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The hydrodynamic performance of the locomotive near the water surface is impacted by its geometrical shape. For marine animals, their geometrical shape is naturally selective; thus, investigating gliding locomotion of marine animal under the water surface may be able to elucidate the influence of the geometrical shape. We investigate three marine animals with specific geometries: the killer whale is fusiform shaped; the manta ray is flat and broad-winged; and the swordfish is best streamlined. The numerical results are validated by the measured drag coefficients of the manta ray model in a towing tank. The friction drag of the three target models are very similar; the body shape affected form drag coefficient is order as swordfish < killer whale < manta ray; the induced wave breaking upon the body of the manta ray performs different to killer whale and swordfish. These bio-inspired observations provide a new and in-depth understanding of the shape effects on the hydrodynamic performances near the free surface.

Studies on buoyancy-driven instabilities are of considerable interest for its wide application in oceanic, environmental and industrial fields. For miscible two-layer stratifications in a Hele-Shaw cell, the base state density profile in the diffusion regime can be predicted by linear stability analysis such that the buoyancy-driven instabilities can be classified in (R, δ) parameter space, where R is the initial density stability ratio and δ is the diffusion coefficient ratio. However, flow dynamics in extended geometry do not satisfy Darcy's law. Hence, linear stability analysis cannot be applied to unbounded flow with nonlinear momentum convection. In this study, we carry out a series of numerical simulations to investigate buoyancy-driven instability for thermohaline stratification in both a Hele-Shaw cell and extended geometry at a high thermal Rayleigh number, Ra T = 7 · 10 6 , with R = 0.05, 0.25, 0.8, 1, 4, and 20. The time evolution of the density stability ratio, temperature, salinity and base state density are calculated and compared. Fingers of unbounded thermohaline stratification have a more distorted shape due to the nonlinear momentum convection. However, the base state density profile of thermohaline stratification in extended geometry is found to be similar to that of a Hele-Shaw cell; thus, the classification of buoyancy-driven instabilities in (R, δ) space for miscible two-layer stratifications in Hele-Shaw cells can be extended to the same flow in extended geometry.

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