# Coupling BES / CFD for natural ventilation

## Why ?

In Building Energy Simulation (BES) softwares, pressure coefficients are generally given by correlations according
to the angle of incidence and the shape of the building [Swami & Chandra 1988]^{1} or tabulated values
that are unsuitable for most real-life situations: for example, Figure 1 below shows the values used by some
BES softwares to determine the pressure coefficients \(C_{\text{p}}\).

The calculated natural ventilation flows are therefore very likely to be incorrect if they are estimated from the default values (see also our article on reducing uncertainties in natural ventilation).

## The workaround

First, the pressure coefficients per façade element must be computed, depending on the wind orientation and magnitude (an illustration of the pressure field on a building is given in Figure 2).

Each pressure coefficient must then be assigned to the correct façade element. To achieve this, we go through
the file defining the BES problem (`*.idf`

files in EnergyPlus) and replace the pressure coefficient for each
opening and each wall, in order to calculate the flow rates related to infiltrations.

This technique has two main advantages:

- Compared to a direct use of the flowrates computed with CFD, this preserves the thermal buoyancy effects related to the temperature difference between inside and outside.
- The number of pressure coefficients per wind direction is increased: by default they are given every 45°, while we take a maximum of 30 °.

The question that arise is then: how many wind directions should we simulation for a better prediction of wind-induced natural ventilation? Next section gives an insight about this topic.

## Influence of angular discretization

Figure 4 below shows the natural ventilation rate in a largely glazed train station atrium. The different lines plotted correspond to an increasing number of wind directions simulated: for instance “4 directions” means one simulation is done every 90° and “24 directions” means one simulation every 15°.

One can see that from 8 directions (*i.e.* \(\Delta\theta\) = 45°), the differences tend to decrease. The
error gap between 12 and 24 simulations is relatively low, which in the presented context would advocate
for only 12 CFD simulations. A more thorough quantitative analysis is available in one of our publication in the
Techniques de l’Ingénieur website
.