Select None when there is no frame change or pitch change.

The frame of reference and/or pitch is changed but the relative orientation of the components across the interface is fixed. The two frames of reference connect in such a way that they each have a fixed relative position throughout the calculation. If the frame changes the appropriate equation transformations are made. If the pitch changes, the fluxes are scaled by the pitch change.

This model produces a steady-state solution to the multiple frame of reference problem, with some account of the interaction between the two frames. The quasi-steady approximation involved becomes small when the through flow speed is large relative to the machine speed at the interface. Frozen Rotor analysis is most useful when the circumferential variation of the flow is large relative to the component pitch. This model requires the least amount of computational effort of the three frame change/mixing models.

The disadvantages of this model are that the transient effects at the frame change interface are not modeled. Modeling errors are incurred when the quasi-steady assumption does not apply. Also, the losses incurred in the real (transient) situation as the flow is mixed between stationary and rotating components is not modeled.

 

5.3.3.1.2.1. Rotational Offset

The Rotational Offset settings control the angle by which the side 1 or side 2 domains are rotated before the interface calculation is performed. The domain axis of rotation is used to perform the rotation. The rotational offset that is applied to the domains on side 1 or 2 depends on whether domains are rotating on side 1, 2, both, or none. The ways in which rotational offsets are stored in results file and are interpreted by CFD-Post are:

• If the side 1 domains are rotating and the side 2 domains are stationary then a rotational offset will be written for the side 1 domains, and interpreted by CFD-Post.

• If the sides 1 and 2 domains are rotating then a rotational offset will be written for the side 1 domains, and interpreted by CFD-Post.

• If the side 1 domains are stationary and the side 2 domains are rotating then a rotational offset will be written for the side 2 domains, and interpreted as a counter-rotation by CFD-Post.

• If the sides 1 and 2 domains are stationary then a rotational offset will be written for the side 1 domains, but not interpreted by CFD-Post; the rotational offset will not be applied.

You can use a rotational offset if you want to change the relative position of the components on each side of the interface without altering the position of the meshes. The positive direction for an offset is determined by the right-hand rule. Figure 5.4: Rotational Offset shows how a rotational offset will alter the relative position of a rotor and stator component.

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Note:  As an alternative to the Rotational Offset parameter, you can also apply a domain rotation in CFX-Pre and create a new CFX-Solver input file.

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Figure 5.4: Rotational Offset

By default, the domain positions are stored in their original (non-offset) location. To have the domains appear in their offset positions in CFD-Post, include the expert parameter rotational offset for post=t in the problem definition and, in CFD-Post, open Edit > Options, then set the CFD-Post > General option Angular Shift for Rotating Locations to Always rotate. Click OK to save the change.

The Stage model (also known as the Mixing-Plane model) is an alternative to the Frozen Rotor model for modeling frame and/or pitch change. Instead of assuming a fixed relative position of the components, the Stage model performs a circumferential averaging of the fluxes through bands on the interface. Steady-state solutions are then obtained in each reference frame. This model enables steady-state predictions to be obtained for multi-stage machines. The stage averaging at the frame change interface incurs a one-time mixing loss. This loss is equivalent to assuming that the physical mixing supplied by the relative motion between components is sufficiently large to cause any upstream velocity profile to mix out prior to entering the downstream machine component. Stage analysis is most appropriate when the circumferential variation of the flow is of the order of the component pitch.

Stage averaging between blade passages accounts for time average interaction effects, but neglects transient interaction effects. Stage analysis is not appropriate when the circumferential variation of the flow is significant relative to the component pitch (for example, a pump and volute combination at off-design conditions).

The Stage model usually requires more computational effort than the Frozen Rotor model to converge, but not as much as the transient rotor-stator model. You should obtain an approximate solution using a Frozen Rotor interface and then restart with a Stage interface to obtain the best results.

This model should be used any time it is important to account for transient interaction effects at a sliding (frame change) interface. It predicts the true transient interaction of the flow between a stator and rotor passage. In this approach the transient relative motion between the components on each side of the GGI connection is simulated. It ultimately accounts for all interaction effects between components that are in relative motion to each other. The interface position is updated each timestep, as the relative position of the grids on each side of the interface changes. It is possible to use a transient sliding interface anywhere a Stage or Frozen Rotor sliding interface could be used.

