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Tutorial: Mean Age of Air in a Reservoir

This tutorial is a step-by-step guide regarding the calculation of the mean travel of air travel through a 5L reservoir. This time is known as “the age of air”, and it can be calculated by setting a passive scalar (T1). The age of air is an important factor in ventilation and comfort, and has a great impact on air quality. For example, in cases of contamination, the time it takes for the air to travel and be replaced in a room can be crucial.

Relevant Projects & Tutorials:

Geometry Preparation

Step 1: Import tutorial geometry in your workbench

Flow Volume Extraction

This CAD model does not contain the fluid volume by default. However, the fluid region can be extracted as a single part, using the ‘Flow volume extraction’ operation within the SimScale workbench. As this is a model that has openings, the ‘Open inner region’ feature is applied.
You can find a detailed tutorial on flow volume extraction here.

  • Click on the options menu next to the CAD name (‘reservoir’)
  • Choose the ‘Add geometry operation’ option.
  • Pick the ‘Open Inner region’ feature, as seen below:
Prepare cad for flow volume extraction and add geometry operation open inner region air reservoir
Adding a flow volume extraction operation.

For the ‘Open Inner region’ operation do the following steps:

  • For the first ‘Assignment’ panel, pick an outer face of the geometry. After picking it, this will appear in a bright blue color.
  • For the second ‘Assignment’ panel, zoom in on the inlet and pick the following face, that will appear pink in color.
  • Click on the ‘Start’ option and wait until the creation is completed.
add geometry operation Open inner region air reservoir face assignment
Assigning outer and inner faces to the ‘Open inner region’ panel.

Simulation Settings

After the flow volume extraction is finished, click on the ‘Create Simulation’.

Create Simulation SimScale air reservoir after Open Inner region flow volume extraction geometry operation
Creating a new simulation.

Choose the ‘Incompressible’ analysis type on the panel that will pop up, as you can see below, and after that, click on the ‘Create Simulation’ option.

Create New Simulation SimScale Incompressible fluid flow
Choosing the correct analysis type for this flow simulation.

Now it is time to fill in the simulation properties:

  • Choose the ‘k-epsilon‘ turbulence model.
  • Apply the ‘Steady-state‘ option (the area of interest is the final state).
  • Change the ‘Passive species’ option from ‘0’ to ‘1’.
Incompressible fluid flow simulation SimScale Dashboard k-epsilon turbulence model steady-state SIMPLE algorithm Adding passive species
Setting the simulation properties panel at the beginning.

Proceed to the Model panel.

  • Set the ‘Turb. Schmidt number’ to ‘1’.
  • Set the ‘Diffusion coefficients’ to ‘1.00e-9’.
Turbulent Schmidt Number Diffusion coefficient setting Model SimScale dashboard passive species air reservoir
Settings for the Model panel.

Click on the ‘+’ next to the ‘Materials’ tab, and choose ‘Air’ at the panel that will pop up. Finish the material set up by clicking on the ‘Apply’ option.

Apply Material assignment SimScale Dashboard air density kinematic viscosity
Applying air as the flow region’s material.

Keep the ‘Initial conditions’ as they are, and make sure the ‘Passive scalar 1’, that was created during the simulation properties setup, has a ‘Global’ value of ‘0’.

Initial conditions SimScale Dashboard air reservoir mean age of air Passive scalar Global
Setting the initial global value for the passive scalar 1, as 0.

Boundary Conditions

The first boundary condition that is applied is a ‘Custom condition’ on the inlet of the flow region, as seen below:

Custom Inlet condition SimScale Dashboard mean age of air in a reservoir Velocity Fixed value Intensity Inlet Dissipation rate Passive scalars gauge pressure
Applying a Custom Inlet boundary condition.

Proceed to setting the outlet to a ‘Pressure outlet’ with a mean value of 0 Pa.

Pressure Outlet Boundary Conditions SimScale  Dashboard for Mean age of air in a reservoir with a mean value of 0 Pa
Applying a Pressure Outlet condition with a 0 Pa mean value on the outlet.

Finally, set the remaining walls to ‘No-slip’ walls.

