## 10.10.1 Setting Up Multiple Reference Frames

To model a problem involving multiple reference frames, follow the steps outlined below.

 The grid-setup constraints for a rotating reference frame listed in Section  10.4 apply to multiple reference frames as well.

1.   Select the Velocity Formulation to be used in the Solver panel: either Absolute or Relative. (See Section  10.7.1 for details.)

Define Models Solver...

(Note that this step is irrelevant if you are using one of the coupled solution algorithms; these algorithms always use an absolute velocity formulation.)

2.   For each cell zone in the domain, specify its translational velocity and/or its angular velocity ( ) and the axis about which it rotates.

Define Boundary Conditions...

(a)   If the zone is rotating, or if you plan to specify cylindrical velocity or flow-direction components at inlets to the zone, you will need to define the axis of rotation. In the Fluid panel or Solid panel, specify the Rotation-Axis Origin and Rotation-Axis Direction.

(b)   Also in the Fluid or Solid panel, select Moving Reference Frame in the Motion Type drop-down list and then set the Speed under Rotational Velocity and/or the X, Y, and Z components of the Translational Velocity in the expanded portion of the panel.

Details about these inputs are presented in Section  7.17.1 and in Section  7.18.1.

3.   Define the velocity boundary conditions at walls. You can choose to define either an absolute velocity or a velocity relative to the velocity of the adjacent cell zone specified in step 2.

If the wall is moving at the speed of the moving frame (and hence stationary relative to the moving frame), it is convenient to specify a relative angular velocity of zero. Likewise, a wall that is stationary in the non-moving frame of reference should be given a velocity of zero in the absolute reference frame. Specifying the wall velocities in this manner obviates the need to modify these inputs later if a change is made in the rotational velocity of the fluid zone.

An example for which you would specify a relative velocity is as follows: If an impeller is defined as wall-3 and the fluid region within the impeller's radius is defined as fluid-5, you would need to specify the angular velocity and axis of rotation for fluid-5 and then assign wall-3 a relative velocity of 0. If you later wanted to model a different angular velocity for the impeller, you would need to change only the angular velocity of the fluid region; you would not need to modify the wall velocity conditions.

Details about these inputs are presented in Section  7.13.1.

4.   Define the velocity at any velocity inlets and the flow direction and total pressure at any pressure inlets. For velocity inlets, you can choose to define either absolute velocities or velocities relative to the motion of the adjacent cell zone (specified in step 2). For pressure inlets, the specification of the flow direction and total pressure will be relative or absolute, depending on the velocity formulation you selected in step 1. See Section  10.7.1 for details. (If you use one of the coupled solution algorithms, the specification is always in the absolute frame.)

Details about these inputs are presented in Sections  7.3.1 and 7.4.1.

5.   Initialize the solution using an absolute frame of reference (Figure  10.10.1).

Solve Initialize Initialize...

Select the Absolute option under Reference Frame. If the Relative to Cell Zone option is selected, the initial flow field can contain discontinuities, which can cause convergence problems in the first few iterations.

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