[MNMK]

Hi,

I simulate a centrifugal ventilator. I would like to know when I set the Domain interfaces what is the different among the Pitch options in cfx- pre Post.

None
Value
Automatic
Specified Pitch angles




Wishes

[AtoHM]

Have you checked the documentation? I remember it being described quite nicely there.
/edit: The modeling guide has approx. 2 pages on this.

[MNMK]

Originally Posted by AtoHM

Have you checked the documentation? I remember it being described quite nicely there.
/edit: The modeling guide has approx. 2 pages on this.

Thanks, just for other members:

5.3.3.2. Pitch Change

To connect dissimilar meshes, an intersection algorithm is used to find theoverlapping parts of each mesh face at the interface; for details, see GGI and MFR Theory in the CFX-Solver Theory Guide. TheCFX-Solver must decide the most appropriate way to perform this intersection. Thechoices are:


•Intersect in physical space. The connection will be made as is with nocorrection for pitch change or rotational offset. This method is usedwhen the pitch change model is set to None.


•Intersection in the radial direction. This method is used when thepitch change model is not None and the radialvariation at the interface is larger than the axial (z-) variation. Forexample, this method is used for the surface of constant z in Figure 5.5: Radial vs. Z Direction.


•Intersect in the axial direction. This method is used when the pitchchange model is not None and the axial variation atthe interface is larger than the radial variation. For example, thismethod is used for the surface of constant radius in Figure 5.5: Radial vs. Z Direction.


If the pitch change model is not None, then theintersection algorithm will fail if the interface has any of thefollowing:


•surfaces of zero radius


•surfaces of constant radius with surfaces of constant z


For example, an interface cannot contain both highlighted regions in Figure 5.5: Radial vs. Z Direction. Instead, two separate interfacesshould be used.

In addition, for best accuracy, the interface should not be highly curved. Forexample, if a small change in radius produces a large change in z and a smallchange in z produces a large change in radius in the same interface, thenseparate interfaces should be used instead.


Radial vs. Z Direction


When there is pitch change, a transformation is also performed betweencomponents to account for the fact that they may not be rotationally aligned.This does not affect the relative position of the components. In Figure 5.6: Misaligned Components the components are not aligned; flowpasses through the overlapping area as expected, but the flow that reaches theinterface at 1 is transformed such that it emerges at 2 having crossed theinterface.



5.3.3.2.1. None

Using None as the pitch change option will cause theconnection to be made “as is.” If you have non-alignedcomponents, as shown in Figure 5.6: Misaligned Components, flowwill only pass through the overlapping region. Therefore, this option shouldonly be used when the overall extent and shape of each side of the interfaceperfectly match and the surfaces are aligned with each other. It maytherefore be suitable when full 360 degree or exactly equal pitch componentsare being analyzed. A pitch change option of Nonecannot be used for a stage interface.


5.3.3.2.2. Automatic

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

For the Frozen Rotor model, the flow variable profiles in the pitch-wisedirection are stretched or compressed to the extent that there is pitchchange 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 byapplying average values calculated from the upstream side of the interfaceto 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 removedacross the interface. The average pressure is maintained at each meridionallocation across the interface, with independent local variation in pressurepermitted.

The computational accuracy degrades rapidly with increasing pitch change.It is recommended that sufficient blade components be analyzed on each sideof the interface to minimize pitch change. Any pitch change will result in anon-physical transient interaction: the entire 360 degree components must besolved for truly “accurate” transient interactions if the pitchratio is non-integer. Small pitch changes will introduce relatively smallererrors, 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, suchas:


•When the vertices on the intersection of the hub and interface arenot all at the same radial and axial position


•When one or both sides of the interface has a 360° or greaterpitch angle


•When one or both sides of the interface has a zero radius


•When one or both sides of the interface has mesh faces normal andparallel to the rotation axis


•When one or both sides of the interface has mesh faces at the hub(that is, low radial or axial position) that are thin in the radialor axial direction


When using the Automatic pitch change option, youshould examine the output file under the MeshInformation heading to confirm that the calculated pitchangles are sensible.

Using the Automatic pitch change option fordetermining the pitch change may result in failure of the solver or asolution that is completely wrong. An example of the flow solution beingwrong is flow exiting a non-overlapping region of the interface or flow notexiting an overlapping area of the interface. In general if the flowsolution through the interface does not make sense, or if the calculatedpitch angles are not sensible in the output file, then you should use the Value or Specified PitchAngles option.


5.3.3.2.3. Value

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


5.3.3.2.4. Specified Pitch Angles

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