Mixsim: Custom impeller by importing ACIS CAD file -- meaning of parameters
I'm using the "Custom by ACIS file" impeller object in order to define an impeller from an existing ACIS (.sat) file.
- What does the parameter "Nom. Diam. (in file)" mean?
- What is the proper contents and orientation of the contents of the ACIS file?
The ACIS file must contain a single ACIS volume, which represents a single impeller blade. From this blade, multiple copies will then be created, as many, as is necessary to yield the number of blades entered in the MixSim GUI for this impeller object. Also, a hub will be added by MixSim according to the parameters defined in the GUI.
The ACIS file geometry will represent the impeller blade for a certain, "nominal" impeller diameter. If, e.g., the blade's tip is located at a distance of 0.2 m from the Z axis, this corresponds to a "nominal diameter" for the impeller as represented in the ACIS file of 0.4 m. This value should be entered in the "Nom. Diam. (in file)" entry field in the MixSim GUI.
The diameter which is to be used for the impeller in the meshed geometry is entered in the "Diameter" entry field. MixSim will then scale the imported geometry by a factor that results from dividing the "Diameter" by the "Nom. Diam. (in file)" parameters.
The correct orientation of the ACIS volume (representing a single impeller blade) in the ACIS file is described in the MixSim 2.0 release notes:
"The impeller blade volume in the ACIS file must be positioned according to the following rules.
+ The coordinate system origin is the center of the impeller.
+ The positive Z axis is the impeller (shaft) axis.
+ Mathematically negative rotation around the (positive) Z axis puts the impeller (blade) in correct operation to pump the liquid in negative Z direction. (If you understand the positive Z axis as upward pointing vertical and look at the impeller from above, clockwise rotation will put the impeller in down-pumping mode.)
Note that the reverse to the above statement is not automatically true. (This would be ``positive rotation around the (positive) Z-axis puts the impeller in correct operation to pump the liquid in positive Z direction".) For many impellers, inverting the sense of rotation is not a proper means of inverting the pumping direction. Instead, the impeller will have to be put upside down AND the sense of rotation be changed.
Note that the ACIS file must be ``clean" in the sense that:
+ the single ACIS volume which it contains must pass the geometry and topology check in GAMBIT.
+ there may not be any sharp angle (less than approximately 30 degrees, depending on the expected mesh quality) between any two edges sharing a common node at their ends.
+ there may not be any edge that is significantly shorter than is necessary in order to capture any flow-relevant geometric detail in its vicinity.
(If your ACIS file contains a complete impeller, maybe even with shaft/hub, you can still use it. In this case, set the number of blades in the MixSim GUI to "1" [one], so that Mixsim doesn't create multiple copies of the imported geometry. NOTE that the ACIS file still *must* contain a *single* ACIS volume. If a shaft and/or hub is included in the ACIS file, avoid MixSim creating its own hub by specifying small [not zero, though!] dimension for the hub in the MixSim GUI -- the created hub will then completely lie inside the imported geometry and thus just disappear.)