A new leg-paddle coupling crablike robot is developed based on the bionic prototype of portunus trituberculatus. The construction with three pairs of walking legs and one pair of swimming legs is adapted for the robot. By the composite propulsion of waking legs and flapping hydrofoil
the robot is endowed with the ability of walking and swimming under water to indicates the characteristics of morphological and functional bionics. Numerical simulations and water tunnel experiments are carried out to obtain the hydrodynamic performance of swimming legs. It is found that both the upstroke and downstroke phases generate propulsive load to lead to much higher propulsive efficiency when the hydrofoil is in the lift-based swimming mode. The average thrust and propulsive efficiency decreases apparently with the increasing flapping amplitude. The relationships between the propulsive efficiency and flapping frequency are like a parabola. The maximum propulsive efficiency of the lift-based swimming mode is achieved at Strouhal number of 0.4-0.6
and the propulsive efficiency reaches the maximum value at Strouhal number of 0.8-1.0 for the drag-based swimming mode. The wake structures of the two modes are characterized by reverse Karman vortex street. Furthermore
the effects of hydrofoil thickness of swimming leg on hydrodynamic performance are investigated
and the results show that the average thrust of much thinner hydrofoil is higher as well as less energy consumption
indicating that thinner hydrofoil is favorable for propulsion performance.
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