Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

Fluid-Solid Interfacial Properties and Drag-Reducing Characterization of the Flexible Conical Microstructured Film Inspired by the Streamlined Body Surface of the Pufferfish

Given the challenges in accurately replicating the surface of the pufferfish, this study employed three-dimensional (3D) printing to create a model based on inverse modeling. The morphology of the pufferfish exhibits a streamlined configuration, characterized by a gradual widening from the anterior oral region to the central ocular area, followed by a progressive narrowing from the midabdominal region toward the caudal extremity. The RNG k-ε turbulence simulation results demonstrate that the streamlined body surface of the pufferfish diminishes differential pressure resistance. This enhancement promotes laminar flow formation, delays fluid separation, minimizes turbulence-induced vortices, and reduces frictional resistance. Moreover, the pufferfish's supple and uneven outer epidermis was simplified into a flexible, nonsmooth planar film to conduct fluid-solid coupling simulations. These revealed that the pufferfish's unique skin can absorb turbulent energy and minimize momentum transfer between the fluid and the solid film, lowering the fluid resistance during swimming. In summary, The high-efficiency swimming capacity of pufferfish stems not only from their streamlined body surface but also significantly from the unique structural characteristics and mechanical properties of their flexible skin. This research provides critical theoretical underpinnings for the design of functional bionic surfaces aimed at drag reduction.

Effects of the Yaw Angle on Air Drag Reduction for Various Riblet Surfaces

Biomimetic riblet surfaces, such as blade, wavy, sinusoidal, and herringbone riblet surfaces, have widespread applications for drag reduction in the energy, transportation, and biomedicine industries. The drag reduction ability of a blade riblet surface is sensitive to the yaw angle, which is the angle between the design direction of the riblet surface and the average flow direction. In practical applications, the average flow direction is often misaligned with the design direction of riblet surfaces with different morphologies and arrangements. However, previous studies have not reported on the drag reduction characteristics and regularities related to the yaw angle for surfaces with complex riblet microstructures. For the first time, we systematically investigated the aerodynamic drag reduction characteristics of blade, wavy, sinusoidal, and herringbone riblet surfaces affected by different yaw angles. A precisely adjustable yaw angle measurement method was proposed based on a closed air channel. Our results revealed the aerodynamic behavior regularities of various riblet surfaces as affected by yaw angles and Reynolds numbers. Riblet surfaces with optimal air drag reduction were obtained in yaw angles ranging from 0 to 60° and Reynolds numbers ranging from 4000 to 7000. To evaluate the effects of the yaw angle, we proposed a criterion based on the actual spanwise spacing (d⁺) of microstructure surfaces with the same phase in a near-wall airflow field. Finally, we established conceptual models of aerodynamic behaviors for different riblet surfaces in response to changes in the airflow direction. Our research lays a foundation for practical various riblet surface applications influenced by yaw angles to reduce air drag.

Bionic research on Paramisgurnus dabryanus scales for drag reduction

Drag reduction is a key problem in marine vehicles and fluid transportation industries. Reducing drag strategies and mechanisms need to be further investigated. To explore a bionic approach for reducing flow resistance, experimental and numerical simulation research was conducted to study the drag reduction characteristics of the Paramisgurnus dabryanus surface microstructure. In this study, the large-area flexible surface of the bionic loach scale was prepared by the template method of one-step demoulding. The water tunnel experiment results show that compared with the smooth surface, the drag reduction rate of the bionic surface ranges from 9.42% to 17.25%. And the numerical simulation results indicate that the pressure gradient and low-speed vortex effect created by the bionic loach scales can effectively reduce the friction drag. The results of experimental data and numerical simulation both prove that the bionic scales of Paramisgurnus dabryanus can achieve the underwater drag reduction function. This research provides a reference for drag reduction in marine industries and fluid delivery applications.

Coupled Bionic Drag-Reducing Surface Covered by Conical Protrusions and Elastic Layer Inspired from Pufferfish Skin

Inspired by the drag-reducing properties of the cone-like spines and elastic layer covering the pufferfish skin, important efforts are underway to establish rational multiple drag-reducing strategies for the development of new marine engineering materials. In the present work, a new drag-reducing surface (CPES) covered by conical protrusions (sparse "k-type" with rough height k⁺ = 13-15) and an elastic layer are constructed on copper substrate via a hybrid method, combining the sintering and coating processes. The drag-reducing feature of the prepared CPES biomimetic surface is achieved by rheometer and particle image velocimetry (PIV) experiments. To comprehensively investigate its drag reduction mechanism, the porous copper substrate (PCS), copper substrate (CS), conical protrusion resin substrate (CPRS), and conical protrusion porous copper substrate (CPPCS) were used for a comparative analysis. In laminar flow, we discovered that the conical protrusion structure and wettability of the elastic surface coupling affect the CPES sample's drag-reducing performance (7-8%) and that the interface produced slip to reduce the viscous drag. In turbulent flow, the CPES biomimetic surface exhibits an 11.5-17.5% drag-reducing performance. Such behavior was enabled by two concurrent mechanisms: (i) The conical protrusions as vortex generators enhance the number of vortices and the wake effect, enabling faster movement of downstream strips, reducing viscous drag; (ii) The conical protrusion elements break and lift large-scale vortices to produce numerous small-scale vortices with low energy, effectively weakening perturbations and momentum exchange. Additionally, the elastic layer shows high adhesion and stability on copper substrate after sandpaper abrasion and water-flow erosion tests. The copper substrate surface formed by the sintering method is also covered with dense porous structures, which gives the elastic layer and conical protrusions excellent combined robustness. Our findings not only shed new light on the design of robust drag-reducing surfaces but also provide new avenues for underwater drag reduction in the field of marine applications.

Rheological Properties and Drag Reduction Performance of Puffer Epidermal Mucus

Most species of fish are covered with mucus, which provides the effect of reduction in swimming drag. In this paper, three concentrations of puffer epidermal mucus were obtained from the epidermal mucosa of puffer. The rheological properties and the drag reduction performance of the puffer epidermal mucus were characterized via a rheometer experimental and numerical simulation method. The relationship between the rheological properties and the drag reduction performance was analyzed and discussed, and the drag reduction mechanism of the puffer epidermal mucus was further explored. The results showed that the best drag reduction rate was 6.2% when the inflow velocity and concentration of puffer epidermal mucus were 0.1 m/s and 18.2 g/L, respectively. The rheological properties of puffer epidermal mucus are viscoelastic, and the mucus forms a sliding surface, which reduces the frictional drag of the fluid. In conclusion, this paper may provide a reference for the development of drag-reducing agents and drag-reducing research studies on other fish mucus.