Fluid dynamics experiments with a passive robot in regular turbulence

This paper presents force measurements of a passive fish robot in a regularly turbulent flow. We placed the robot into a controlled hydrodynamic environment, in running water behind a cylinder which created alternately shed vortices (von Kármán vortex street). We monitored the flow field using digital particle image velocimetry and recorded the force measurements using a force plate. The measurements taken at different locations in the turbulent flow show that the lateral force (perpendicular to the flow stream) experienced by the robot increased significantly in the turbulent flow. On the other hand the drag (force along the flow stream) was reduced up to 42% with respect to swimming in the uniform flow. The drag reduction was mainly due to the shadowing effect of the cylinder. However robots didn´t gain any advantage through their passive interaction with the vortex street. The drag-position relationship had a single minimum along both longitudinal and lateral axis highlighting a favorable location for energy saving. We interpret the results as an evidence that the turbulent flows can provide rewarding opportunities to derive more energy efficient and stable behavioral strategies for underwater robots.