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Documentation

How to Prepare the CAD for Simulating Rotating Zones?

When performing a simulation with rotating parts, such as pumps and turbines, we need to use rotating zones. This requires some additional steps to prepare the CAD before importing the model into the SimScale Workbench.

In short, a CAD model for simulations with rotating zones needs to contain one flow volume region, and one additional volume per rotating zone. This article outlines the best practices that users should follow to prepare their CAD models.

Important

It is possible to generate rotating zones in SimScale or in a CAD software. For demonstration purposes, we are going to showcase the workflows in SimScale and Onshape.

Please note that, as far as CAD preparation goes, multiple workflows will lead you to the same result. This article does not intend to be exhaustive: it only shows one of the possible workflows.

Approach

The most important requirement when you prepare the CAD for a simulation involving rotating motion is the rotating zone: a cylindrical volume surrounding all rotating parts. For information regarding the setup of a rotating zone in SimScale, please visit this documentation page.

Below, you will find examples showing the workflow to prepare the CAD model for a simulation with rotating zones for internal and external applications. Examples are a drone, a water pump, and a wind turbine.

Example 1: Drone (SimScale)

This example features the geometry from the drone simulation tutorial. The geometry initially contains the drone propeller and the arm. Using CAD mode the flow volume region and the rotating zone can be created within the SimScale environment without having to leave the platform.

cad mode access
Figure 1: Drone is a good example for external aerodynamics simulation involving rotating parts. This figure shows how to enter the CAD mode environment fro the SimScale Workbench.
cylinder operation in cad mode
Figure 2: The list of operations supported in CAD mode. Choose Cylinder to create rotating zones.

The Cylinder operation in CAD mode allows the creation of cylindrical volumes that can be used when dealing with simulation domains having rotating zones. There are two methods to create cylinders: Custom and From faces.

When using the Custom method, the center of rotation, axis of rotation, radius, and height of the cylinder need to be specified by the user.

custom cylinder operation
Figure 3: A preview of the cylinder is visible in the viewer before accepting the cylinder settings

The Custom approach requires global coordinates to be provided for the creation of the cylinder. In case you don’t have those readily available, the From faces approach might be preferred.

When using the From faces approach to create a cylinder, the user needs to select the Faces which should be covered by the cylinder:

generating cylinder in cad mode from faces
Figure 4: The box selection tool, highlighted by the blue arrow, can help to quickly select the faces of interest

With this approach, a cylinder that envelops all selected faces is created. Furthermore, a Clearance factor is applied, ensuring that the cylinder is slightly larger than the assigned faces. For simulations involving rotating machinery, a Clearance factor of 1.1 is recommended.

As a result of the Cylinder operation, a brand new volume is created:

generated cylinder in cad mode from faces
Figure 5: A new volume named Cylinder will be visible in CAD mode after running the operation

Creating a rotating zone using CAD mode is suited when dealing with cylindrical rotating regions. For more complex blade shapes and orientations refer to the following examples.

Example 2: Wind Turbine (SimScale)

In this example, a wind turbine geometry containing only the solid turbine parts is used as a starting point.

wind turbine geometry rotating zone
Figure 6: Initial CAD model for the wind turbine, showing the rotor (1), nacelle (2), and tower (3).

Upon bringing this model to SimScale, we can enter the CAD mode environment to perform the following operations:

  1. Create a cylinder to represent the rotating zone
  2. Create a flow region for the external aerodynamics simulation
  3. Delete the solid parts of the wind turbine

The blades from this geometry are at an angle. With the From faces method for the Cylinder operation, the algorithm will automatically generate a rotating zone with the correct angle:

selecting faces for a cylinder cad mode operation
Figure 7: Generating a MRF rotating zone for angled blades in the CAD mode environment with a Cylinder operation

An easy way to quickly select the rotor faces is by having an explicit volume for the rotor (separate from the rest of the geometry). The result will be an angled rotating zone:

rotating zone angle cad mode
Figure 8: CAD mode automatically adjusts the rotating zone to the angles of rotating parts.

