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Age of Fluid. Tutorial and Validation Case

In this tutorial, you will learn how to get the “Age of fluid” while setting up a CFD simulation in SimScale. A validation case for the feature is also included.

A visualization of the air travel inside the domain(age of fluid).


An important parameter for thermal comfort and indoor quality other than air temperature, relative humidity, velocity and species concentrations, is the age of fluid.  These fundamental parameters are very important for the evaluation of the comfort conditions, especially for working offices, where people spend a lot of time.

The age of air is the average time for air to travel from a specified inlet to any point in the room. This parameter is very important as new air must be supplied to the occupants, such that they get fresh, clean air and the contaminants are evacuated as quickly as possible.

For a room with volume \(V\), where fresh air is supplied at a volumetric rate \(q\), the mean age of air is given by \(\tau\):

\(\tau [s]=\large \frac{V [m^3]}{q [m^3/s]}\)


Before following the steps you need to upload a geometry and have a valid mesh for your simulation. You can use the next case as a template.
The geometry used is a cubic room with two openings. The first one is the inlet, from where the air enters, and the other one is the outlet. The latter will be used to calculate the air travel.

Import this project into your workspace
You can also take a look at the respective public project.

Simulation Setup

Start this project by creating a new simulation.

create simulation for age of air in the simscale dashboard
Creating a new simulation for the room.

Choose an ‘Incompressible’ or ‘Convective Heat Transfer’ simulation.

Create a new simulation for incompressible fluid flow
Choosing the incompressible analysis for this simulation.

Then set the ‘Passive species’ equal to 1.

Incompressible fluid flow simulation setup with a k-epsilon turbulence model
Choosing a passive scalar in the In-compressible analysis.

In the simulation tree, go to Model, set the \((\small Sc_t)\)Turb. Schmidt number to 1 and the Diffusion coefficient to a very small value e.g. 1e-9 \(\small m^2/s\). Ideally this value should be zero, as we are interested in the transport of a scalar by purely advection and not diffusion. However, setting it to zero may lead to numerical instability.

turbulent schmidt number diffusion coefficient setting with passive species air reservoir
Setting up the Turbulent Schmidt number and the Diffusion coefficient.

Choose ‘Air’ for the whole fluid domain during the material assignment.

Apply Material assignment SimScale Dashboard air density kinematic viscosity
Applying air as the material of the fluid domain.

Initial Conditions

Within the ‘initial conditions’, set the Global value for your passive scalar to be zero.

Initial conditions for the age of air passive scalars
Setting the initial value of the passive scalar at 0.

Boundary Conditions

Set the upper opening to ‘Pressure Outlet’ with a mean value of 0 Pa.

pressure outlet boundary conditions simscale  dashboard forage of air in with a mean value of 0 pa
Setting a mean pressure value of 0 Pa at the outlet of the room.

Add the remaining faces as ‘Walls’ with a no-slip condition.

no-slip wall boundary condition for age of air in a room
Applying no-slip walls condition on the walls of the room.

At the inlet, add a ‘Custom’ boundary condition as the following:

custom inlet condition in the simscale dashboard age of air velocity fixed value intensity inlet dissipation rate passive scalars gauge pressure
Setting the custom boundary condition for the inlet opening of the room.

Advanced Concepts

Lastly in the advanced concepts, add a Volumetric passive scalar source by choosing the entire volume, and assigning a Flux of 1 \(\small [m^3s]^{-1}\).

advanced concepts for age of air add volumetric passive scalar source with φ flux.
Setting up a passive scalar volume source.

Simulation Properties

Before moving on to the Mesh options, set the ‘Simulation control’ panel as below:

simulation control for age of air scotch decompose algorithm
Filling in the simulation control panel.

Result Control

In order to export the mean time of travel from the inlet to the outlet, apply the ‘Area Average’ calculation in the ‘Result control’ panel on the face of interest (the face of the outlet).

result control surface data area average on the outlet age of air.
Setting the result control so that the area average values of the outlet will be calculated.


Choose the Hex-dominant Automatic for the mesh, starting with the ‘Very Fine’ level. Later, you can switch it to ‘Fine’ if you wish, with a smaller computational cost and a reduction of overall core number.

mesh panel hex-dominant internal meshing mode automatic mesh sizing very fine automatic boundary layers
Setting up the automatic mesh properties.

Click on the ‘+’ next to the ‘Refinements’ option, and apply a ‘Surface Refinement’ condition for the walls except from the two openings.

add surface refinement for age of fluid walls
Adding a surface refinement for the walls of the model.

Finally, add another ‘Surface Refinement’ for the inlet and outlet.

add surface refinement age of air inlet and outlet
Adding another surface refinement, for the inlet and outlet of the room.

The mesh will be created after the setup of the whole simulation, and right before the simulation run.

Simulation Run & Post Processing

In order to create a new run, click on the ‘+’ icon next to the ‘Simulation Runs’ option, then apply ‘Start’.

start a new simulation run
Starting a new simulation run.

Post Processing

After the run is finished, your passive scalar variable represents the age of fluid with the units of seconds. This variable is given by T1 in the post-processor. 

Initially, a ‘Cutting Plane’ is added, normal to the ‘Y’ axis, as it is seen below:

add cutting plane passive scalar time travel age of air post processing environment
The distribution of the passive scalar, T1, showing the age of fluid through the domain.

For better visualization of the T1, the ‘Continuous Legend’ option is checked.

smoothing the visualization of the T1 with the 'Continuous legend' feature.
Smoothing the visualization of the T1 with the ‘Continuous legend’ feature.

By applying the T1 value to be demonstrated on the whole model, and then rotating it, the time of the air travel through the whole model can be inspected.

visualization of air post processing environment
Visualization of the age of air through and around the room.

Finally, the mean age of fluid on the outlet was calculated to be 395.69 sec.

Validation Case

The previous project is a validation case of the feature after comparing results with the below publication:

Bartak M, Cermak M, Clarke J A, Denev J, Drkal F, Lain M, Macdonald I A, Majer M & Stankov P 2001, Experimental and numerical study of local mean age of air, Proc. Seventh International IBPSA Conference (Rio de Janeiro, Brazil).

Additional Notes

This method for calculating the age of fluid is limited for simulations which do not have other passive scalars sources. The Schmidt number assigned in the previous steps may not be physical for them.

Further Reading

C. Buratti, R. Mariani & E. Moretti 2011. Mean age of air in a naturally ventilated office: Experimental data and simulations. Journal of Energy and buildings.

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