To study the flow structure around the reef, water flume experiment was carried out. The following results were obtaind by flow visualization.
1. Granular dye scattering method is very useful to visualize the flow patterns and fine vortices around a three-dimensional obstacle under Re ?? 1600.
2. The length between an obstacle and its reattachment point of the flow varies with the shape of the obstacle.
3. An obstacle whose length is between 6 and 8 times of its height generates strong upwelling vortex behind the circlet zone. The height of upwelling vortex is more than 4 times of the height of the obstacle.

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It is well-known that the dimples on the surface of a golf ball increase its flying distance. But the detailed mechanism of the effect of flow by the dimples has not been made clear. This paper deals with the effect of the dimples represented by grooves that were put on the surface of a circular cylinder. For solving the Navier-Stokes equation, the computation was performed using the QUICK method. The grooves were formed to the same sectional shape as the dimples and the number of grooves is 32. The results by computation of the flow pattern around the circular cylinder with and without grooves showed a wide margin of difference.

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