Adding a Mean Age-of-Air Contour
A variable not yet mentioned that can be used to help determine where stagnant air exist in the facility is the Mean Age-of-Air. The mean age of air looks at how “old” particles are if they are released from the upstream boundary of the domain. Once a contour is defined, if you zoom out and look in plan view (as in the below figure), then you will see the mean age-of-air gradually increase in the direction of the wind. The value at the end of the domain, if it was empty, would roughly be (Domain Length) / (Wind Speed) i.e. how long has it taken the air to flow through the domain.
The age-of-air is simply how old the air is since it started at the upstream boundary.
In more complex flows with geometry air can stagnate either due to re-circulation, or simply due to a region of lower wind speed. In these regions, air can enter them from any number of historical paths so depending on the model and wind scenario, the air can have a range of ages. This is why the variable is the ‘mean age’ not just ‘age’.
The actual value of the mean age of air is not really very important. It would be higher, for example, if you just moved the inlet boundary further upstream, or made the domain bigger – neither of these would actually affect the flow on the site, but the maximum value of mean age would increase.
This variable can be visualized in contours and isosurfaces. The mean age-of-air variable is also available in the various monitor data windows.
Adding a Mean Age-of-Air contour to the project
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Turn off any visualizations that are already showing in the 3D window
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From the Add Items tab, select Contour from the Select Item dropdown menu.
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Set the Simulation as Westerly, 5m/s
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From the Variable dropdown menu, select Mean Age-of-Air, s
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Using the XY Plane as the On Plane value, enter a height of "2" as the Offset At value.
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Set the Relative to value as Domain and click the Add Item button
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In the Project Items tab, right-click on Mean Age-of-Air at z = 2 meters and select
Link to Legend to have the legend sync to this contour and appear on the 3D window.
Tutorial 4 - Figure 13 - Top view of Mean Age-of-Air contour added to the project for the Westerly wind case
The orange and red areas, downwind and east of the large tanks, show that the air flow in this direction gets trapped. These regions are more prone to re-circulation and would cause difficulties if a gas cloud were to be near or in these regions.
Notice also in Figure 13 above that the outer bounds of the contour show the gradual progression with time.
The real value of the mean age of air variable is qualitative, not quantitative. In some CFD applications it is common to use a mean age of air variable to get a 3D view of where the stagnation regions are. This is not an exact science as you have to tune the value of the mean age to make it look decent, but does give good insight into where the site is most poorly ventilated.
One method or type of analysis would look at these poorly ventilated areas and then simulate gas leaks pointed towards these regions as they may be considered a worst case scenario. For the westerly wind case used in this section, one may choose to investigate a westerly pointing leak from the same flange as 'HP Release 01' as it is a poorly ventilation region, indicated by the orange a red regions in the figure below.
Tutorial 4 - Figure 14 - Indication of one of the poor ventilation areas for the Westerly wind case
Save your Tutorial 4 file as it will be used at the start of Tutorial 5. Note that the file size for the nine total simulations with the example CAD file is around 21 MBs and contains all the data for the 9 CFD simulations.
