FLOW PAST A ROUNDED SQUARE CYLINDER ? EFFECTS OF ROUNDING ON FLOW CHARACTERISTICS

초록

Flow past a rounded square cylinder has been numerically studied in order to elucidate the effects of rounding the sharp corners of a square cylinder on the flow topology and flow-induced forces. Rounding the corners was done by inscribing a quarter circle of radius r in each corner of a square cylinder of height d (Fig. 1). Nine cases of rounding were considered ranging from a square cylinder (r/d=0) to a circular cylinder (r/d=0.5) with an increment of 0.0625. Each cross-section was numerically implemented in a Cartesian grid system by using an immersed boundary method. The Reynolds number (Re) and the corner radius ratio (r/d) are the key parameters. Rounding the sharp edges of a square cylinder significantly affects the flow topology (Fig. 2(a)). As a result, a small rounding delays the flow separation towards the trailing rounded edges, resulting in lower lift fluctuation than those of the two extreme cases (square and circular). The minimum mean drag also occurs when the sharp corners of a square cylinder are “partially” rounded (Fig. 2(b)). The drag reduction is achieved because rounding reduces pressure force on the upstream cylinder face, especially near the stagnation point. However, this reduction is offset by the decrease of the base-point pressure with increasing r/d, yielding the “optimal” r/d for drag reduction (Fig. 2(b)). The optimal corner radius ratio (r/d) for lift fluctuation or for mean drag depends upon Re, and high-Re flow tends to be more sensitive to small rounding. We also performed simulations with leading-edge rounding only, and with trailing-edge rounding only, respectively. We found that the main topological changes resulting from four-edge rounding can be obtained by leading-edge rounding alone. Trailing-edge rounding, however, plays a positive role in drag reduction and stabilizing the flow when the two types of rounding are combined.