Protection of the coast has been widely considered by many researchers and ocean engineers. Breakwater or submerged obstacles is one of the method used for protection of the coast. Breakwater, by generation of turbulence flow and vortices causes dissipation of wave energy. In present study, the effect of the distance of two submerged rectangular obstacles on the flow structure, vortex generation, and evolution were investigated using Particle Image Velocimetry (PIV) technique. The PIV is an experimental method that tracking particles seeded to the fluid and tracked by laser light are captured frame by frame using a video camera. The PIV technique was used to calculate the fluid velocity over the flow field is calculated by means of applying image processing techniques on captured frames. First, a channel with dimensions of 600*60*30 cm and piston wave maker were designed and constructed. Second, two rectangular obstacles with interval of 4, 8 and 16 cm are placed inside the channel. Then, velocity field of passing solitary wave over the placed obstacles were obtained by using PIV techniques. Flow pattern showed that the distance of second obstacles from the first one has a direct effect on the size and strength of vortices formed on lee side of front obstacle and weather side of rear obstacle. The results show that when the distance between two obstacles is 4 cm the vortex is not formed on the second obstacle and when the distance between two obstacle increase the larger and more powerful vortex is formed on the second obstacle. Also, when the distance between obstacles increases the larger and more powerful vortex is formed behind the first obstacle due to have more space. Moreover, the reduction of wave height before and after obstacle is measured through wave gage altitudes. The result demonstrated that the maximum wave reduction occurs when the distance of two obstacles from each other is 8 cm which dissipates almost 11% of wave energy. Furthermore, the drag force generated over obstacles was calculated by solving momentum differential equation on and the results showed that the maximum force is about 40 Newton. The horizontal component of velocity was obtained at different distances when the shear layer formed behind the first and second obstacles. The results show that the distance between two obstacles does not effect on profiles of horizontal velocity component of second obstacle but the distance between two obstacles is effective on profiles of horizontal velocity component of behind the first obstacle. Keywords: Solitary Wave, Velocity fields, Vortex generation, PIV technique, Breakwater, Wave Maker