No-slip wall boundary condition for mean age of air in a reservoir SimScale Dashboard with wall functions
‘No-slip’ wall condition for the rest of the flow region’s faces.
  • Move on to the ‘Advanced concepts’ and click on the ‘+’ next to the ‘Passive scalar sources’.
  • Add a ‘Volumetric passive scalar source’.
  • Set the ‘(φ)Flux’ to ‘1’
  • Choose the whole flow region and click on the blue checkmark to proceed.
Advanced concepts SimScale Dashboard for mean age of air in a reservoir add volumetric passive scalar source with φ flux.

Leave the ‘Numerics’ at their default state, and proceed to the ‘ Simulation control’ panel. Fill it in as the following one:

Simulation control SimScale Dashboard for mean age of air in a reservoir scotch decompose algorithm
Setting the Simulation control panel.

For the final step of the Simulation settings, go to the ‘Results control’ panel and add an ‘Area Average’ calculation. Choose the face of the outlet, so the variables that we need, like the passive scalar T1, that displays the travel of air, can be examined at the end.

Result control SimScale dashboard surface data area average on the outlet for mean age of air in a reservoir.
Adding an Area average calculation for the outlet of the reservoir.


After the simulation settings are all applied, go on to the Mesh panel.

  • Switch to the ‘Hex-dominant(only CFD’ algorithm.
  • Make sure the ‘Meshing mode’ is switched to ‘Internal’.
  • Change the ‘Fineness’ to ‘Very Fine’.
  • Pick a ‘Number of processors’ according to your needs. The 16 cores are suggested.
Mesh panel hex-dominant internal meshing mode automatic mesh sizing very fine automatic boundary layers SimScale
The Mesh panel for a Hex-dominant (CFD only) algorithm and a very fine mesh.

Add a refinement by clicking on the ‘+’ icon next to the ‘Refinements’ tab. Choose to apply a ‘Surface refinement’ as the following on the inlet and outlet of the flow region, that are of great significance for this simulation.

Add surface refinement SimScale mean age of air in a reservoir inlet and outlet
Adding a ‘Surface Refinement’ to the inlet and the outlet.

For the automatic mesh, the ‘Generate’ button is disabled, and the meshing starts after a ‘New Run’ is created, as it is described in the next section.

Simulation Run & Post-Processing

Create a New Run

In order to set the simulation in motion, click on the ‘+’ icon next to the ‘Simulation Runs’, and select the ‘Start’ option at the panel that will appear on the interface, after you name your simulation as you wish.

Start a new simulation run SimScale
Naming the simulation run before hitting the ‘Start’ button.

Initially, the meshing procedure will begin, and after the mesh is created, the analysis will take place as well.


After the simulation run is finished, click on the ‘Post-process results’ option to be redirected to the Post-processing environment.

Post-Process results finished simulation run SimScale mean age of air in a reservoir
Redirection to the post processing environment after the run is completed.

Cutting planes can be used to visualize the variables’ distribution through the part. Click the ‘+’ on the ‘Cutting Planes’ options, then pick the ‘Z’ axis that will be normal to the plane.
Switch the ‘Scalar’ to ‘T1’ so you can examine the time of travel of the air inside the flow region.

Add cutting plane Passive scalar time travel mean age of air in a reservoir post processing environment SimScale scalar
A cutting plane normal to the Z axis showing the time travel of the air inside of the reservoir.

The option ‘Continuous legend’ adds smoothing to the gradual heat transfer visualization.

Add continuous legend Passive scalar time travel mean age of air in a reservoir post processing environment SimScale scalar
Enabling the ‘Continuous legend’ feature for the passive scalar distribution.

Finally, you can also check the mean age of air in the reservoir, by clicking on the ‘Area Average 1’ that you set earlier, and then on the ‘T1’. That way you can examine the mean time of travel from the inlet to the outlet.

Area average simulation results passive scalar mean age of air in a reservoir outlet time of travel SimScale
The average value of the passive scalar on the outlet.

For the Mean age of air in a reservoir project, it was found to be 9.8736 sec, as you can see in the description in the comment section.

Last updated: March 27th, 2020

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