Understanding the rotating frame of reference

Rather than modeling the moving machinery passing through the fluid, the software solves the flow equations of the selected domain in a rotating frame of reference (RFR). This is done by introducing the centripetal and Coriolis terms in the momentum equation.

The analysis measures the perturbations in the moving fluid as it passes around the stationary object, and produces CFD results that correspond to the actual flow pattern that would result from the moving machinery or object. Because a frozen disk methodology is used, no averaging is done at the interface between frames of reference. The interface condition neglects the temporal variation of blade position with respect to the geometry in the stationary frame. Generally, this modeling error is greatly diminished the closer the outer geometry in the immediate vicinity of the RFR is to being circumferentially symmetric. Therefore:

• In the case of weak interaction between flow patterns around moving and stationary objects, this approach is reasonable.

• In the case of a strong interaction, the flow solution is valid only for the chosen relative position of the moving and stationary components. To model a strong interaction for the fixed and relative positions, you must use a different approach. For example, to model a rotor and a stator, consider using a mixing plane methodology at the interface. A mixing plane is useful in cases where the rotation rate is very large compared to the velocity in the bulk streamwise direction, so that the flow quantities may reasonably be taken as well-mixed in the circumferential direction.

For example, if a portion of a fan blade was close to a non-rotating surface in one position, but significantly farther from it in another, results may be inaccurate.

The rotating volume may or may not be enclosed by a larger inertial fluid volume. For certain models, you will need to model the adjacent walls of the moving section. To apply a velocity on these walls, use a Flow Surface simulation object.

Example 1

In the following figure, an inertial fluid volume surrounds a fluid volume that is defined in a rotating frame of reference in relation to stationary flow surfaces. At the boundary between the two fluid volumes, the software calculates the flow between the volumes in the rotating and the inertial frames. The rotation of the fluid volume is converted and displayed for post processing.

	(1) Inertial fluid volume(2) Fluid volume in a rotating frame of reference(3) Fan(4) Non-meshed surface between fluid volumes

Example 2

In the following figure, an axial fan displaces flow along a pipe. Three fluid volumes sections are necessary. The moving frame of reference is applied to the section that includes the fan. To model the fixed walls of the pipe on the rotating frame of reference, a spinning surface must be applied on the rotating fluid volume pipe walls.

	(1) Pipe(2) Inertial fluid volumes(3) Fan(4) Fluid volume in a rotating frame of reference

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