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How to Create Flow Volume Extractions?

The simulation domain for a fluid dynamics simulation is the region occupied by the fluid (gas or liquid). In many cases the CAD model, however, will not contain the fluid volume by default. In such cases region can be extracted as a single part, using the Flow volume extraction operation within the SimScale workbench.
This can be done on the SimScale platform and is called Fluid Volume Extraction.

In order to illustrate the problem further, see the basic use case of a flow simulation through a pipe below.

example of how flow volume extraction works
Left: Pipe geometry. The goal is to simulate the fluid flow through this pipe intersection. The model only contains the walls of the pipe however. Right: The extracted flow volume through the pipe. This is the region that the fluid occupies and is required as input geometry for the simulation.

There are three different flow volume extraction modes available within SimScale:

  1. Open Inner Region
  2. Closed Inner Region
  3. Enclosure

Depending on your geometry and the fluid region that needs to be extracted, one of the three modes can be used. Find examples below about the application for each of the three types.

Open Inner Region

This operation generates a fluid volume for an open inner region. Like the pipe example above, an open inner region is defined by an enclosed space with a limited number of openings to the outside void. The extraction is performed by:

  1. Selecting the boundary faces that lie between the external environment and the internal (highlighted in blue)
  2. Selecting one internal seed face (pink)
  3. Running the extraction
Flow volume partially created
Flow Volume being created within an exhaust silencer.
extracted fluid volume
Successfully extracted fluid volume.

By default, the original geometry is removed when the fluid volume is extracted. It is possible to keep all original parts by enabling the ‘Keep Existing Parts’ toggle.

toggle option
Keep existing parts option (highlighted in Red)

This will result in a geometry that contains both the fluid volume as well as the outer wall geometry as two separate parts. However, for a simple fluid flow simulation, only the fluid volume is required. Keep the original geometry only in case of a Conjugate heat transfer simulation in which the transfer of heat between the solid parts and the fluid domain needs to be simulated.

Closed Inner Region

This feature enables the creation of fluid regions within closed volumes. The operation here is similar to the case above, except only one internal seed face is required. An example of this extraction mode is demonstrated below for the case of a theater hall. Here the model does not have any openings to the outside, but the space occupied by the fluid is currently void inside the model. Therefore it needs to be extracted for simulation.

theater model 1
Selecting an internal seed face for a theater model
Resulting fluid volume


This option is generally used for external flow problems. The operation is performed by sizing the enclosure and then running the extraction operation. The example below shows a half symmetry air domain being created for a race car CAD geometry.

Half symmetry domain created
Creating an External Fluid Domain
Resulting External Fluid Domain

In the case of an enclosure flow volume extraction it is recommended to size the enclosure box in a way that the turbulent wake behind the model as well as the pressure region in front of the model do not reach the limits of the box.

A demo of the above cases can be found in the link below:


If the above steps are not followed properly the user can run into errors. The possible reasons for these errors are elaborated here.

Last updated: October 9th, 2020

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