Shark Skin Inspired Surfaces for Aerodynamically Optimized High Temperature Applications - Fabrication, Oxidation, Characterization (2011)

Among other things, the high speed of sharks is caused by reduced friction between water and skin due to riblet structures on the shark scales. When using a surface with riblet structures, the turbulent momentum transfer at the wall, which is responsible for the skin friction, is hampered. This works for all turbulent flowing fluidic media such as liquids and gases. The effect of drag reduction has already been used for airfoil wings that were laminated with polymer based riblets. A reduced flow resistance was obtained, resulting in less fuel consumption. A potential new application field for riblet surfaces are blades and vanes for aeroengines. Riblet structured coatings on the blades would act as oxidation and corrosion protection and additionally reduce the skin drag on the surface. The reduced friction of the air flow would lead to higher efficiencies of engines. This dissertation deals with the fabrication, oxidation and characterization of riblet structures with micrometer dimensions for aerodynamically optimized high temperature applications. First, the necessary riblet sizes were calculated for the different areas in the aeroengine. The size of the riblets depends on the surrounding conditions, namely the velocity, temperature and pressure of the flowing gas. The necessary riblet sizes for compressor inlet, compressor outlet, turbine inlet and turbine outlet were calculated on the basis of one example aeroengine. Calculations show that riblet sizes on blades and vanes for aeroengines vary between single-digit micrometers up to several tens of micrometers, depending on local temperature, pressure and gas velocity. For studying the general oxidation behavior of the riblet candidate materials, different coating materials such as titanium, nickel, chromium, aluminium, silicon and platinum were deposited on high temperature substrate materials such as nickel and titanium based alloys and were oxidized at various temperatures between 400 and 1000°C, depending on a later application in compressor or turbine. Based on the calculations of the necessary riblet dimensions, the feasibility of various methods for the fabrication of riblet structures was investigated. As riblet fabrication methods direct structuring methods such as laser material removal and diamond cutting were investigated as well as structuring methods via a mask. As masks metal masks and resist masks were applied. Different coatings were deposited via thermal evaporation, sputter deposition and electrodeposition. For the direct structuring, best results were obtained via laser structuring. Photolithography and subsequent electrodeposition of nickel showed the best results for the structuring methods with the help of a mask. All tested riblet fabrication methods were evaluated concerning their applicability for structuring parts in aeroengines. The fabricated riblets were oxidized between 900 and 1100°C for simulating the high application temperatures. The structures were still intact after oxidation. The effectiveness of the riblet structures was exemplarily tested for one as-fabricated riblet design. An scaled-up model was measured in an oil channel for determining the reduction of the wall shear stress. The measured riblet design showed a significant wall shear stress reduction of up to 4.9%. Therefore, this doctoral thesis identifies and evaluates possible methods for the design and development of effective riblet structures for high temperature applications such as for parts in aeroengines.