Extension

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Extension

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The designed geometry can be extended in meridional direction at the outlet.

The extension defines the Rotor-Stator-Interface (RSI). It is only used for CFD purposes and its position is not taken into account by neighboring components. Typically, the RSI is located in the middle of the rotating and the non-rotating component.

Using the extension is recommended, because otherwise the trailing edges of the blades would just lie on the rotor-stator interface, which can cause both meshing problems and numerical simulation errors. Meshing problems could occur, especially for small values of the blade angle ß2.

The drop down menu Direction sets the direction of the extension. If it is set to tangential, hub and shroud will be tangentially extended. If it's set to connected, the extension will match the next component inlet.

Below you can specify the Length of the extension and whether the length should be measured radial or absolute (i.e. in the direction specified above).
Furthermore, you can set Extension outlet at r = constant or Extension outlet at z = constant, which means that the outlet of the extension is forced to be horizontal (parallel to the z-axis) or vertical (parallel to the r-axis).

The designed extension will be displayed in the diagram automatically.

Limitations

The extension is not available for stators. For CFD simulation of vaned stators, some space after the trailing edge might be needed. This can be achieved by unchecking the Trailing edge fixed on outlet checkbox in the meridional contour dialog.

Possible warnings

Problem

Possible solutions

Length of extension is smaller than the model tolerance.
This is problematic for "Flow domain" creation during model finishing.

The length of the extension is smaller or equal to the distance tolerance. This might cause geometrical defects when sewing faces during Model finishing.

If geometrical problems occur, change the distance tolerance or the length of the extension.

Extension outlet has nearly constant radius. Selecting "Extension outlet at r = constant" is recommended.

The endpoints of the hub and shroud extension have a slightly different radius. This can result in almost flat cone surfaces for the adjacent RSI connection, which may be problematic to import into other CAD/CFD systems.

Set the endpoints of the hub and shroud extension to the same radius by checking the "Extension outlet at r = constant" checkbox.

Extension outlet is nearly vertical. Selecting "Extension outlet at z = constant" is recommended.

The endpoints of the hub and shroud extension have a slightly different z-coordinate. This can result in almost flat cone surfaces for the adjacent RSI connection, which may be problematic to import into other CAD/CFD systems.

Set the endpoints of the hub and shroud extension to the same z-coordinate by checking the "Extension outlet at z = constant" checkbox.

Extension is overlapping neighboring component.

The interface defined by hub and shroud extension overlaps neighboring component.

Reduce extension length or increase gap between impeller and static neighboring component.

Extension is not well defined.

Under certain circumstances it is possible that geometric interface and CFD-interface (extension) are misaligned.

Check configuration of extension regarding:

intersection

matching

inversion (negative extension)

See diagram for its visual representation.