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.
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:
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.
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’.
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.
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’.
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’.
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.
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’.
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:
Applying a Custom Inlet boundary condition.
Proceed to setting the outlet to a ‘Pressure outlet’ 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 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.
Leave the ‘Numerics’ at their default state, and proceed to the ‘ Simulation control’ panel. Fill it in as the following one:
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.
Adding an Area average calculation for the outlet of the reservoir.
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.
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.
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.
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.
Post-Processing
After the simulation run is finished, click on the ‘Post-process results’ option to be redirected to the Post-processing environment.
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.
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.
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.
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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