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:

• Mean Age of air in a reservoir – public project
• Age Of Fluid

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:

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.

Simulation Settings

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

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’.

Proceed to the Model panel.

• Set the ‘Turb. Schmidt number’ to ‘1’.
• Set the ‘Diffusion coefficients’ to ‘1.00e-9’.

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.

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’.

Boundary Conditions

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

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

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

• 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.

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

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.

Mesh

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.

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.

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.

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

Post-Processing

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

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.

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

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.

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