The principle disadvantage of this method is that the computer resources required may be large, in terms of simulation time, disk space and quantitative postprocessing of the data. The resource requirement problem is exacerbated if unequal pitch between components occurs. In these situations, spatial periodicity cannot formally be used to limit the analysis to a single blade passage per component. Often the problem of unequal pitch is addressed by modifying the geometry to the nearest integer pitch ratio, which may affect the validity of the analysis. In practice, components of unequal pitch can be treated by solving N passages on one side and M passages on the other side, with N and M determined such that the net pitch change across the interface is close to unity. As with a Frozen Rotor interface, pitch change is automatically accounted for by scaling of flows by the pitch ratio.

It is possible to start a Transient Rotor-Stator computation from a simple initial guess, or from an existing prediction. If you are interested in the start-up transient of the machine, then start from the appropriate physical initial conditions. If you are interested in simulating a periodic-in-time quasi-steady-state, then it may be helpful to first obtain a steady-state solution using Frozen Rotor interfaces between components. This solution will contain most of the overall flow features, and should converge to the desired transient simulation in the fewest transient cycles.

Translational relative motion is not supported at a transient sliding interface (only rotational motion is supported).

Using None as the pitch change option will cause the connection to be made "as is." If you have non-aligned components, as shown in Figure 5.6: Misaligned Components, flow will only pass through the overlapping region. Therefore, this option should only be used when the overall extent and shape of each side of the interface perfectly match and the surfaces are aligned with each other. It may therefore be suitable when full 360 degree or exactly equal pitch components are being analyzed. A pitch change option of None cannot be used for a stage interface.

The Automatic pitch change option is the standard option for accounting for pitch change between components. The pitch ratio is taken to be the ratio of the areas of the two components. The two sides must each have the same radial or axial extent and can therefore only differ in extent in the direction of rotation of the frame of reference. This means that the surfaces on each side of a frame change interface must sweep on the same surface of revolution.

For the Frozen Rotor model, the flow variable profiles in the pitch-wise direction are stretched or compressed to the extent that there is pitch change across the interface. All flows (mass, momentum, energy, and so on) are scaled accordingly, based on the pitch change.

For the Stage (Mixing-Plane) model, the pitch change adjustment is made by applying average values calculated from the upstream side of the interface to the downstream side. The averaging is performed in a conservative manner. Meridional variation of the flow is maintained across the interface. Circumferential variation of all flow variables except pressure are removed across the interface. The average pressure is maintained at each meridional location across the interface, with independent local variation in pressure permitted.

The computational accuracy degrades rapidly with increasing pitch change. It is recommended that sufficient blade components be analyzed on each side of the interface to minimize pitch change. Any pitch change will result in a non-physical transient interaction: the entire 360 degree components must be solved for truly "accurate" transient interactions if the pitch ratio is non-integer. Small pitch changes will introduce relatively smaller errors, large pitch changes will introduce larger errors.

There are a number of scenarios where the algorithm used by the Automatic pitch change option is not valid, such as:

When using the Automatic pitch change option, you should examine the CFX-Solver Output file under the Mesh Information heading to confirm that the calculated pitch angles are sensible.

Using the Automatic pitch change option for determining the pitch change may result in failure of the solver or a solution that is completely wrong. An example of the flow solution being wrong is flow exiting a non-overlapping region of the interface or flow not exiting an overlapping area of the interface. In general if the flow solution through the interface does not make sense, or if the calculated pitch angles are not sensible in the CFX-Solver Output file, then you should use the Value or Specified Pitch Angles option.

This option enables you to explicitly provide the pitch ratio. This is the ratio of the pitch angle from the side with the larger pitch angle to the side with the smaller pitch angle across the domain interface. You can use this option when the precise pitch ratio is critical to the analysis; for example, in a closed system containing multiple pitched components.

This option enables you to specify the pitch angle on side 1 and side 2 of the domain interface. You can use this option when the precise pitch angle is needed or when the area ratio of the two sides is not equal to the pitch ratio.