Manufacturing
Riblet structures add an artificial roughness to a surface, which can be seen through a microscope or be sensed by hand. However, from a simplified fluid-dynamic perspective, Riblet surfaces are smoother than smooth. With their contribution, surface friction is significantly below the friction of an ideally smooth surface.
There are different possible ways to manufacture Riblet surfaces, which all have distinct characteristics. These basic processes include adhesive plastic films, laser machining and coating. Big efforts are therefore put into the advancement of these technologies and the development of new processes.
The industrial use of laser technology has become increasingly common nowadays. Laser beams are employed for tasks such as welding, cutting, ablation and various other specific processes.
Simplified, laser machining uses optically focused light beams to reach needed manufacturing characteristics. The ongoing development progresses in this field make laser technology economically viable for machining Riblets.
Laser machining offers several advantages regarding Riblet manufacturing:
- Laser machining enables the treatment of every discrete surface point on a microscale, which makes the manufacturing of 3D Riblets with variable geometries possible.
- With laser technology, the machining of any surface geometry becomes possible. Riblets can hereby be applied to complex, non-developable surface areas, which cannot be covered with foil.
- Most materials can be treated with laser equipment, while other processes are bound to their original surface material. Hereby, Riblets become available for numerous additional operating conditions and can be optimized regarding heat resistance or durability.
The roll-to-roll (R2R) film coating technology provides a possibility to create functional films with Riblet structures. These Riblet films find applications in the aerospace industry and in the wind energy sector.
One notable program in this field is called “AeroSHARK”. It is a cooperation between BASF and Lufthansa Technik and aims for applying Riblet film to airplanes. The technology received its aerospace license in 2022. In the same year, SWISS became the first airline who flies with Riblet covered passenger planes. In 2023, Lufthansa equipped their cargo planes with the AeroSHARK technology.
Regarding the 12-airplane fleet of SWISS, over 1% of fuel is saved by efficiency gains through Riblet surfaces. This translates to yearly savings of 4800 tons of fuel and 15200 tons of CO2.
The third basic Riblet application process is coating. Its respective device is basically constructed of two main components, the coating system and the application system.
The application process itself is conducted with a specially designed rolling device. The respective paint is sprayed onto the surface. Afterwards, a silicone foil, covered with the negative Riblet structure, embosses the microscopic shapes into the coating, which are then cured from behind with an UV light source.
The continuous process combines the application of paint, embossing the Riblet structure and its curing in one single step and hereby reaches an application speed of 1 m² per minute.
