Velocity field measurements above the roof of a low-rise building during peak suctions

• Flow field peak events investigated with synchronized pressure and PIV measurements.
• Peaks on small areas near leading edge correlated with smaller reattachment lengths.
• Peak pressures are related to locally accelerated flow above separated shear layer.
• Temporal development of peaks consistent with process described by Melbourne.

The flow over a low rise building has been investigated through synchronized pressure and velocity measurements. Peak suctions on the upper surface were investigated utilizing ensemble averages, conditioned on the peaks. Near the leading edge, it was found that the reattachment length scales with the size of the roof surface area over which the pressures are integrated, with small areas being associated with reattachment lengths that are significantly smaller than the mean. However, within the separation bubble, but further downstream, the dependence of the reattachment length with the size of the surface area is not significant. Peak suctions are associated with locally accelerated flow near the leading edge of the building, which scale with the size and location of the roof surface area over which the pressures are integrated. Quasi-steady theory under-predicts the peak suctions that would result from these locally accelerated flows. The scale of these accelerated flows is consistent with Melbourne׳s (1979) small scale turbulence parameter. As the instant of the peak suction on a small area near the leading edge is approached, the position of the separation bubble decreases in both length and height as suctions near the lead edge increase. Higher speed flow also emerges along the separated shear layer above the leading edge. After the peak suction the separation bubble grows and large suctions both decrease in magnitude and span a larger area, while high speed flows decrease in magnitude and become dispersed.