Evaluation of effect of micro-topography on design wind velocity

Abstract

Wind flow around micro-topographic features is very complicated. It includes convergent flow, separated flow and reverse flow. Since wind directions and velocities vary greatly around such features, it is very important to estimate accurately the flow fields around them. We have utilized a triple-split probe that can measure wind directions and velocities in complicated flow fields, and investigated micro-topographic effects on design wind velocity by conducting wind tunnel tests. This paper introduces micro-topographic effects, “topographic multipliers”, for several kinds of up-slope cliffs. When no flow separation is observed, for slope gradients of 7.5° and 15°, topographic multipliers for mean wind velocities take maximum values of 1.3–1.4 at x/ H=0, but those for fluctuating wind velocities are small. When flow separation is observed, for slope gradients of 30°, 45° and 60°, topographic multipliers for mean components take maximum values of 1.3–1.4 at 0⩽ x/ H⩽1, and those for fluctuating components become very large in a separation bubble, and the influence of topography reaches x/ H=6 and z/ H=2 for each slope. No flow separation is observed for a curved surface whose mean slope gradient is 24°, and distributions of topographic multipliers resemble those for a slope gradient of 15° at x/ H⩾0. Thus, topographic multipliers for a curved surface can be predicted by its mean slope gradient.