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1. Hydrodynamic analysis of fin–fin interactions in two-manta-ray schooling in the vertical plane

   https://doi.org/10.1088/1748-3190/ad1b2e  

   Huang, Zihao; Menzer, Alec; Guo, Jiacheng; Dong, Haibo (January 2024, Bioinspiration & Biomimetics)

   Abstract This study investigates the interaction of a two-manta-ray school using computational fluid dynamics simulations. The baseline case consists of two in-phase undulating three-dimensional manta models arranged in a stacked configuration. Various vertical stacked and streamwise staggered configurations are studied by altering the locations of the top manta in the upstream and downstream directions. Additionally, phase differences between the two mantas are considered. Simulations are conducted using an in-house developed incompressible flow solver with an immersed boundary method. The results reveal that the follower will significantly benefit from the upstroke vortices (UVs) and downstroke vortices depending on its streamwise separation. We find that placing the top manta 0.5 body length (BL) downstream of the bottom manta optimizes its utilization of UVs from the bottom manta, facilitating the formation of leading-edge vortices (LEVs) on the top manta’s pectoral fins during the downstroke. This LEV strengthening mechanism, in turn, generates a forward suction force on the follower that results in a 72% higher cycle-averaged thrust than a solitary swimmer. This benefit harvested from UVs can be further improved by adjusting the phase of the top follower. By applying a phase difference of $\pi /3$to the top manta, the follower not only benefits from the UVs of the bottom manta but also leverages the auxiliary vortices during the upstroke, leading to stronger tip vortices and a more pronounced forward suction force. The newfound interaction observed in schooling studies offers significant insights that can aid in the development of robot formations inspired by manta rays. 

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2. Passively actuated, triangular radiator fin array

   Cannon, Josh R (August 2020, NASA Thermal and Fluid Analysis Workshop, August 18-20, 2020)

   null (Ed.)

   Thermal control is a challenge for spacecraft as they must maintain internal components within operating limits despite significant fluctuations in external and internal thermal loads. Satellites often rely on dynamic thermal control to manage internal temperatures depending on the thermal environment. However, many of these systems are actively managed, relying on the satellite’s internal electronics to control the radiator’s behavior. The problem of thermal control is compounded for small satellites, such as CubeSats, which have high power dissipation per unit surface area, stringent size/weight restrictions, and reduced thermal mass. Passive thermal control is particularly attractive for such small systems, potentially offering increased reliability and simplicity. Attempts at passive, dynamic thermal control of spacecraft radiators have been demonstrated in the literature using louvers actuated by bimetallic coils and radiators deployed by shape memory alloys. In this work, we propose a dynamic thermal control method for CubeSats by using bimetallic coils to passively deploy an array of four triangular radiator fins that, when folded, comprise the external face of a CubeSat. This approach differs from previous approaches as it uses mass efficient, triangular radiative fins as well as bimetallic coils to passively actuate the panels, as opposed to shape memory alloys. The advantages of this design include reduced complexity, cost, volume, and weight when compared to traditional deployable radiators in addition to increased redundancy by using an array of panels. An experimental demonstration of the proposed design is presented indicating the ability to passively deploy a single radiator fin using custom bimetallic coils at a rate of approximately 3.9° of angular rotation per 1 °C with minimal hysteresis. A preliminary model of our design indicates the possibility to achieve a turndown ratio of greater than 7:1. Experimental and numerical prediction results are presented as a motivation for exploration of the proposed design in ongoing work. 

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3. A ferroelectric-gate fin microwave acoustic spectral processor

   https://doi.org/10.1038/s41928-023-01109-5  

   Hakim, Faysal; Rudawski, Nicholas G.; Tharpe, Troy; Tabrizian, Roozbeh (February 2024, Nature Electronics)
4. Deep evolutionary origin of limb and fin regeneration

   https://doi.org/10.1073/pnas.1900475116  

   Darnet, Sylvain; Dragalzew, Aline C.; Amaral, Danielson B.; Sousa, Josane F.; Thompson, Andrew W.; Cass, Amanda N.; Lorena, Jamily; Pires, Eder S.; Costa, Carinne M.; Sousa, Marcos P.; et al (July 2019, Proceedings of the National Academy of Sciences)

   Salamanders and lungfishes are the only sarcopterygians (lobe-finned vertebrates) capable of paired appendage regeneration, regardless of the amputation level. Among actinopterygians (ray-finned fishes), regeneration after amputation at the fin endoskeleton has only been demonstrated in polypterid fishes (Cladistia). Whether this ability evolved independently in sarcopterygians and actinopterygians or has a common origin remains unknown. Here we combine fin regeneration assays and comparative RNA-sequencing (RNA-seq) analysis of Polypterus and axolotl blastemas to provide support for a common origin of paired appendage regeneration in Osteichthyes (bony vertebrates). We show that, in addition to polypterids, regeneration after fin endoskeleton amputation occurs in extant representatives of 2 other nonteleost actinopterygians: the American paddlefish (Chondrostei) and the spotted gar (Holostei). Furthermore, we assessed regeneration in 4 teleost species and show that, with the exception of the blue gourami (Anabantidae), 3 species were capable of regenerating fins after endoskeleton amputation: the white convict and the oscar (Cichlidae), and the goldfish (Cyprinidae). Our comparative RNA-seq analysis of regenerating blastemas of axolotl and Polypterus reveals the activation of common genetic pathways and expression profiles, consistent with a shared genetic program of appendage regeneration. Comparison of RNA-seq data from early Polypterus blastema to single-cell RNA-seq data from axolotl limb bud and limb regeneration stages shows that Polypterus and axolotl share a regeneration-specific genetic program. Collectively, our findings support a deep evolutionary origin of paired appendage regeneration in Osteichthyes and provide an evolutionary framework for studies on the genetic basis of appendage regeneration. 

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5. Thyroid hormone regulates proximodistal patterning in fin rays

   https://doi.org/10.1073/pnas.2219770120  

   Harper, Melody; Hu, Yinan; Donahue, Joan; Acosta, Benjamin; Dievenich Braes, Flora; Nguyen, Stacy; Zeng, Jenny; Barbaro, Julianna; Lee, Hyungwoo; Bui, Hoa; et al (May 2023, Proceedings of the National Academy of Sciences)

   Processes that regulate size and patterning along an axis must be highly integrated to generate robust shapes; relative changes in these processes underlie both congenital disease and evolutionary change. Fin length mutants in zebrafish have provided considerable insight into the pathways regulating fin size, yet signals underlying patterning have remained less clear. The bony rays of the fins possess distinct patterning along the proximodistal axis, reflected in the location of ray bifurcations and the lengths of ray segments, which show progressive shortening along the axis. Here, we show that thyroid hormone (TH) regulates aspects of proximodistal patterning of the caudal fin rays, regardless of fin size. TH promotes distal gene expression patterns, coordinating ray bifurcations and segment shortening with skeletal outgrowth along the proximodistal axis. This distalizing role for TH is conserved between development and regeneration, in all fins (paired and medial), and between Danio species as well as distantly related medaka. During regenerative outgrowth, TH acutely induces Shh-mediated skeletal bifurcation. Zebrafish have multiple nuclear TH receptors, and we found that unliganded Thrab—but not Thraa or Thrb—inhibits the formation of distal features. Broadly, these results demonstrate that proximodistal morphology is regulated independently from size-instructive signals. Modulating proximodistal patterning relative to size—either through changes to TH metabolism or other hormone-independent pathways—can shift skeletal patterning in ways that recapitulate aspects of fin ray diversity found in nature. 

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