Uncertainties due to the use of surface averaged wind pressure coefficients

A common practice, adopted by several building energy simulation (BES) tools, is the use of surface averaged wind pressure coefficients (Cp) instead of local Cp values with high resolution in space. The aim of this paper is to assess the uncertainty related to the use of surface averaged data, for the case of a cubic building with two openings. The focus is on wind-driven ventilation and infiltration, while buoyancy is not taken into account. The study is performed using published empirical data on pressure coefficients obtained from wind tunnel tests. The method developed to calculate the uncertainty is based on comparison of: the flow rate calculated using the averaged values (fAV), and the one calculated using local values (fLOC). The study considers a large number of combinations for the opening positions in the facade. For each pair of openings (i), the values of fLOC_i and fAV_i are calculated. Based on the ratio between fLOC_i and fAV_i the relative error (ri) is calculated. The relative error is presented statistically, providing probability density graphs and upper and lower bounds for the confidence interval (CI) of 95%. For this CI, the conclusion is that 0.24 fAV <fLOC <4.87 fAV