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How to Prepare Your 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 which will be covered in this article.


The most important requirement is the rotating zone; a cylindrical volume surrounding all rotating parts. Use the links, if you would like to know more 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 areas of the fluid, as shown in the picture below:

Prepare CAD for simulating Rotating Zones SimScale Geometry before and after preparation
Original CAD Model and CAD Model for the Simulation

A) Make Your Model Watertight

In the first step, 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. The following picture describes this workflow:

Prepare CAD for simulating Rotating Zones SimScale Create a box covering the flow volume
Creating a box covering the flow volume

Create a new plane and extrude it to the dimensions necessary.
Now, we need to follow the two steps in the following image:

Prepare CAD for simulating Rotating Zones SimScale subtract the rotor blades and housing from the box
Subtract the housing and the rotor blades from the box and delete all unnecessary parts
  1. Subtract the rotor blades and the housing from the box
  2. Delete everything except the fluid region

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 as the picture below demonstrates:

Prepare CAD for simulating Rotating Zones SimScale Extract in and Outlets
Extrude the inlets and outlets

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: October 12th, 2020

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