# Advances in Riblets Design
###### Abstract  
Continuous evolution in nature has created optimum solutions for creature survival that have inspired many innovative engineering designs. Riblet geometries, passive flow control devices, have been studied, which were inspired by the skin of fast-swimming sharks. Turbulent boundary layer research reveals the positive effect of riblets in reducing drag by manipulating turbulent structures. Reducing drag is an important topic for the aviation industry, as it directly relates to fuel savings and reductions in carbon footprints. Aircraft noise represents another significant area of concern. When riblet designs modify turbulent structures, they can also impact pressure sources within the boundary layer, consequently influencing the generation of self-noise. Earlier research studies have demonstrated the favorable outcomes of riblet configurations on the variations in wall pressure, resulting in reduced levels of noise propagation. The current review paper is mainly devoted to the application of riblets in the aviation industry, focusing on studies that are performed in wind tunnels, flight tests, and using numerical techniques. Proving the desired performance of micro-grooves, their method of fabrication and implementation on aircraft surfaces are important topics that are also discussed. In addition, the effect of durability on the performance and required maintenance intervals was previously investigated and is also presented. Finally, recommendations for future activities in the relevant fields of study are provided. r 2023 13 10893 1  
trailing position of the wings, wavelet-like surfaces, and grooves over the feathers on lower parts of the wings would have a positive effect on reducing radiated noise. The optimum design of shark skin that has contact with water is another example that has evolved by nature. As a multi-objective evolutionary process, the anti-fouling and hydrophobic ability also evolved besides its reduction in drag. The idea of using shark skin structure on the surface that is in contact with the fluid flow is introduced as a solution to reduce frictional drag [14; 15; 16; 17; 18]. In this regard, many innovative surfaces replicated from shark skin made their way to the industry, facilitating improvements in engineering designs, especially in friction drag [19]. In aerodynamics, drag is a directional force that acts against movements between fluid and solid body. It always resists movements by producing two main types of resisting forces, named form drag, also known as pressure drag and skin friction drag. Form drag is the opposite force generated by air molecules when flowing past the solid body and pushing it harder against forward rather than backward motion [20]. Skin friction drag, or viscous drag, is the resisting force generated through the moving of fluid flow over the object's surface. Flow viscosity and velocity are the main factors that act on the value of skin friction drag. In addition to the above-mentioned fluid property factors, roughness and surface area exposed to the flow would affect the friction drag value. Riblets can be considered bio-inspired replications of fast-swimming sharks that acceptably mitigate friction or viscous drag through the surface [14; 15; 21; 22]. These aligned to the flow micro-size geometries can potentially lift high energetic, turbulent vortices from the surface to the riblet tips, leading to less interaction between the surface and turbulent vortices, hence resulting in friction drag reduction [2; 23; 24; 25; 26]. Since riblet performance shows the possibility of drag reduction, several numerical and experimental activities have been conducted to investigate different sizes and configurations over these geometries. There is also a wide range of analyses, including simple extrusions as well as complex 3D geometries, combined with various other methods for reducing drag [26; 27; 28; 29; 30].