A comparison of generic pressure coefficient data for indoor air flow studies

Pressure coefficients across the building envelope (Cp) are an important source of uncertainty in building simulation. Wind tunnel experiments are generally the primary source of Cp data, but in infiltration and ventilation studies, generic values from databases or analytical models are commonly used. Building Energy Simulation (BBS) and Air Flow Network (AFN) programs include several sources of generic Cp values. Those generic values show large differences between each other, which might lead to significant differences in the simulation results depending on the type of building, the building location and the performance indicator under analysis. This paper presents a comparison between five sources of generic pressure coefficient data used for infiltration and natural ventilation studies, namely: the database provided by AIVC, that by ASHRAE and the analytical models CpCalc, CpGenerator and the model by Swami & Chandra. The first part of the paper describes each data source. In order to compare the values provided by each data source, a cubic building is adopted. We focus on three points on the facade: one at the center, one near the upper right corner and one near the lower left corner. Two different shielding configurations are considered: sheltered and unsheltered. The comparison is made for a range of wind attack angles to the facade from 0 to 180 degrees. The higher variation is found in points far from the facade center, especially when the wind is parallel to the facade. Variations in Cp from different sources are significantly higher for the sheltered than for the unsheltered case. The differences found in the Cp values indicate the uncertainty involved. This knowledge might be used to guide research on Cp for ventilation and infiltration purposes, as well as uncertainty studies in building simulation.