Abstract—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. I. INTRODUCTION TATE-of-the-art underwater robots do not model or navigate with respect to the flow. Research robots in laboratory conditions are tested in still water. Field robots are exposed to the flow but they treat the current or turbulence as a drift or disturbance to be compensated for. There is no underwater robot that takes advantage of the flow for better localization and navigation. While robot builders consider flow as an annoying disturbance to be compensated by control algorithms, the biological evidence suggests that aquatic animals know how to turn it into an advantage. For example it is suggested that salmonoids migrating upstream in turbulent rivers spend time behind an object to recover from fatigue [1]. The metabolic consumption of oxygen by the rainbow trout is lower while entraining in the vortex wake [2]. Furthermore it is shown in many occasions that fish can minimize their energy consumption by adjusting their locomotion patterns to the vortex patterns. The best known study is by Liao et al. demonstrating a dead fish floating upstream behind a vortex wake of a bluff object [3]. This study suggests that the vortices encountered by the fish can be beneficial to reduce, This work is supported by European Union 7 th Framework program under FP7-ICT-2007-3 STREP project FILOSE (Robotic FIsh LOcomotion and SEnsing), www.filose.eu 1 Centre for Biorobotics, Tallinn University of Technology, Estonia 2 Dept. of Computer Science, University of Verona, Italy neutralize or even overcome the drag experienced by the fish due to the flow. A similar experiment has been repeated with a high aspect-ratio passive hydrofoil [1]. Today it is not well defined under which circumstances such positive fluid-body interactions can take place. When size of the vortices are too big or too small with respect to the body size or if the body does not interact with the vortices in the “right way'', it is expected that vortices can have negative impacts on the interaction such as increasing the drag on the body or obstructing its stability [4]. To gain insight into how to interact with the vortices many studies have investigated the phenomenon of Kármán gaiting, the tendency of fish to synchronize with the periodically shed vortices. Fish adjust their tail beat frequency with the vortex shedding frequency. There is no conclusive evidence and established consensus about whether the flow-exploiting behaviors, such as entraining and Kármán gaiting are passive or active (whether fish activate their muscles or they are simply actuated by the external forces due to the flow.) Also it is not known if fish need flow sensing to have a control on these behaviors. Moreover it is not possible to directly measure the drag of a swimming fish and the indirect measurements are rather imprecise. It is therefore only possible to indirectly estimate the energy consumption of fish in different flow regimes [2]. In this paper we develop a case for the exploitation of the same energy saving phenomena by an underwater fish robot. In robot applications we see two ways of taking advantage of the flow: i) to seek a position in the flow at which the drag is lower, ii) to interact with the flow “properly'' to capture the energy which is readily available in the flow. We describe experiments with passive underwater fish robots attached to the force plate in a regular turbulence. We record the force measurements both in downstream and lateral directions and we visualize the flow using digital particle image velocimetry. We analyze the relationship between the drag (proportional to the energy consumption of the robot) and flow speed at different locations in the vortex wake. We demonstrate that by choosing a convenient position inside the vortex wake, the robot can decrease the perceived drag. We furthermore propose a method to evaluate if favorable drag conditions arise from choosing the right spots in the flow or additional energy harvesting as a result of passive flow-body interaction also takes place. Fluid Dynamics Experiments with a Passive Robot in Regular Turbulence Gert Toming 1 , Taavi Salumäe 1 , Asko Ristolainen 1 , Francesco Visentin 2 , Otar Akanyeti 2 , Maarja Kruusmaa 1 S 532 Proceedings of the 2012 IEEE International Conference on Robotics and Biomimetics December 11-14, 2012, Guangzhou, China