Aerodynamic characteristics of rectangular cylinders in steady and accelerating wind flow

Abstract

Aerodynamic actions on three two-dimensional rectangular cylinders were studied in steady and accelerating wind tunnel experiments. The aim of these experiments was to determine how body aerodynamics is modified during flow acceleration. The acceleration rate studied roughly scales to that experienced during a strong thunderstorm downburst. As such, conclusions are drawn about wind loading during these events. Cylinders with side ratios of B/D = 1/3, 1 and 3 were tested. Results show that when accelerating flow from initially quiescent conditions, the onset of wake instability is delayed to higher Reynolds numbers than expected in steady flow. Following this, 5 or 6 vortex shedding cycles are required for most aerodynamic characteristics to collapse onto expected steady flow values. During this period, drag and lift force coefficients are generally less than in steady flow conditions. Cases that accelerated flow from an initial Reynolds number of 9,300 also led to modified aerodynamic loads, but no such modification occurred when initial Reynolds numbers were larger. This suggests that initial flow conditions, as well as body side ratio, play an important role in defining how flow acceleration modifies aerodynamic loads. Despite the existence of aerodynamic modifications throughout these experiments, pressures or forces greater than experienced during steady flow conditions at the target velocity (as observed by others in impulsively started flow tests) were not observed. As such, when considering the practical application of this work, for the body configurations tested, the level of flow acceleration experienced during strong thunderstorm downbursts are not expected to lead to larger wind loads than in steady flow conditions.