After creating the rotating zone volume, the next step is to generate a flow region. In this case, an External Flow Volume operation is the best option:

generating a flow region geometry with rotating zone
Figure 9: A very important step is to assign the rotating zone as an Excluded Part. This will ensure a correct behavior in the simulation run.

Finally, before exporting the new geometry back to the Workbench, the solid parts (tower, nacelle and rotor) need to be deleted with a Delete Body operation:

deleting solid parts simulation with a rotating zone cad mode
Figure 10: For analysis types such as incompressible, only the flow region and the rotating zone volume need to be in the final CAD model. Therefore, the solid parts need to be deleted.

After deleting the solid parts, the CAD model will be correctly constructed for a simulation involving rotating zones.

Did you know?

In a CAD model that is correctly prepared for simulations with rotating zones, the flow region will contain the negative of the solid parts. Note that the solid parts (e.g. the rotor blades) should not be part of the final geometry.

correct cad preparation rotating zone simscale
Figure 11: A CAD model for a simulation with a single rotating zone should consist of a flow region and a rotating zone volume.

Example 3: Water Pump (Onshape)

This example features the geometry from the water pump tutorial. The initial pump model contains the rotor blade, housing, and a back cover. Water enters the pump through the front inlet and leaves through the top outlet.

prepare cad for simulating rotating zones simscale geometry before preparation
Figure 12: Initial geometry for a water pump simulation, highlighting the rotor blades (1), housing (2) and back cover (3)

In the CAD tool, before importing the model to SimScale, we need to create a rotating zone around the rotor blades. Note that the rotor blades volume is centralized at the origin. This will make the next steps much easier to follow.

The first operation consists of creating a Sketch, using one of the planes at the origin. In this case, we will use the Front plane:

creating a sketch on onshape rotating zone
Figure 13: To make the operation easier to perform, you can only keep the rotor blades volume visible

Now, we can use sketch tools to draw a profile for the rotating zone, which will be revolved afterward. Note that the profile should be slightly larger than the rotor in all directions. As a rule of thumb, you can make the rotating zone extend further than the blades in all directions by a distance of \(0.05D\), where \(D\) is the diameter of the blades.

sketch cad rotating part
Figure 14: Since we will revolve the sketched profile, we need to capture only half of the rotor blades in the sketch.

After accepting the sketch, we are ready to use the Revolve operation to create the rotating zone volume, with the following steps:

revolve operation mrf rotating zone
Figure 15: The revolve operation allows you to revolve a profile around an axis.
  1. Create a ‘Revolve’ operation
  2. Select the faces of the sketch as the profile to revolve
  3. The revolve axis will be the baseline of the sketch, which goes through the point (0, 0, 0)
  4. Make sure that you are creating a ‘New’ volume in this operation

At this point, the geometry consists of 4 parts: the back cover, the housing, the rotor blades, and the rotating zone. This geometry can be imported to SimScale for further CAD preparation. To see all CAD import options, please refer to this documentation page.

Note

If you are interested in seeing a different workflow in CAD to create the rotating zone, then consider checking this article.

CAD Mode

The geometry is not yet ready to run a CFD simulation. We need to create a flow region, and also delete the solid parts of the domain.

Both of these operations can be performed in SimScale’s CAD Mode:

cad mode environment simscale
Figure 16: Entering the CAD Mode environment in SimScale

Within CAD Mode, we have two main objectives:

  1. Create an internal flow region
  2. Delete all solid parts from the geometry

Therefore, the first step is to select an ‘Internal flow volume’ operation, and follow the steps below:

internal flow volume extraction cad mode
Figure 17: By excluding the rotating zone from the operation, its volume will be ignored for the flow volume extraction operation.
  1. Create an ‘Internal’ flow volume extraction operation
  2. As Seed face, assign one internal face that will be in contact with the flow region
  3. Under Boundary faces, assign the boundary faces around the openings. Make sure to check this article if you are unsure about the boundary faces of your geometry
  4. Assign the Rotating Zone volume in the Excluded parts tab
  5. Press ‘Apply’ to run the operation

