Resistance Performance Study Of The Bionic Structured Surface

After natural selection for one hundred million years, many creatures in the nature have suited the cruel survive principle, and as for their own continuously evolution, they have gained the unsmooth surface structures to prevent from getting pasted, take off the sticky things, and reduce friction. Based on these functional animals'and plants'unsmooth surface configurations in the nature, combined with the writer' own experience, this paper have designed a kind of convex bag form of the structured unit cell, which can be applied to these smooth surfaces taking water as the medium, and named bionic structured surface.Estimate the structured unit cell's height (H), reasonably design the model size and the solution domain. Use structured and unstructured hexahedron grids for spatial discretisation of the solution domain. The grids near the wall area are refined by using boundary layers. The k-oSST turbulence model was used to close the Reynolds-averaged Navier-Stokes(RANS) equations. The SIMPLE algorithm with first-order upwind schemes are adopted to ensure the accuracy and stability. The numerical methods used, Fluent, is one of the computational fluid dynamic (CFD)softwares. The study was undertaken for smooth flat models. Computational results agree well with the calculation results of experience formulas for almost all studied models, and so the train of thought could be proved correct.The numerical simulation was undertaken to calculate the resistance coefficient of bionic structured surfaces in liquid-solid interface system. Various situations have been simulated:five different unit cell sizes, two array ways, one array spacing value and five Reynolds numbers. To confirm bionic structured surfaces'ability in reducing friction resistance as compared with smooth surfaces, computational results have been analyzed, the conclusions are:1) Bionic structured surfaces could reduce friction resistance effectively.2) The rhombic interlaced arrangement is better than the queue arrangement in resistance reducing.3) Model m2has the highest resistance reducing rate(4.35%) among those five models using rhombic interlaced arrangement,then follows model ml(4.2%).The results show clearly the smaller model'diameter(D) is when the unit cell height (H) is a certain value, the better it works; in the D/H value certain circumstances, with smaller H the friction reduction effect decreased gradually.4) When it comes to the queue arrangement, the unit cell height(H) matters, whereas D/H is not highly sensitive to friction reduction.5) Compared with smooth flats, though bionic structured surfaces could decrease friction resistances, they increased pressure drags, eventually resulted in bigger total resistances.After comparison and analysis of models'physical quantities such as surface shear stress, velocity, turbulence statistics and vorticity, the mechanism of friction reduction for bionic structured surfaces is obtained:1) The low-speed flows between structured unit cells make high-speed flows separated from the wall, form the liquid-liquid contact and realize friction reduction through decreasing speed gradient; in addition, there's backflow in unit cells'downstream, the backflow would lower the velocity of the low-speed flow and make the lowest wall shear stress happen there.2) Friction reduction can be implemented by increasing the wall's boundary layer thickness, reducing the speed gradient, and effectively controlling or fixing boundary layer through structured unit cells.3)By reducing the amount of small scale vortex brought by vortex lines'pulling and slenderizing, limiting vorticity's accretion, the energy dissipation would decrease and so the friction can be reduced.