Form, function, and divergence of a generic fin shape in small cetaceans

Tail flukes as well as the dorsal fin are the apomorphic traits of cetaceans which appeared during the evolutionary process of adaptation to the aquatic life. Both appendages present a wing-like shape associated with lift generation and low drag. We hypothesized that the evolution of fins as lifting structures led to a generic wing design, where the dimensionless parameters of the fin cross-sections are invariant with respect to the body length and taxonomy of small cetaceans (Hypothesis I). We also hypothesized that constraints on variability of a generic fin shape are associated with the primary function of the fin as a fixed or flapping hydrofoil (Hypothesis II). To verify these hypotheses, we examined how the variation in the fin’s morphological traits is linked to the primary function, species and body length. Hydrodynamic characteristics of the fin cross-sections were examined with the CFD software and compared with similar engineered airfoils. Generic wing design of both fins was found in a wing-like planform and a streamlined cross-sectional geometry optimized for lift generation. Divergence in a generic fin shape both on the planform and cross-sectional level was found to be related with the fin specialization in fixed or flapping hydrofoil function. Cross-sections of the dorsal fin were found to be optimized for the narrow range of small angles of attack. Cross-sections of tail flukes were found to be more stable for higher angles of attack and had gradual stall characteristics. The obtained results provide an insight into the divergent evolutionary pathways of a generic wing-like shape of the fins of cetaceans under specific demands of thrust production, swimming stability and turning control.

Grammatonotus bianchi, a new species of splendid perch (Percoidei: Callanthiidae) from Myanmar, northeastern Indian Ocean

A new splendid perch, Grammatonotus bianchi sp. nov. is described on the basis of two specimens (45.9–68.7 mm SL) collected at 184 m depth in the Andaman Sea off the coast of Myanmar during bottom surveys conducted by the R/V Dr Fridtjof Nansen in 2018. The new species can be distinguished from all congeners by its large head (37.7–38.6% SL), large orbit (14.4–15.3% SL), caudal-fin shape, and fresh coloration. A key to Indian Ocean species of Grammatonotus is provided.

Optimal Shape of Non-Linear Partially Wet Annular Fins for Maximum Efficiency

Heat exchangers with annular finned-tube type and partially wetted condition are utilized widely in engineering systems, such as air-conditioning systems and refrigeration systems. In addition, the physical properties of fin materials should be considered as functions of temperature in reality and thus become a non-linear problem. Based on the above two conditions, an optimal partially wet annular fin design problem, with temperature-dependent thermal properties of the fin, to yield optimal fin efficiency was investigated in the present work, which has not been examined previously and it is the novelty of this study. An iterative regularization algorithm using the conjugate gradient method (CGM) is considered as the optimization tool based on the desired fin efficiency under a fixed fin volume constraint. The partially wet annular fin condition can result if the relative humidity of surrounding air is between 80 and 90%. Finally, the optimal fin shape, with the highest computed efficiency among examined fins under identical operational conditions, can be obtained. It is found that when the Biot numbers for ambient air (Bia) and relative humidity (φ) increased, the optimum computed fin efficiency and interfacial radius between wet and dry fin domains (rwd) will be increased, and the estimated optimum fin shape also changed. However, the shape of optimal fin remained approximately unchanged when the Biot numbers for the inner tube (Bii), the thermal conductivities of the tube (kw) and fin (kf) varied. It reveals that Bii, kw and kf have an insignificant influence on the optimal shape of the annular fin in a partially wet condition.

Effect of Active and Passive Curvature on the Hydrodynamic Performance of Flapping Fins

Ray-finned fish swim by flapping their fins, which are composed of bony rays connected by an inextensible membrane. Throughout the flapping cycle, the fins typically undergo both ‘passive’ deformation due to hydrodynamic loading, and ‘active’ deformation arising from internal musculature deforming the fin against the flow. To systematically analyze the impact of fin shape on hydrodynamic performance, a parametric definition of the fin geometry and its modes of deformation is required, consistent with the fin’s material and mechanical properties. In this paper we present a model and algorithm to determine the fin shape corresponding to an arbitrary out-of-plane curvature distribution for each ray. The shape is computed by iteratively enforcing constraints corresponding to membrane inextensibility, and negligible torsional stiffness of the rays. Based on this model, we present a low-order parametrization of fin shapes that capture the predominant deformation modes due to combined hydrodynamic loading and intrinsic actuation, as compared to experimental observations. To demonstrate the model’s ability to provide insight into the effect of curvature on hydrodynamic fin performance, we integrate our algorithm into a 3D Navier-Stokes solver Using this framework, we present initial results on the cycle-averaged thrust coefficient of a passively and actively deforming generalized trapezoidal caudal fin model at Reynolds number 1500 and Strouhal number 0.3. The results demonstrate that our model, algorithm, and integration with the flow solver form a useful framework to understand the effect of 3D curvature on hydrodynamic performance of flapping fins.