Drag reduction capacity of multi-scale and multi-level riblet in turbulent flow

For high-speed moving objects,drag reduction has been a prolonged major challenge.To address this problem,passive and negative strategies have been proposed in the preceding decades.The integration of creatures and nature has been continuously perfected during biological evolution.Unique structure characteristics,material properties,and special functions of marine organisms can provide inexhaustible inspirations to solve this intractable problem of drag reduction.Therefore,a simple and low-cost laser ablation method was proposed.A multi-scale and multi-level riblet(MSLR)surface inspired by the denticles of the sharkskin was fabricated by controlling the density of the laser path and ablation times.The morphology and topographic features were characterised using an electron microscope and a scanning white-light interfering profilometer.Then,the drag reduction capacity of the bionic riblet surface was measured in a circulating water tunnel.Finally,the mechanism of drag reduction was analysed by the computational fluid dynamics(CFD)method.The results show that the MSLR surface has a stable drag reduction capacity with an increase in Reynold(Re)number which was contributed by high-low velocity stripes formed on the MSLR surface.This study can provide a reference for fabricating spatial riblets with efficient drag reduction at different values of Re and improving marine antifouling.

Flow field characteristics and drag reduction performance of high-low velocity stripes on the biomimetic imbricated fish scale surfaces

Improving energy efficiency and cost reduction is a perennial challenge in engineering.Natural biological systems have evolved unique functional surfaces or special physiological functions over centuries to adapt to their complex environments.Among these biological wonders,fish,one of the oldest vertebrate groups,has garnered significant attention due to its exceptional fluid dynamics capabilities.Researchers are actively exploring the potential of fish skin's distinctive structural and material characteristics in reducing resistance.In this study,models of biomimetic imbricated fish scale are established,and the evolution characteristics of the flow field and drag reduction performance on these bionic surfaces are investigated.The results showed a close relationship between the high-low velocity stripes generated and the fluid motion by the imbricated fish scale surface.The stripes'prominence increases with the spacing of the adjacent scales and tilt angle of the fish scale,and the velocity amplitude of the stripes decreases as the exposed length of the imbricated fish scale surface increases.Moreover,the biomimetic imbricated fish scale surface can decrease the velocity gradient and thereby reduce the wall shear stress.The insights gained from the fish skin-inspired imbricated fish surface provide valuable perspectives for an in-depth analysis of fish hydrodynamics and offer fresh inspiration for drag reduction and antifouling strategies in engineering applications.

Controllable deformation based self-adaptive drag reduction for complex surface

Reduction of energy consumption and improvement of cruising speed are greatly necessary for underwater vehicles.Previously,regular riblets have been machined and the drag reduction has been verified;however,the riblet parameters are not adjusted like the denticles of sharkskin,which adapt quickly to the complex changing fluid flow.To achieve an improved drag reduction effect on the complicated shape surface,a simple,low-cost,and timesaving stretching approach was proposed to adjust the riblet parameters on the underwater vehicle surface by controllable deformation.Nature latex rubber membrane with regular micro-riblets was prepared as a stretching flexible film,and the spacing and height of the micro-riblets were adjusted by adaptive control of the stretching ratio.The circulating water channel experiment verified the effectiveness and feasibility of the selfadaptive drag reduction by the controllable deformation method.The results demonstrated that the drag reduction rate of the controllable deformation bionic fish skin was 4.26%compared with a smooth surface at 0.25 m/s with an angle of attack of 0°,which is better than any other angle.The controllable deformation bionic fish skin provides a feasible method for the drag reduction of complex surface adaptive underwater vehicles.

Bio-inspired drag reduction surface from sharkskin

Nature evolution provides nature surface treasures with special and fascinating surface function to inspiredesign,such as the drag reduction of sharkskin.In this overview,the morphology and mechanism of the sharkskinexplained from different aspects,and various methods of fabricating surfaces with sharkskin morphology areillustrated in details,and then the applications in different fluid engineering are demonstrated in brief.This overviewwill improve the comprehension of the morphology and mechanisms of sharkskin,and methods of fabricating thesurface with morphology,and the recent applications in engineering.

Hydrogen embrittlement of grain boundaries in nickel:an atomistic study

The chemomechanical degradation of metals by hydrogen is widely observed,but not clearly understood at the atomic scale.Here we report an atomistic study of hydrogen embrittlement of grain boundaries in nickel.All the possible interstitial hydrogen sites at a given grain boundary are identified by a powerful geometrical approach of division of grain boundary via polyhedral packing units of atoms.Hydrogen segregation energies are calculated at these interstitial sites to feed into the Rice-Wang thermodynamic theory of interfacial embrittlement.The hydrogen embrittlement effects are quantitatively evaluated in terms of the reduction of work of separation for hydrogen-segregated grain boundaries.We study both the fast and slow separation limits corresponding to grain boundary fracture at fixed hydrogen concentration and fixed hydrogen chemical potential,respectively.We further analyze the influences of local electron densities on hydrogen adsorption energies,thereby gaining insights into the physical limits of hydrogen embrittlement of grain boundaries.