Basic Research On Drag Reduction Mechanism And Replication Technology Of Micro-riblets On Shark Skin

With the spreading of energy crisis, drag reduction technology has been an important part of science and technology competition between countries. There are several drag reduction methods which have been taken into practice, however all these methods have limitation in application. Natural creatures provide rich knowledge resource to help human beings to solve engineering problems, such as "shark skin effect" which provides human being with a perfect drag reduction template. With the development of hydrodynamics and turbulence researching, it was fount out that a certain shape of groove on surface could help to reduce drag. Shark skin replica had been produced through amplification and simplification. Artificial shark skin replica would be widely applied in aerospace, ocean shipping, oil pipeline. However, the production of replica which has the similar riblet and scale structure with real shark skin is still a problem.Firstly, drag reduction mechanism of shark riblet was studied by numerical analysis. Fluent software was applied in numerical simulation. Because of the difficulty of creating a similar 3D shark skin computing modeling, two parts of the drag reduction numerical simulation of shark skin, riblet drag reduction numerical simulation and scale drag reduction numerical simulation, were researched separately. The results indicated that the convection movement of secondary vortex is the main reason of riblet drag reduction. Another reason of riblet drag reduction is the existence of small space between the scales which can hold low speed fluid. The small vortex in the space can turn sliding friction into rolling friction and prevent high speed fluid rushing the wall of shark skin scale.Then, two methods, heat-embossing electroforming method and vacuum casting method were attampted in shark skin replicating. Process of heat-embossing electroforming method includes three steps:Firstly, micro-replication template was produced with PMMA by heat-embossing. Then, PE shark skin replica was produced by polymer casting based on shark skin template. Finally, taking the PE replica as an electroforming template, the shark skin nickel mould was finished after electroforming. Process of vacuum casting method includes two steps:Firstly, replication material was casted onto shark skin and put into vacuum tank. After solidification, the mould was finished. Then, liquid silicone rubber shark skin replica was produced by using of this mould in vacuum environment also. It could be concluded as following after comparison and analysis. Clear and similar shark skin riblet and scale was nearly replicated on shark skin metal mould by electroforming. Template made by vacuum casting had clear groove and scale structure. Nevertheless, the phenomenon of the existing of fillet on shark skin metal mould was serious, which had relationship with the complex structure on shark skin. So we can draw the conclusion of that, vacuum casting method is a relatively perfect shark skin replication method.