The applicability of quasi-steady theory to pressure statistics beneath roof-top vortices

During cornering winds, dual conical vortices form in the separated flow al ong the leading edges of flat roofs. These vortices cause the most extreme wind induced suction forces found anywhere on the building, so it is import ant to predict them accurately. The quasi-steady theory is commonly used to predict building surface pressures using upstream flow conditions. However , many studies have concluded that the quasi-steady theory should not be us ed in the separated flow regions of a building, because it underpredicts th e peak and rms pressure coefficients, C-pv and C-prms. A wind tunnel study of a low-rise building is performed to examine why C-pv and C-prms are unde rpredicted. The study uses simultaneous pressure and velocity measurement t o assess the basic assumption of quasi-steady theory in this situation, whi ch is that an instantaneous change in wind direction (omega) will have the same effect on vortex position and strength as a long-term change in omega. This assumption is found to be valid only for wind angles of 45 degrees +/ - 10 degrees, and primarily for low-frequency changes in omega. This ought to actually result in an overprediction of C-pv and C-prms, as quasi-steady theory is shown to overestimate the effects of vortex motion due to latera l turbulence. However, the quasi-steady theory ignores the contributions to C-pv and C-prms from random vortex motion and random changes in vortex str ength. The authors apply an analytical model of the vortex flow that links vortex behaviour to surface pressure to assess these contributions, and sho w that their absence results in the net underprediction of C-pv and C-prms, even when the quasi-steady theory is applied fully, with no linear simplif ications. (C) 2001 Elsevier Science Ltd. All rights reserved.