# Advances in Riblets Design
## 4 Manufacturing and Fabrication of Riblets  
During the past few decades, the fabrication of riblet geometries has been implemented through several different methods in order to fulfill the requirements of experimental research studies. An array of research activities introduced different procedures of fabrication as a part of their studies on the effectiveness of riblets [55; 65; 80; 108; 109; 110]; however, a few studies have been specifically devoted to investigating different methods of industrial fabrication [45; 66; 84; 86; 104]. Research works that are developed in academic environments have been mostly conducted with cost-effective fabrication techniques, using small-size flat plates or extruded airfoils, while in commercial applications, some other parameters such as weight, time of implementation, durability and maintenance would be more important [18; 56]. An overview of manufacturing methods with their advantages and disadvantages is presented in Table 3. \begin{table}
\begin{tabular}{l l l l} \hline \hline
\begin{tabular}{} \end{tabular} } \\  & & & & \\ \cline{1-1} \cline{5-5}  & & & & \\ \cline{1-1} \cline{5-5}  & & & & \\ \cline{1-1} \cline{5-5}  & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._ \begin{table}
\begin{tabular}{l l l} \hline \hline
**Method** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{Milling [34; 65; 90; 110; 111]} & High precision \\  & Ability to make different patterns & Time-consuming \\  & Wide material compatibility & Material removal \\  & Suitable surface finished & Possibility of misalignment \\ \hline \multirow{4}{*}{Grinding [109]} & \multirow{4}{*}{\begin{tabular}{} \end{tabular} } & \multirow{4}{*}{\
\begin{tabular}{} \end{tabular} } \\  & & & \\ \cline{1-1} \cline{5-5}  & & & \\ \cline{1-1} \cline{5-5}  & & & \\ \cline{1-1} \cline{5-5}  & & & \\ \cline{1-1} \cline{5-5}  & & & \\ \cline{1-1} \cline{5-5}  & & & \\ \hline \multirow{4}{*}{
\begin{tabular}{} \end{tabular} } & Ideal for deformable materials \\  & No material removal & Limited to deformable materials \\ \cline{1-1} \cline{5-5}  & Uniform riblet patterns & Initial tooling setup \\ \cline{1-1} \cline{5-5}  & Quick application for industrial use & Possibility of misalignment \\ \hline \multirow{2}{*}{3D printing [33; 51]} & High customization \\  & Complex shape compatibility & Material limitations \\ \cline{1-1} \cline{5-5}  & & Suitable for research & Variable implementation time and cost \\ \hline \end{tabular}
\end{table} 
Table 3: General strategies in riblets fabrication. From the laboratory tests point of view, riblets have been designed and manufactured in-house with specific strategies to cover the requirements of the test. Milling is a widely used method for producing riblet geometries [25; 110]. This approach allows for the creation of various riblet configurations. In a study by [PERSON] et al. [55], PVC pipes were employed, and lathe machining was utilized to produce triangular-shaped riblets. Subsequently, the pipes were flattened to create flat riblet plates with equal height and spacing. [PERSON] et al. [110] implemented a CNC five-axis milling micro-machining for producing different sizes and configurations of 2D and 3D riblets. Their method was able to produce modular ribbed plates, which were tested in a wind tunnel. The authors developed an iterative process to finalize the commands and toolpaths by use of CAM design and CNC machining and evaluated the quality of production by laser confocal microscope. Similar to [PERSON] et al. [110], [PERSON] et al. [111] applied the micro-machining technique by use of a diode-pumped, regenerative-amplified ps-Nd:YVO4-laser source on eight different types of metal. During the riblets fabrication process, the positioning of instruments should be strategically considered. [PERSON] et al. [65] used machine cuts of soft steel flat plates to produce bladed riblets, and pin-hole probes were mounted for pressure measurements between the ribs.