# Advances in Riblets Design
## 6 Crossflows and Riblet Surfaces  
The effect of crossflows on the ribbed surfaces has been investigated in a number of studies [26; 53; 70; 74; 131]. Supercritical airfoils and swept wings are commonly employed in commercial aircraft to enhance stability and enable high-speed flights. However, one draw \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Reference/Year** & **Geometry** & **Method** & **Solver** & **Result and Discussion** \\ \hline [PERSON] et al. [123] & Flat plate, triangular & DNS & In-house spectral method code & Drag reduction: 4\% \\ \hline [PERSON] et al. [127] & Flat plate, semi-circular & LES & In-house code & \\ \hline [PERSON] et al. [58] & Flat plate, semi-circular, triangular & RANS & Modified k-\(\omega\) and Spalart–Allmaras ONERA & Adverse pressure gradient \(\text{Re}_{\text{-channel}}=5000\), h/s = 0.3 Drag reduction: 9\% in APG (\(\beta=0.25\)) \\ \hline [PERSON] et al. [59] & Flat plate, semi-circular & LES & In-house code & Use of riblet experiments, sweep angle: 0, 10, 15, 20, 30 different s/h values \\ \hline [PERSON] et al. [32] & Flat plate, semi-circular, Blade, triangular & LES & ANSYS Fluent 14.5 & ZPG and mild APG, (\(\beta=0.5\)), h/s = 0.5 \\ \hline [PERSON] et al. [129] & Flat plate, bladed, semi-circular, triangle & LES & & \(h^{+}=8\) and \(s^{+}=8.3-41.1\) Drag reduction in bladed: 11.6\% Drag reduction in semi-circular: 5.6\% Drag reduction in triangular: 4.1\% \\ \cline{2-5}  & Airfoil Eppler E374, triangular riblet film, numerical trip & LES & & Comparing two grid sizes and Choi DNS data Streamwise riblet, Re = 2800 \(s=0.1135\delta\), \(h=0.0892\delta\) \\ \hline [PERSON] et al. [38] & Flat plate, triangular & DNS & In-house code incompressible Newtonian fluid governed by NS & M = 0.2, \(\alpha=3^{\circ}\), \(Re=2.0\times 10^{5}\) Drag reduction of about 9.5\% in drag at 45\({}^{\circ}\) of sweep angle \\ \hline [PERSON] et al. [125] & Flat plate, triangular & DNS & In-house code & \(Re_{\tau}=186\) and \(Re_{\tau}=547\) Different riblets tip angles: 45\({}^{\circ}\), 60\({}^{\circ}\), 75\({}^{\circ}\), 90\({}^{\circ}\), 105\({}^{\circ}\) \\ \hline [PERSON] [69] & Flat plate, triangular, transonic airfoil, NASA CRM & RANS & FLOWer & \(s^{+}\)= 30.82 \(h^{+}\)= 15.41 \\ \hline [PERSON] et al. [96] & Flat plate, TRA2022 aircraft, spaced triangular & RANS & FaSTAR & s = 100\(\upmu\)m for flat plate \(50<s_{opt}<150\) for the aircraft \\ \hline [PERSON] et al. [37] & UAV, triangular & RANS & OpenFOAM & Cruise speed: 22 m/s Re chord base: \(5\times 10^{5}\) Drag reduction about 3\% \\ \hline [PERSON] et al. [130] & UAV, triangular & RANS & ANSYS Fluent & Cruise speed = 39 m/s Re chord base: \(1.9\times 10^{6}\) Drag reduction: 5\% \\ \hline \hline \end{tabular}
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Table 4: _Cont._back of these design features is the occurrence of crossflows on the wing surface, which contributes to increased drag generation. In this regard, a number of research activities have been devoted to investigating the misalignment between streamlines and riblets. [PERSON] et al. [103] installed 3M riblet films, with an angle of 15\({}^{\circ}\) of misalignment to the flight direction, on the wing of a T-33 aircraft with up to a M = 0.7 flight speed. It was shown that the effectiveness of riblet surfaces was reduced by half when compared to those aligned with the flow. In the same study with 3M riblet films, [PERSON] et al. [26] indicated unchanged characteristics of riblets until 15\({}^{\circ}\), which were reduced to the smooth flat plate properties up to 30\({}^{\circ}\). [PERSON] [88] conducted tests at supersonic speed (M = 1.25), with ribbed geometries between 0\({}^{\circ}\) to 90\({}^{\circ}\), by measuring drag from the misaligned flow with 5\({}^{\circ}\) steps. At 20\({}^{\circ}\), the drag reduction effect of riblet geometries became halved, and at 30\({}^{\circ}\), their effectiveness was lost completely. [PERSON] et al. [74] tested four different geometries in various yaw angles in the range of 0-20\({}^{\circ}\).