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How to Create a Rotating Zone?

When performing a simulation with rotating parts such as pumps and fans, we need to use rotating zones. The current article focuses on one of the many workflows to create a rotating zone once the CAD has been prepared accordingly.

Another workflow for creating rotating zones along with some necessary CAD preparation, for both internal and external rotatory flows, is covered in this documentation:


The rotating zone is an axially symmetric volume which encloses all rotating parts. At the same time, it must not touch any walls which do not rotate. In this article, we will prepare the rotating volume for a pump impeller to be covered by the rotating zone using Onshape, although the discussed practices will be applicable to all CAD software solutions.

comparison of rotating zone geometry and impeller
Figure 1: Impeller geometry (left) and rotating zone (right)

Best Practices to Create a Rotating Zone

A) Create a Surface Offset of the Impeller

The following image shows how to create an offset of the impeller geometry:

Prepare CAD for simulating Rotating Zones SimScale surface offset
Figure 2: Adjust the size of the offset to keep the impeller off the flow volume geometry.
  1. Hide everything except your impeller.
  2. Select the outer surfaces of the impeller and create an offset. To pick the right distance, you need to make visible the flow volume geometry that covers everything.
  3. Adjust the distance of the offset. Make sure that the offset is entirely within the flow volume (make it so small that you cannot see it anymore when the flow volume is visible) .

As a result, we have generated a surface which we will now convert to a volume.

B) Convert the Surface Into a Solid

There are several different workflows to convert the surface into a volume. Some CAD tools have the option to find open surfaces so that you can simply close them and get a solid part as a result. If your CAD tool cannot do that, stick to the following workflow:

Prepare CAD for simulating Rotating Zones SimScale convert surface to solid
Figure 3: Steps 1 and 2 to creating a rotating zone. Convert the surface to a solid body.
prepare CAD for simulating rotating zones, SimScale convert surface to solid
Figure 4: Steps 3, 4, 5 to creating a rotating zone. Convert the surface to a solid body.
  1. Select the top surfaces
  2. Thicken them
  3. Select the bottom surface and extrude it to the top surface
  4. Subtract the thickened part from the extrusion
  5. Delete everything that is not the rotating zone

Congratulations! You have successfully created a rotating zone!


If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

Last updated: March 30th, 2022