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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.

## Overview

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 $$\tau$$:

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

## Geometry

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.

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

Then set the ‘Passive species’ equal to 1.

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.

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

### Initial Conditions

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

### Boundary Conditions

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

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

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

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

### Simulation Properties

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

### 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).

## Mesh

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.

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

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

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’.

### 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:

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

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.

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).