallel to the translational velocity vector.

• Strictly speaking, the use of multiple reference frames is meaningful only for steady flow. However, ANSYS FLUENT will allow you to solve an unsteady flow when multiple reference frames are being used. In this case, unsteady terms (as described in this section in the separate Theory Guide) are added to all the governing transport equations. You should carefully consider whether this will yield meaningful results for your application, because, for unsteady flows, a sliding mesh calculation will generally yield more meaningful results than an MRF calculation.

• Particle trajectories and pathlines drawn by ANSYS FLUENT use the velocity relative to the cell zone motion. For massless particles, the resulting pathlines follow the streamlines based on relative velocity. For particles with mass, however, the particle tracks displayed are meaningless. Similarly, coupled discrete-phase calculations are meaningless.

  An alternative approach for particle tracking and coupled discrete-phase calculations with multiple reference frames is to track particles based on absolute velocity instead of relative velocity. To make this change, use the define/models/dpm/ options/track-in-absolute-frame text command. Note, that the results may strongly depend on the location of walls inside the multiple reference frame. The particle injection velocities (specified in the Set Injection Properties dialog box) are defined relative to the frame of reference in which the particles are tracked. By default, the injection velocities are specified relative to the local reference frame. If you enable the track-in-absolute-frame option, the injection velocities are specified relative to the absolute frame.

• You cannot accurately model axisymmetric swirl in the presence of multiple reference frames using the relative velocity formulation. This is because the current implementation does not apply the transformation used in this equation (in the separate Theory Guide) to the swirl velocity derivatives.

• Translational and rotational velocities are assumed to be constant (time varying $\omega$, $v_t$ are not allowed).

• The relative velocity formulation cannot be used in combination with the MRF and mixture models (see this section in the separate Theory Guide). For such cases, use the absolute velocity formulation instead.

• You must not have a single interface between reference frames where part of the interface is made up of a coupled two-sided wall, while another part is not coupled (i.e., the normal interface treatment). In such cases, you must break the interface up into two interfaces: one that is a coupled interface, and the other that is a standard fluid-fluid interface. See Section  6.4.4 for the steps involved in setting up a coupled interface.

You can switch from the MRF model to the sliding mesh model for a more robust solution. See Section  11.2 for details on how to make this change in the fluid's boundary conditions. Currently, this switch is not possible when running in parallel.