This paper is to discuss experimentally the flow condition and the vortex formation and shedding around a submerged sphere under wave action. The flow pattern around the sphere can be classified into two types, such as the oscillating and the rotating flow patterns. It is found that the vortex formation and shedding for each flow pattern are quite different. In the typical case of the oscillating flow pattern, the vortex ring is formed in every half cycle. On the contrary, in the rotating flow pattern, wake region is very turbulent and the vortex ring is not formed.
Investigation of flow condition around the coastal and offshore structure is very important for discussion of the wave force generation mechanism and accurate estimation of the acting wave force. Many researchers have been conducted on the flow situation around the circular cylinder (e.g., Ikeda et a1., 1884, Masuda et a1., 1985, Tsuzuki et a1., 1988). However, only a few investigations have been performed on the flow properties around a submerged sphere in time dependent flows (e.g., Jenkins and Inman, 1976). Nakamura et a1. (1983) investigated the flow around the sphere located in a sinusoidally oscillating flow. They showed that the vortex ring is formed on the sphere surface of the downstream side and that the three dimensional structure of the vortex ring plays an important role to the lift force. In the wave field, flow conditions are more complicated since the water particle has two dimensional motion, different from the oscillatory flow field. Iwata and Mizutani (1989) discussed the wave force acting on a submerged sphere In connection with the flow pattern. They pointed out that the wave force and force coefficients are remarkably affected by the flow condition around the sphere.