Experimental studies on drag reduction of the circular cylinder with an upstream rod in tandem

The aerodynamics characteristics and the flow structure of the circular cylinder with an upstream rod in tandem were examined. The pressure distribution on the cylinder was obtained through the Hyscan2000 electronic pressure scanner in a wind tunnel, and the force was obtained by the pressure integration. The flow structure around the cylinder was visualized by the hydrogen bubble technique in a water tunnel. With the different stream-wise spacing ratio between the rod and the cylinder, the flow structures have two patterns: the cavity flow mode and the wake impinge mode. In the cavity flow mode, the drag of the cylinder was dramatically reduced by the upstream rod. With the rod diameter d/D = 1/2(d and D are the diameters of the rod and cylinder, respectively), the maximum drag reduction of the cylinder is 98% that of the cylinder in free-stream. According to the flow structure and the pressure variation on different parts on the cylinder, the mechanism of the drag reduction of this passive control method was discussed. There are two reasons for the drag reduction method: one is the shield effect of the rod on the downstream cylinder, the other is the enhancement of turbulence intensity of the approaching flow. With the rod diameter increased, the latter has more contributions to the drag reduction.