Abstract
A loach has unique propulsion technique by bending its long body and using its tail fin. Its motion and flow around it have been experimentally visualized by particle image velocimetry (PIV). Vortices around a loach and the interactions between the loach motion and water flow are analyzed. Generating and growing vortices by bending its body, it pushes water backward to gain repulsing force, at the same time it moves through vortices reducing the resistance force. When a vortex reaches to the tail fin, it accelerates both sides of the vortex pushing water back and gaining propulsion utilizing the tail fin. After moving forward it leaves a vortex street like reverse Karman vortices which means loach gains propulsion. The role of tail fin and vortex structure was studied in detail by both of horizontal and vertical views (two 2D2v PIV). Flows with and without tail fin, different species of loach were also studied.
