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How To Choose the Seed Face Properly?

A Seed face occurs in the context of flow volume extractions operations. If you are not yet familiar with the extraction of flow volume using SimScale, you can look it up here.

Seed Face Selection

In a flow volume extraction operation, the algorithm doesn’t know where the flow region should be created, as oftentimes there will be multiple options. The Seed face serves exactly this purpose: it indicates where to create the flow region, by acting as the source face for the flow volume.

Let’s say that we have a cube that is hollow inside. We can create a fluid volume to fill the hollow region, or we can create a fluid volume external to the cube. Depending on the configuration of the flow volume extraction operation, both internal and external volumes can be created in SimScale.

flow volume creation internal or external
Figure 1: By selecting the appropriate seed for the flow domain, you are telling the algorithm where to create the fluid region.

There are two ground rules for the seed face selection:

  1. It must be in contact with the flow volume that you wish to create
  2. If the geometry has open boundaries, the seed face must be connected to the Boundary faces. For instance, we cannot select a face from a floating object

We will show some examples below, using operations available in the CAD Mode environment.

Internal Flow Volume

When the geometry has open boundaries, the best choice for a seed face is an internal face that is adjacent to one of the boundaries. For example, in Figure 2, we have a meeting room with a single open boundary, representing a window.

seed face for internal flow regions with boundaries
Figure 2: The best option for a seed face (in blue) is an internal face that is adjacent to the boundary face (in white).

Note that multiple seed faces are valid choices in the case above. For simplicity, the easiest choice is to select a face that is both adjacent to the boundary face and in contact with the flow volume that will be created.

For geometries without boundary faces, you can simply select any of the internal faces, which will be in contact with the flow region. Taking the same cube geometry from Figure 1, if we want to create a fluid region internal to the cube, the necessary steps are in Figure 3:

seed face definition closed geometries cad mode
Figure 3: Before assigning a seed face (in blue), you will have to hide an external wall by right-clicking on it.
  1. Right-click on an external face
  2. Select the hide face option
  3. Assign one of the internal walls, in contact with the internal void, as a seed for the flow volume


Please note that no boundary faces are defined for fully closed volumes.

External Flow Volume

For External flow volume operations, the Seed face definition is always optional. In case the user does not assign any face, the algorithm will automatically create an external flow volume.

cad mode seed face external flow volume
Figure 4: External flow volume operation without a seed face (left), creating an enclosure around the hollow cube (right).

As the image above shows, the default flow region is always external to the geometry. Note that if we define a seed face internal to the cube (as in Figure 3), we are going to end up with the internal flow region instead:

seed face for external flow volume cad mode
Figure 5: If we select a seed face for an external flow volume, it will dictate where the fluid region is created.

Last updated: May 10th, 2021