Shark skin inspired riblet structures as aerodynamically optimized high temperature coatings for blades of aeroengines

Abstract

This paper deals with different structuring methods for high temperature resistant nickel alloys. The ideal structured surface for a possible application on the blades of aeroengines combines high oxidation resistance with low drag in a hot gas flow. The effect of drag reduction due to riblet structured surfaces was originally inspired by shark scales, which have a drag reducing riblet structure. The necessary riblet sizes for effective drag reduction depend on the temperature, pressure and velocity of the flowing medium (gas or liquid). These riblet sizes were calculated for the different sections in an aeroengine. The riblets were successfully produced on a NiCoCrAlY coating by picosecond laser treatment. This method is suitable for larger structures within the range of some tens of micrometers. Furthermore, experiments were performed by depositing different materials through polymer and metal masks via electrodeposition and physical vapor deposition. All fabricated structures were oxidized at 900-1000 • C for up to 100 h to simulate the temperature conditions in an aeroengine. The resulting shape of the riblets was characterized using scanning electron microscopy. The most accurate structures were obtained by using photolithography with a subsequent electrodeposition of nickel. This method is suited for single digit micrometer structures. The reduction of the wall shear stress was measured in an oil channel. The riblet structures prior to oxidation showed a reduction of the wall shear stress of up to 4.9% compared to a normal smooth surface. This proves that the fabricated riblet design can be used as a drag reducing surface.

Biomimetic shark skin: design, fabrication and hydrodynamic function

“…The structure of shark skin denticles and their possible effect on the pattern of water flow over the body has attracted considerable interest from biologists interested in the microstructure and distribution of denticles (Kemp, 1999;Meyer and Seegers, 2012;Motta et al, 2012;Reif, 1978;Reif, 1982a;Reif, 1985), engineers focused on how the surface roughness may reduce drag forces during locomotion (Bechert et al, 1997;Bechert et al, 2000;Lang et al, 2011), and researchers interested in generating biomimetic models of shark skin to reduce locomotor drag on humans and human-designed structures (Büttner and Schulz, 2011;Mollendorf et al, 2004). Bioengineering studies of shark skin denticle function have focused on the effects of denticle-like structures using scaledup models of denticles to study how surface roughness affects drag forces (Dean and Bhushan, 2010;Lang et al, 2008;Lang et al, 2011;Luchini et al, 1991;Walsh, 1980;Walsh, 1983).…”

The hydrodynamic function of shark skin and two biomimetic applications

“…For example, riblets are fine rib-like surface geometries with sharp surface ridges that can be aligned either parallel or perpendicular to the flow direction and might reduce drag. A diversity of riblet shapes and sizes has been investigated experimentally and theoretically (Bechert and Bartenwerfer, 1989;Bechert et al, 2000;Bechert et al, 1997;Büttner and Schulz, 2011;Koeltzsch et al, 2002;Luchini et al, 1991;Luchini and Trombetta, 1995;Neumann and Dinkelacker, 1991), and drag reduction of stiff bodies covered with riblet material has been shown to occur (Bechert et al, 1997;Bechert et al, 1985;Dinkelacker et al, 1987). Experiments with an adjustable surface with longitudinal blade ribs and slits revealed the highest stiff-body drag reduction of 9.9%, with a groove depth of half the size of lateral riblet spacing (Bechert et al, 1997).…”

Laser-Based biomimetic functionalization of surfaces: from moisture harvesting lizards to specific fluid transport systems

“…Such kind of inspirations found in nature became more and more famous for an innovative product development [12,13,14].…”