Archsim is a multi-zone simulation program for Rhino3D using Grasshopper3D algorithmic modeling. Archsim can simulate natural ventilation in 3 different modes.

1. Scheduled
2. Simple wind and stack equations
3. Airflow network

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The first option is great for coupling Archsim with other tools that return a schedule of air-changes in hourly format. Just link this schedule in the zone settings under the ventilation tab or specify a fixed air change rate.This model does not model airflow from zone to zone.

The second option uses simple formulas explained on the slides below. This model is extremely robust ? especially for buoyancy driven flow. Wind driven flow always assumes an ideal exhaust and basically simulates cross-ventilation ? even if you only model one window ? this can easily lead to overestimation of the wind driven ventilation potential. This model does not model airflow from zone to zone.

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The third option is the airflow network. It provides the possibility to model natural ventilation and more complicated airflow paths through the building. Thus, this is the preferred option ? however ? it might slow down your simulations.

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Performance Network
http://www.performance.network