Flow Characteristics Analysis of Artificial Reef Wake Region Using Recirculating Efficiency Index

A wake region is a recirculating water flow zone formed behind an artificial reef (AR). The region is richer in nutrients and organic matter than the surrounding water mass, and has lower water flow, which helps promote the feeding and resting activities of marine organisms. Studies on the artificial reef wake region have mainly focused on its size. However, the performance of the wake region cannot be limited to the size, and the velocity of the recirculating flow should be also evaluated as an important factor. Thus, we proposed a recirculating efficiency index of a cube-type AR and examined ways to increase the efficiency index by modifying the geometric characteristics of the reef. When considering seven variations of the basic design (AR1) of the reef, the inclination of the corner members or the increase in the overall thickness of the members due to the reduction of the perforated area did not improve the efficiency index. On the other hand, in the design plan that maintained the thickness of the basic but reduced the perforated area or increased the area of the structural element that blocks the inflow, the efficiency index increased by more than 40%. This is because the flow-blocking function of the design plan was strengthened, generating a relatively low-speed recirculating flow inside and at the rear of the AR. Thus, it can be said that increasing the volume of the reef as small as possible while increasing the area of the structural element that blocks the flow into the reef is beneficial for improving the recirculating efficiency.