# Advances in Riblets Design
## 8 Fluctuating Terms of the Flow
### Riblet Effects on Velocity Fluctuations  
Low-speed streaks are coherent structures that play a significant role within turbulent boundary layers. They are particularly relevant in the transition zone and contribute to the formation of turbulence. These streaks are characterized by regions of reduced velocity compared to the surrounding flow. They are closely associated with the bursting phenomenon, which marks the transition from laminar to turbulent flow. The presence and behavior of low-speed streaks influence the overall turbulence characteristics, including turbulence intensity and flow mixing. Studying the formation and dynamics of these streaks is crucial for a better understanding of turbulent flows and their impact on various flow properties. The spanwise motion of low-speed streaks under the viscous sublayer would be restricted by using an optimized size and shape of riblet geometries. Optimized riblet geometries could also possibly produce lower levels of velocity perturbations [56; 91; 92]. This reduction has been studied in both streamwise [56; 126] and spanwise [92] directions by considering different boundary layer heights. Several analyses showed that the turbulence intensity would be reduced for a specified amount of non-dimensional height above the riblet geometries. [PERSON] [62] pointed out an increase in the amount of mean velocity over the bladed riblet surface, with a reduction of about 10% in the turbulent intensity compared to the smooth surface. The author reported the reduction in turbulence intensity just for the \(y^{+}<70\). The results showed a reduction in maximum turbulence intensity for a smooth surface in \(y^{+}=18\) with \(u^{\prime}/U_{\infty}=0.104\), which was reduced to 0.0934 at \(y^{+}=19\). [PERSON] et al. [92] observed lower values of Root Mean Square (RMS) velocity fluctuations in the flat plate, inside semi-circular riblet valleys in drag reduction cases and \(y^{+}<30\), while there were slightly lower or similar to smooth surface behavior at \(y^{+}>30\). [PERSON] et al. [56] indicated a reduction in turbulence intensity between 10-15% for \(y^{+}<100\) in the trailing edge position of a NACA 0012 airfoil for 0-4\({}^{\circ}\) of inlet flow angels, but such reductions were observed for 5-6\({}^{\circ}\) in the \(y^{+}<40\). [PERSON] et al. [91] investigated the fluctuating terms of U velocity at 2 mm and 20 mm, with a boundary layer height of approximately 10 mm, and observed that turbulent structures would become favorable by reducing skin friction drag. Their measurements inside the boundary layer indicated an 8.3% reduction in turbulence intensity compared to the smooth plate, while it remained nearly unchanged in the outer boundary layer. The reduction of turbulent intensity also affects the boundary layer transition time. [PERSON] et al. [126] indicated that the growth rate of the momentum thickness during the non-linear stage of the transition over a smooth surface is greater than over the ribbed surface. Additionally, the turbulence intensity is reduced by riblets, supporting the fact that the transition has been delayed. The evaluation of energy dissipation rate has been investigated by [PERSON] et al. [91] and [PERSON] et al. [63]. Energy dissipation is related to the small scale of turbulence by assuming a relation between producing and dissipating among large and small-scale turbulence structures. Riblet surfaces showed lower dissipation (about 18%), while large-scale turbulent structures became larger in the streamwise direction. [PERSON] et al. [63] observed the reduction in energy levels in a NACA 0012 airfoil for all ranges of incidence angle, with \(\beta=0.5-1.06\) at \(y^{+}=20\), and low-frequency range (lower than 200 Hz) on the riblet surface. The effect of riblets on Reynolds shear stress profiles has also been studied by [PERSON] et al. [56], [PERSON] al. [68] and [PERSON] [29]. [PERSON] [29] reported a reduction of about 10% in the \(y/\delta\approx 0.1\) for ZPG flows. [PERSON] et al.