Calculation of facade pressures for natural ventilation in semi-open spaces

With the reduction of energy consumption in buildings and the increasing use of natural ventilation as a means of cooling, particular attention needs to be paid to the estimation of pressures on the envelope, which allow us to estimate air renewal linked to wind through openings or by infiltration.

The difficulty of calculating them has led to the development of widely-used rules of thumb that allow them to be obtained from simple correlations.

To characterize the thermal behavior of a building through simulation, we need to take into account the coupling between thermal and aeraulic phenomena. This requires knowledge of the pressures at the facade and at the openings. Thus, for each angle of incidence of the wind, we introduce the notion of pressure coefficient (C _p ), fraction of the dynamic pressure in the flow undisturbed by the building. The C _p eliminates the need for a systematic calculation of the stagnation pressure on openings for all incident wind amplitudes, and provides pressure differentials between facades.

In current Dynamic Thermal Simulation (DTS) software, pressure coefficients are generally given by correlations as a function of the angle of incidence and the shape of the building . These relationships are valid for isolated buildings of low height and regular shape. Corrective tables exist for regular urban environments , but these are limited to regular environments, aligned or offset, for two configurations:

• twice as small as the building under study ;

• of the same height as the building under study.

Additional empirical correction factors, ranging from 0.33 to 0.99, are applied directly to natural ventilation rates if significant masking effects are to be taken into account. Additional corrections exist for geometry, when the building of interest has a simple geometric shape: an "L"-shaped building, or an open courtyard ("U"-shaped).