Lei Jin

Surface geometry can significantly modify turbulence characteristics and thereby influence drag in turbulent boundary-layer flows. While compliant surfaces have shown promise for drag reduction, the hydrodynamic role of the deformation-induced surface geometry itself remains insufficiently understood. In the present study, a deformation-inspired surface profile previously obtained from a single flexible-wall unit is adopted as a prescribed geometry for rigid surface arrays, and the resulting turbulent flow is investigated using improved Delayed Detached-Eddy Simulation (IDDES) based on the shear stress transport k–ω model. A series of array configurations with systematically varied unit width and spacing are examined to clarify their influence on drag characteristics and flow-modulation behavior. The results reveal a strong competition between friction-drag reduction and pressure-drag generation. Net drag reduction is achieved only when these competing mechanisms are balanced. The findings highlight the importance of geometric parameter tuning in arrayed surface designs and provide qualitative insight into the comparative flow-modulation behavior induced by deformation-inspired surface structures. The present IDDES framework partially resolves the turbulence structures while modeling part of the near-wall flow. Therefore, the turbulence analyses presented here are primarily intended to reveal comparative trends among different configurations rather than fully resolved near-wall turbulence mechanisms.

Turbulent boundary layers critically influence energy consumption and performance in ships. Conventional active drag reduction methods can achieve significant resistance reduction but require high energy input and are difficult to implement at large scales, while passive techniques such as microstructured surfaces or polymer additives exhibit limited effectiveness, poor durability, and restricted environmental compatibility under practical operating conditions. Inspired by biological surfaces, this study investigates drag reduction using an adaptively deforming flexible plate in high-Reynolds-number turbulent flow. A shape-based approach is adopted: the deformation that minimizes instantaneous pressure drag and total drag is determined via two-way fluid–solid interaction and then fixed to isolate geometric effects. Since the extracted geometry corresponds to simultaneous minima of pressure drag and total drag, the fixed surface modifies the near-wall velocity gradient and wall shear, thereby reducing the frictional contribution to the total drag while maintaining pressure drag within a controllable range. These coordinated effects reorganize near-wall turbulence structures and ultimately lead to a net reduction in overall drag. These modifications reorganize near-wall turbulence structures and lead to a net reduction in wall shear stress. The results provide quantitative insights into turbulent drag reduction mechanisms and offer conceptual guidance for the development of passive surface strategies in high-Reynolds-number flows.

Due to the interaction between the hull and fluid, a wide range of interface breaking occurs around the ship. For the interface breakage around the sailing ship, the numerical simulation method is used to study the interface breakage characteristics and the distribution characteristics of entrained bubbles under the quasi-steady state. The momentum-conservation volume of fluid method is adopted to capture the two-phase interface. The governing equation is based on the incompressible Navier–Stokes equations. The octree adaptive refinement mesh is adopted to refine the two-phase interface. The simulation results can well reproduce the scars, bow wave overturning breaking, and wake interface breaking in the real environment. Through the numerical simulation of the water surface breaking around the ship model with three different Froude numbers, the characteristics of interface breaking, flow field, and bubble evolution are studied. For the bubble distribution characteristics, based on the bubble distribution in the range of the ship model, the simulation results show that the bubbles at different speeds present the phenomenon of bubble sweepdown, the fan-shaped bubble region, and the obvious bubble transport from bow to stern. And the bubbles present obvious symmetrical distribution along the width direction.

A numerical investigation is conducted on the propulsive performance and acoustic characteristics of two flapping foils in tandem at Re = 200 in uniform flow using the lattice Boltzmann method combined with the immersed boundary method. The effects of phase difference and horizontal spacing on hydrodynamics and acoustics are studied. The results indicate that the propulsive performance and acoustic characteristics of the in-phase foils and anti-phase foils are completely different. As the horizontal spacing increases, the change trend of their propulsive performance is completely opposite. The propulsive performance of the in-phase foils shows a trend of first weakening and then enhancing, while the anti-phase foils show a trend of first enhancing and then weakening. For sound field, the in-phase foils exhibit a dipole-like pattern similar to that of a single foil, while it exhibits a quadrupole-like pattern for anti-phase foils. Compared to the in-phase foils and single foil, the maximum effective sound pressure value of the anti-phase foils is smaller. Meanwhile, the relationship between propulsive performance and acoustic characteristics is analyzed, and it is found that the correlation coefficients follow the same variation pattern. The presented work can offer valuable insights for designing and controlling new bio-inspired autonomous underwater vehicles aimed at suppressing flow noise.