Study On Turbulent Drag Characteristics Of Non-smooth Surfaces Using Large Eddy Simulation

As the resources on the earth are running out and the ecological environment deteriorates,it is imperative to save energy and reduce carbon emissions. Transportation is a high energyconsumption industry, especially for every kind of vessels. A large shipping company willburn several billion dollars of oil each year, and great benefits will be gotten even if a litter ofoil consumption is saved for each vessel in each day. Recently, reducing the turbulentboundary layer drag to improve the overall economic efficiency of vessels has induced greatinterest in the field of fluid mechanics. NASA&DLR have paid more attention to the ribletsurface of sharks from1980s, and they try to get some inspiration from this fascinatingstructure for reducing the turbulent boundary layer drag. It has been noticed that when theshark quickly moves, the structure of ribs will align to streamwise direction to reduce frictiondrag of the turbulent boundary. As a passive drag reduction technology without powerrequirement, riblet surfaces have a great potential in attenuating drag. Researchers have madesignificant efforts to study this kind of mechanism of riblet surfaces, and10%total dragreduction has been reported.In this thesis, non-smooth surface drag reduction technology is investigated. Based on theboundary layer theory, turbulent coherent structure theory and other fluid dynamic theories,the bionic non-smooth surface turbulence and especially the wall-bounded turbulencestructure characteristics under shark-skin effects are studied, comprehensively. Interactionbetween the non-smooth surface and turbulence is examined through numerical simulation,and the drag reduction mechanism is discussed by the flow field characteristics. In this study,the Large Eddy Simulations (LES) of Localized Dynamic Kinetic-energy Model (LDKM) isused to predict the micro-scale riblet flow field, to validate the drag characteristics of differentblade riblets with specific shape parameters and scale, and to calculate the flow velocity,velocity fluctuation, and Reynolds shear stress of the riblets in different positions. By visualanalysis, the drag reduction mechanism is demonstrated, and the drag reduction criteria ofblade riblets are summarized for getting a better drag reduction result. In this paper, transversenon-smooth surfaces with other configurations are also studied. Although the viscosity dragdoes not reduce, the discussions about the drag increase mechanism are also beneficial tocomprehensively understand the physics insight of the fluid dynamics of non-smoothsurfaces.We have found that LES of LDKM is reliable to study turbulent drag of riblets and other non-smooth surfaces.9%drag reduction has been achieved by the blade riblets in this study,and the shark-skin effects of wall-bounded turbulent boundary push-away and spanwise flowimpeding are observed. It is found that the drag increase over the surface of a transversehalf-circle concave surface versus its rough height is a parabolic profile. Finally, the dragcharacteristics of riblets embedded in wavy wall are investigated, and it is clear that the flowfield there is dominated by the coupling of flows over riblet surface and wavy walls.