Fundamentals
Shark skin looks smooth and ordinary from a regular perspective. As soon as it is inspected through the magnification lens of a microscope, it reveals a whole different shape. Tiny teeth and grooves cover the surface and give it a look and feel, similar to the surface of sandpaper. These “dermal denticles” are scales which vary in their size, depending on their location on the shark. Furthermore, different species show different denticle allocations and shapes. A common characteristic of these grooves is their orientation parallel to the water flow over the skin.
The various capabilities of the shark skin structure provide sharks with significant benefits and are a reason for their evolutionary superiority. Benefits of the denticle scales are on the one hand a better protection against competitors, or on the other hand advantageous fouling properties, which means less attachment of organic material like bacteria or algae.
The main function of the special surface structure is to reduce skin friction while swimming. Hereby, sharks have an increased swimming speed and save energy. The discovery of this capability was the starting point for studying Riblets and led to development of technical applications.
“Riblet surface” describes the adaption of shark skin and its principal geometry for a technical application. The micro-scaled grooves are oriented longitudinally to the flow and can reduce skin friction in turbulent flows, if properly designed and dimensioned. They interact with the turbulent boundary layer, resulting in a reduction of up to 10% in viscous drag. While the occurring effects are complex and not entirely explained, the basic working principle of Riblets can be divided into two main mechanisms.
First, cross-stream flows and their velocity fluctuations are hampered, which reduces the ejection of vortices into outer areas of the boundary layer and hereby reduces associated Reynolds shear stresses of the turbulent flow. Second, the streamwise vortices are lifted away from the surface and mainly interact with the Riblets’ tips. This leads to a total reduction of momentum transport although the wetted surface area is increased with Riblet structures, because high shear stress, due to high velocities, mostly occurs at the tips. To work properly, the spacing and sizes of Riblet structures must be adjusted to the vortices’ diameters, which depend on flow velocity and viscosity. Otherwise, the skin friction is increased by a larger effective surface area for wall near vortices.
Groundbreaking research in the field of Riblet technology was conducted at the NASA Research Center Langley, starting from 1976 by the scientist Michael J. Walsh and his team. With their initial experiments they tested V-shaped Riblets on flat aluminum plates in a wind tunnel. This work caught the attention of 3M, who suggested, that the application of Riblets would be easier and superior on molded plastic film. In collaboration with 3M, they developed and manufactured the respective film, which was later evaluated by the NASA Research Center. The evaluation confirmed the technology’s advancement and demonstrated that a drag reduction of up to 10% compared to an untreated surface is possible by an adapted turbulent airflow.
Based on these results, Doug McLean, who was an engineer at the Flight Research Institute of Seattle, the Boeing company and the University of Washington conducted tests of Riblet foil on Olympic rowing shells. Equipped with the regarding Riblet foil, an American rowing team won a silver medal at the Olympics in 1984 in Los Angeles.
Following these achievements, the mentioned actors continued their activities on Riblet technology with a specific focus on aerospace applications.
The other important scientific foundation for Riblet technology was laid in Germany at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt – DLR). Parallel to ongoing research activities in the USA, the German researchers aimed to investigate the mechanisms and working principles of shark skin. After first tests in wind tunnels, the construction of an oil tunnel was started in the 1990s in Berlin to deal with the very small size of Riblet structures. Compatible oil properties allow the application of similitude laws, meaning the validity of test results from upscaled models for original sized objects. Hereby, this new testing facility enabled much more precise measurements and facilitated experiments significantly.
Beside their theoretical investigations, the DLR focused on examining and comparing different Riblet geometries, including sharks’ dermal denticles or Sawtooth Riblets, which correspond to the triangular shaped Riblets investigated by the NASA Research Center. They also developed their own structure called Trapezoidal Riblets, which is the most applied technology nowadays and for instance used for the Lufthansa AeroSHARK film. The technical optimum was found with so called Blade Riblets, which make drag reductions of up to 10% possible.
The development of 2D Riblet geometries, which mean constant cross section geometries along the flow direction, is basically finished, however, 3D Riblet structures offer the next big optimization potential. Novel application methods enable the manufacturing of such structures, which could enhance potential drag reductions by up to 15%.
Prospective work in this sense will mainly focus on issues of practical appliance of Riblet surfaces, like the development of manufacturing methods or application processes. Furthermore, associated benefits of Riblet surfaces become subjects of growing interest. These regard for instance advantageous acoustic effects, heat transfer interactions or anti-fouling properties for applications in water.
