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How to Prepare the CAD for Simulating Rotating Zones?

When performing a simulation with rotating parts such as pumps and fans, we need to use rotating zones. This requires some special CAD preparation in your local CAD tool before importing the CAD model into the SimScale Workbench.

This article outlines the best practices that users should follow while developing their CAD model for simulating rotating zones using Onshape, although they are applicable to all CAD software solutions in general.


The most important requirement when you prepare the CAD for simulation is the rotating zone; a cylindrical volume surrounding all rotating parts. Use the links if you would like to know the background about rotating zones and how to create them.

Because of the rotating zone volume, flow volume extraction is not available for flows with rotating regions. Therefore, the flow volume will have to be prepared prior to importing into SimScale.

Best Practices

The pump in our example contains the housing, rotor blades, and a back cover making the pump watertight. Water enters the pump through the front inlet and leaves through the top outlet. What we need is a volume which represents the regions of the fluid i.e., water, as shown in Figure 1:

prepare cad for simulating rotating zones simscale geometry before and after preparation
Figure 1: Original CAD Model (left) and cleaned CAD model ready involving only the fluid region for the rotation simulation

A) Make Your Model Watertight

To prepare the CAD for simulation, you need to delete unnecessary parts, as well as merge as many parts as possible. Make sure that your model is watertight and that there are no gaps in the geometry except the inlets and outlets. In our case, we needed to merge the back cover to the housing. Once complete, we proceed to the next step:

B) Create the Flow Volume

Now we need to create a box covering the locations which will later be the flow volume. Create a new plane and extrude it to the dimensions necessary.

prepare cad for simulating rotating zones simscale create a box covering the flow volume
Figure 2: Creating a box covering the flow volume


  1. Subtract the rotor blades and the housing from the box
  2. Delete everything except the fluid region
for simulating rotating zones  substract rotor blades and delete unnecessary parts
Figure 3: Subtract the housing and the rotor blades from the box and delete all unnecessary parts.

Now that we have the fluid region, called ‘BOX’, we can rename it to ‘fluid region’, so that we can address it properly later in the process.

C) Extrude the Inlets and Outlets

In order to obtain accurate results, it is best practice to lengthen the inlets and outlets a couple of pipe diameters. The reasons for this practice can be found here while the final result is as demonstrated below:

extruding inlets and outlets avoids divergence
Figure 4: Extrude the inlets and outlets to ensure the flow is fully developed and to avoid possible divergence.

D) Create the Rotating Zone and Export Your Files

Finally, we will create a rotating zone; you can take a look at this article to see how. Now all you need to do is to save the flow volume and the rotating zone in a file.

Congratulations, now you can upload this assembly to SimScale!


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

Last updated: November 10th, 2020

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