@chris, @RafaelA sorry for delayed response here, I’m in the middle of multiple deadlines. But I’ve managed to slowly chip away and have two updates.
1. Reason for mismatch between AFN and target room infiltration
First off, I can confirm that @Chris’s initial speculation was correct, the envelope reference delta pressure is the source of most of the mismatch between the AFN infiltration rate and the target IFREA. My guess about moving the aperture edge air flow into the facade area air flow calculation ended up having only a marginal impact on AFN infiltration.
I did some tests on two test AFN models (multizone and single zone). With no changes, the IFREA target (0.000569 m3/s-m2) was an order of magnitude higher than the AFN infiltration rate (0.000085 m3/s-m2) (or ~15% of the target value). This roughly corresponds to the difference between the 4 Pa reference delta pressure and the “actual” simulated delta pressure, as reported in the ‘AFN Linkage Node 1 to Node 2 Pressure Difference’ EP output.
Accordingly, I’ve updated the code to expose a delta_pressure input that can be used to enter a custom reference delta pressure if there’s a need to match the AFN infiltration with the IFREA target.
2. How to calculate the reference delta pressure
While using an average of the output from the ‘AFN Linkage …’ report will get the AFN room infiltration to the same order of magnitude as the IFREA, it’s still fairly imprecise. This is because those output pressures are, in turn, a function of our coarsely approximated reference delta pressure of 4.
The good news is I found a way to calculate the reference delta pressure needed to hit the target IFREA with high accuracy. The bad news is that you need to run at least one simulation in order to capture the ratio that governs the relationship between the reference delta pressure and output pressures. I don’t think there’s any alternative to this, based on my understanding of the problem, so there’s no easy solution that I can include in the code to get AFN rates to match the IFREA automatically. But I can at least explain the formula to calculate it with at just one simulation, so there’s no need to iterate over parameters manually until results converge. The final formula is a little counterintuitive but the derivation is fairly straightforward:
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Start with the power law function between any AFN-simulated infiltration \dot{Q}_{afn} and the AFN-derived delta pressure \Delta{P}_{afn}, and take the log of both sides to make our math simple:
\dot{Q}_{afn} \cdot \rho = C \cdot \Delta{P}_{afn}^n
log(\dot{Q}_{afn} \cdot \rho) = log(C) + n \cdot log(\Delta{P}_{afn}) -
Substituting the flow coefficient C with the formula used in the
solve_norm_area_flow_coefficientfunction. This exposes \dot{Q}_{ifrea} the IFREA target value:
\begin{align} log(\dot{Q}_{afn} \cdot \rho) &= log(C) + n \cdot log(\Delta{P}_{afn}) \\ &= log(\dot{Q}_{ifrea} \cdot \rho) - n \cdot log(\Delta{P}_{ref=4}) + n \cdot log(\Delta{P}_{afn}) \end{align} -
Finally, rearrange to solve for the AFN-simulated delta pressure and get rid of the log:
log(\Delta{P}_{afn}) = -\frac{1}{n} \cdot log(\dot{Q}_{ifrea} \cdot \rho) + \frac{1}{n} \cdot log(\dot{Q}_{afn} \cdot \rho) + log(\Delta{P}_{ref=4})
\begin{align}\Delta{P}_{afn} &= exp(-\frac{1}{n} \cdot log(\dot{Q}_{ifrea} \cdot \rho) + \frac{1}{n} \cdot log(\dot{Q}_{afn} \cdot \rho) + log(\Delta{P}_{ref=4})) \\ &= \sqrt[n]{\frac{\dot{Q}_{afn}}{\dot{Q}_{ifrea}}} \cdot \Delta{P}_{ref=4}\end{align}
And that’s pretty much it. \Delta{P}_{afn} provides the reference delta pressure required for any target room infiltration rate \dot{Q}_{ifrea}. The initial AFN-simulated infiltration rate \dot{Q}_{afn} and assumptions for n, \rho and \Delta{P}_{ref=4} (0.65, 1.225 and 4), are constants in this formula, so only have to be calculated once for a given structure.
The only other thing I’ll add is that I found this worked well for simple AFN models (98% of the target), but got slightly less reliable as model complexity increased in my multizone setup (I guess the data gets noisier with more complex multizone flows). Luckily, the formula above tells us that the logs of \Delta{P}_{afn} and \Delta{P}_{ref=x} are linearly related, so the most accurate way to approximate true values is simply by fitting a straight line to the noise. Specifically, you can iterate through a couple of reference delta pressures, capture the output infiltration rates and plug the logs into a linear regression. In practice I found that my multizone model (the AFN apartment from the sample files) was able to get to 98%+ of the target with just two data points.