Numerical Simulation On Drag Reduction Of Bionic Squamous Cavity Non-smooth Surface

With the worsening of environmental pollution and the gradually exhausted non-renewable energy resources, the sustainable development model with low carbon environmental protection has been a common goal for human. It has been a main way to reduce energy consumption by reducing surface friction to decrease the total flow resistance in vehicle, pipe transportation and so on. Based on bionics principle, the squamous cavity non-smooth surface used in field of fluid drag reduction was presented.Bionic non-smooth surface drag reduction is an important branch of non-smooth surface theory. After decades of development, many results have achieved. But so far, the main works of scholars are focus on grooved shape surface, the research on drag reduction of other forms of non-smooth structure is less. Inspired by squamous concave wear scar on flow surface of Hydraulic Machinery and combined with the study of physiological structure and surface properties of carp scales, designed squamous concave surface. The drag reduction effect of squamous concave surface was studied by the way of numerical simulation with Fluent6.3.16. Besides, the influence on flowing property in near-wall region and the mechanism of resistance decrease of squamous concave surface was researched.Firstly, the three-dimensional morphology of fish scale-shaped cavity wear scar over flow surface on typical hydraulic machinery and its generation mechanism were analyzed. What’s more, the surface morphology and surface wettability of scar scales were measured. Based on the analysis results of the aforementioned, the simplified model of squamous concave was built.Numerical results showed that squamous concave surface with special morphology had an obvious drag reduction effect. Compared with the smooth surface, squamous cavity surface produced pressure resistance, while the friction resistance accounted for about80%of the total drag reduced substantially. Finally, it had a drag reduction effect reflection on the total drag coefficient. Orthogonal test analysis indicated that the shape of squamous cavity is the most factor for drag reduction effect, followed by velocity, maximum depth, diameter and longitudinal spacing. The best parameter combination was:the deepest point of concave advance0.025mm, velocity is4m/s, the maximum depth of squamous cavity is0.02mm, the diameter of squamous concave is0.1mm, the longitudinal spacing of squamous pits is0.2mm. By analyzing the influence on flow characteristics with near-wall region of squamous concave surface, it can found that squamous concave decreased the wall shear stress of certain areas by reducing the velocity gradient of near-wall region. Compared with spherical cavity surface, the adverse pressure gradient in the squamous concave region was less than that in spherical cavity region, which led to its pressure drag was much less than that of spherical cavity surface. This was the reason why the drag reduction effect of squamous concave surface is superior than that of spherical cavity surface. There is an obvious low-speed rotating flow (known as secondary vortex) in squamous pit. The "secondary vortexes" existing in pits like a series of "miniature roller bearing" arranging on the surface, turning the fluid-to-solid sliding contact into the fluid-to-fluid rolling contact, and then forms the "vortex mat effect". This effect can significantly reduce resistance of surface.