Abstract
The reduction of drag of a circular disk with the axial-length/diameter ratio c/d<0.5 was investigated. The drag coefficient is 1.17 to 1.0 in the range 0< c/d <0.5 regardless of Reynolds number. To control the flow field around the circular disk, the front edge was cut to a stepwise shape with smaller-diameter d1. The step height and step length, a= (d-d1) /2 and b, are a/d =0.1 and b/d=0.2, respectively. Experiments were performed c/d=0.30, 0.40 and Re= (0.8-8) ×104. The shear layer separated from the corner of the small diameter-section reattaches on the front corner of the large-diameter section. Then, an annular vortex is formed at the step region and the pressure coefficient on the step region becomes negative. This causes thrust on the step region of the disk. In addition, the drag force decreases due to the smaller area of the front surface. As the results, the wake width decreases and the base pressure coefficient rises. The drag coefficient of the circular disk with a step decreases by about one-forth of a standard circular disk.
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