At this point, one volume named Flow region is created in the scene tree on the top right. We are now ready to delete the unnecessary parts of the geometry.

delete body operation cad mode simscale
Figure 18: With this operation, we are only maintaining in the geometry the flow region and the rotating zone volumes.
  1. Pick the ‘Delete’ operation under BODY
  2. Select the rotor blades, the housing, and the back-cover volumes. Note that two volumes remain unselected: the flow region and the rotating zone
  3. Hit ‘Apply’

Before exporting this model to the Workbench, we can also extend the inlet and outlet faces, which is a good practice for simulations with pumps. The Extrude face operation can be used for this purpose:

exporting a cad model from cad mode
Figure 19: Once you hit Export, the current state of the geometry in the CAD Mode is exported to the Workbench.
  1. Select a ‘Move’ operation
  2. Select inlet and outlet faces
  3. Set the Move method to ‘Distance’, with an appropriate Distance of interest
  4. Hit ‘Apply’ to run the operation
  5. Click on ‘Export’ to export the model to your Workbench

This geometry is correctly constructed for simulations with rotating zones in SimScale.

geometry ready for simulations with rotating zones water pump
Figure 20: Correctly constructed geometry for simulations with rotating zones, containing a flow region and a rotating zone volume

Example 4: Wind Turbine (Onshape)

In this external aerodynamics example, the same wind turbine model will be used, but this time the MRF rotating zone will be constructed in Onshape.

The easiest way to prepare the CAD model is by sketching a cylinder, which will be used to construct the rotating zone:

wind turbine mrf creation
Figure 21: Note that the sketched circle is slightly larger than the diameter of the blades.

After creating the initial sketch, we are ready to extrude the MRF rotating zone as a new part. In this step, it’s important that the rotating zone fully envelops the rotating parts:

mrf creation external aerodynamics
Figure 22: Notice that the rotating zone volume is always created as a New volume.

At this point, you can rename the rotating zone volume appropriately, and the model is ready to import to SimScale.

CAD Mode

Once in SimScale, it’s necessary to perform a couple of operations before using the geometry for a simulation. The objectives from this stage are:

  • Generating the flow domain for the simulation
  • Deleting the unnecessary solid parts from the model

Both of these operations can be performed within the CAD Mode environment, which can be accessed from the Geometries tab:

accessing cad mode in simscale
Figure 23: To access the CAD Mode, click on the highlighted icon from the image

Initially, we are first interested in creating an external virtual wind tunnel. The image below shows the steps:

flow volume creation in cad mode
Figure 24: Creating an external wind tunnel within the CAD Mode environment
  1. Select an ‘External’ flow volume operation, which will construct an enclosure
  2. Define the enclosure dimensions according to your geometry and wind direction
  3. Exclude the Rotating zone volume that was created in your CAD tool from the operation. This will ensure that the rotating zone is correctly constructed for the simulation
  4. Hit ‘Apply’ to run the operation

At this point, the geometry is almost ready to run a simulation. To avoid multi-region mesh errors, it’s necessary to delete the solid parts from the geometry. In CAD Mode, we can proceed as follows:

deleting parts rotating zone cad preparation
Figure 25: All solid parts should be deleted from the geometry. The only parts that should remain are the flow region and the rotating zone.
  1. Pick the ‘Delete’ operation under BODY
  2. Select the rotor blades, the nacelle, and the tower. Note that two volumes remain unselected: the flow region and the rotating zone
  3. Hit ‘Apply’
  4. Click ‘Export’ to export the new CAD version to your Workbench

Now that we have only the two necessary volumes to run simulations with rotating zones, we can proceed to creating a simulation.

Note

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

Last updated: September 12th, 2022

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