In industrial CFD consulting, my company has found the “Mean Age of Air” concept to be incredibly useful. It helps leverage a steady-state solution to show some transient-like flow behaviors. The steady-state solution already has characterized the primary and secondary flow structures, so adding in the Mean Age of Air calculation can enable some time-based information to be gleaned from the solution. We also have noticed that the concept is not well-documented in the CFD community, so we put together a workshop on how to incorporate this calculation into your CFD predictions. The workshop is also a great overview on convective vs. diffusive driven flow, so if you are a CFD beginner, it’s worth a listen. A summary and how-to steps are also available here: https://www.azorecfd.com/blog/mean-age-of-air Some applications: - Tank purging/cleaning operations - Ventilation of enclosed spaces - Air exchange rates in HVAC models - Identification of stagnant/bypass flow