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.


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.

Import this project into your workspace

Step 1.

Create an Incompressible or Convective Heat Transfer simulation and choose Passive species equal to 1.

Choosing a passive scalar in the In-compressible analysis.

Step 2.

In the Simulation tree, in 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. But setting it to zero may lead to numerical instability.

Setting up the Turb Schmidt no. and diffusion coefficient.

Step 3.

In the Initial conditions, set the Global value for your Passive scalar to be zero.

Step 4

At the inlet, set the Passive scalar value to be Fixed value and zero.

Step 5.

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

Setting up a passive scalar volume source.

Now you are all set up and can create a run. 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. 

The passive scalar variable T1, represents the age of fluid in seconds.

Validation case

The previous project is a validation case of the feature after comparing results with the next 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 does 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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