# Investigation of the Pressure Coefficients Correlation Field for Low-Rise Building Roofs
## 3 Experimental Wind-Tunnel Results
### Mean Wind Direction 90\({}^{\circ}\)  
For this wind angle there was in most cases a substantial symmetry of correlation functions for symmetrical pressure taps with respect to the mean flow direction, corresponding to the transverse symmetry axis of the models. Examples can be seen in the comparison of the level curves Aa and Ea in Figure 13, Ba and Da in Figure 16, Ab and Eb in Figure 14, and Aa and Ea in Figure 15. However, the experimental results for the wind angle of 90\({}^{\circ}\)sometimes showed a different correlation trend for certain pairs of nominally symmetrical pressure taps (e.g., Ac and Ec in Figure 13, Ab and Eb in Figure 15), due to physical statistical variations of the phenomena under examination, as well as unavoidable inaccuracies in the measurement chain, as already highlighted in a previous work by two of the authors [41].  
For a wind direction of 90\({}^{\circ}\), the aerodynamic behavior of this kind of roof is very different than for the other two wind angles (0\({}^{\circ}\) and 45\({}^{\circ}\)), because the flow impacts on the higher and wider side of the model.  
The flow detached from the longer edge of the model, and on the flatter roof it continued in an almost straight line after detachment, according to the trend of pressure coefficients [29]. For the curved model, the flow tended to follow the downward curvature along the direction of the wind and to reattach, although only partially. Consequently, the correlation between the pressure taps on the roof and their surrounding areas was larger for the curved models, as for example in the comparison of pressure taps Cc and Cd in Figures 13 and 15 with corresponding pressure taps Cc and Cd in Figures 14 and 16.  
On the other hand, the correlation of the internal pressure taps was also significantly influenced by the height of the model; for a greater height, the previously described phenomena due to the detachment of the flow were accentuated. For the taller models examined here, there were a larger differences of correlation relative to models with different curvatures, compared with the lower models; e.g., pressure taps Cd and Ce in Figure 15 (RHC) and in Figure 16 (RHF), compared with pressure taps Cd and Ce in Figure 13 (RLC) and in Figure 14 (RLF).  
Similarly, in the case of curved models a greater height emphasized the sudden reduction of the correlation for pressure taps on the edge directly hit by the wind (for example pressure taps Ca, Ba, and Da of model RHC in Figure 15 compared with corresponding pressure taps Ca, Ba, and Da of model RLC in Figure 13). For the flatter models, meanwhile, the effect of different heights was substantially irrelevant to the trend of the correlation function for pressure taps along the leading edge.  
More difficult to interpret is the trend of the correlation for pressure taps contained in the areas near the two orthogonal edges of the model where two different curvatures meet, because for this geometry such zones are always disturbed by the detachment of vortices [29]. In these regions, therefore, the curvature and height of the model both played a relevant role for the angle of approaching wind considered in this section (90\({}^{\circ}\)), as indeed was also found for the different wind angles discussed in the previous sections.  
In conclusion, for a wind angle of 90\({}^{\circ}\) the height and curvature of the model both affected the correlation for most of the roof, except for the areas near the leading edge, where the height of the model's windward surface played a predominant role compared with the effect of its curvature.