# Best way to model interzone airflow through Air Walls with AFN?

This can be further broken down into two questions, first, what is the best way to model interzone airflow through Air Walls with the EnergyPlus AirflowNetwork, and secondly, (assuming cracks are a good way to achieve interzone air flow) how can I derive an appropriate flow coefficient for this scenario?

**1. What is the best way to model interzone airflow through Air Walls with the AFN?**

The EnergyPlus 'Tips and Tricks' section on Air wall, Open air connection suggests using a large vertical opening between zones with the AFN, which seems to realistically capture both the volume and two-way flow of air between two zones open to each other. However I have since learned that EnergyPlus does not support using the SimpleOpening with BuildingSurfaces (only with FenestrationSurfaces). Our team would like to use the Air Wall, rather then a FenestrationSurface, to capture radiant exchange between zones.

The next best strategy seems to be to model the airflow using an extremely leaky crack. There seems to be some precedent for this, referenced in the DetailedOpening Input/Output documentation, which suggests using an AFN Crack with a "large air mass flow coefficient" for large horizontal openings, since regular horizontal openings won't model two-way flow in exterior surfaces. While this method won't capture the two-way flow that occurs in the AFN SimpleOpening, its seems like I should be able to reproduce the volume (with a correctly chosen flow coefficient), while maintaining the radiant exchange properties of the Air Wall.

**2. How to derive a Cq?**

If the second strategy seems reasonable, the second question is: what is the best approach to derive a mass flow coefficient that would replicate the air flow quantity of a large-ish vertical opening? My instinct is to try and approximate the mass flow coefficient from the formula used for the surface EffectiveLeakageArea?

The derivation I have in mind is as follows:

```
The crack formula is:
m = Cq * dP^n
where:
m_crack = mass flow rate kg/s
Cq = mass flow coefficient
dP = pressure difference across crack
n = mass flow exponent
```

The ELA formula is:

So to derive the Cq from the ELA formula:

```
m_crack = m_ela
Cq * dP^n = ELA * Cd * sqrt(2 * density) * dP_r^(0.5 - n) * dP^n
The dP^n cancels out, I assume n = 0.5 (for turbulent flow) which makes dP_r(0.5-n) = 1, and rearrange to get:
Cq = ELA * Cd * sqrt(2 * density)
```

So I plug in a large opening area for the ELA parameter, and some assumption for air density to get the Cq for the interzone crack. I realize I can use the Surface:ELA object to achieve the same result, but we've already programmed infiltration into our software using the AFN Crack method, so would like to avoid introducing a new EP object, if possible.

Please let me know if this seems reasonable, of if there's a better way of modeling interzone air mixing with an Air Wall. Thanks in advance.