Force Production on the California Sea Lions' (Zalophus californianus) Propulsive Surfaces
Open AccessUnlike most fish and mammals that utilize Body and/or Caudal Fin (BCF) swimming, California sea lions (Zalophus californianus) rely on their pectoral flippers to generate thrust. Thrust production is achieved by moving these foreflippers from high above its head and against its abdomen, a motion termed as a 'clap'. This clapping motion is followed by a glide, which is highly unusual for an animal of this size. Due to the absence of an oscillating frequency, resulting wake structure will not resemble the reverse von Karman street, commonly seen in BCF swimming. This study fills the gaps in bio-inspired literature regarding sea lion swimming by studying them from a fluid dynamics perspective.To extract in-field 3D kinematics of the sea lion foreflippers during the clap, a protocol is developed to be used in large uncalibrated volumes of interest. Based on the bending angles of the foreflipper, a soft robotic foreflipper robot is built using silicone outerskin and 3D printed bone structure. Laser scans on an excised foreflipper specimen show that the cross-sections resemble airfoils with a camber ranging from 2.5% to 9.2% of the chord length, and also a variation in the local angle of attack, (-3 to 7 degrees) along the span. This is contrast to the traditionally held viewpoint in literature that sea lion foreflipper cross-sections are symmetric in nature. To achieve near-surface Particle Image Velocimetry (PIV) data on the static foreflipper, a combination of rhodamine and layout fluid-based paint was applied to the model to attenuate reflections. Both the Surface Oil Flow Visualizations (SOFV) and PIV show that the flow over the foreflipper is three-dimensional and vortex dominated. Force measurement experiments on a 3D printed anatomical foreflipper reveal that the foreflipper produces positive lift from angle of attack = -5 to 16 degrees before encountering a gradual stall. Unlike the well-studied leading edge tubercles of the humpback whale flipper that help to delay stall, the trailing edge crenelations of the sea lion foreflipper have no effect in a static sense. PIV experiments on the robotic foreflipper clapping into a flat plate show a distinct vortex on the dorsal side of the foreflipper. The results show that the shed vortex contributes to the momentum in the direction of thrust, rather than forcing a jet of fluid between the foreflipper and the plate. We find that faster clap maneuvers produce higher fluid velocities in an absolute sense, however slower clap maneuvers produce higher conversion of angular to linear velocity. Additionally, phase averaged PIV flowfields show the presence of a dual jet system contributing to the thrust production. Finally, the role of hindflippers during turning maneuvers is studied by calculating the forces generated by it at a wide range of angles of attack. Force measurements indicate the hindflippers function as a biological delta wing by generating added lift due to the dorsal side vortices.
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Kulkarni_gwu_0075A_15400.pdf | 2020-12-17 | Open Access |
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