# Advances in Riblets Design
## 8 Fluctuating Terms of the Flow
### Effects of riblets on Wall Pressure Fluctuations  
The generation of noise by airfoils interacting with turbulent flows is a significant concern in the field of aviation. Understanding how riblets can affect velocity and pressure fluctuations contributes to the exploration of noise reduction strategies and the development of quieter and more efficient aircraft designs. Wall pressure fluctuations due to the turbulent boundary layer associated with the application of riblets were investigated by a few researchers [51; 62]. [PERSON] [62] compared wall pressure fluctuations inside the turbulent boundary layer with and without riblets. He found that the RMS intensity of the wall pressure would be reduced by about 4% on the riblets' surface, representing a positive effect on the dissipation of turbulent structures [65]. The author used the Variable Interval Time Averaging (VITAL) method to evaluate the near-wall bursts in a three-dimensional view. Results showed that burst signatures of wall pressure fluctuations over the riblet surface would be dissipated rapidly when compared to the smooth surface. [PERSON] shows the results for the trapezoidal bladed surface and the effect of wall pressure spectra over the flat plate. The energy level was decreased near the wall for the low-frequency region (below 20 Hz), while it increased at higher frequencies (between 20 to 100 Hz) and remained unchanged for frequencies above 100 Hz. [PERSON] performed similar analyses with near-wall miniature hot film sensors with a frequency rate lower than 300 Hz. The results showed a similar low-frequency reduction in the wall pressure fluctuations spectrum [62], as illustrated in Figure 12.  
Similar to [PERSON] [38], another investigation was conducted by [PERSON] et al. [51] to evaluate the energy signatures of the turbulent boundary layer in three different ranges of  
Figure 12: Wall skin friction fluctuations on a smooth and riblet surface [62].  
frequencies. The frequency range \(0.2\leq f\leq 2\) kHz was defined as low/mid frequencies, and \(2\leq f\leq 6\) kHz was considered the high-frequency range. The large size of turbulent eddies refers to the low-frequency spectrum and considers spectrum decay of about \(f^{-1/2}\), while \(f^{-5/3}\) corresponds to the mid-range and \(f^{-5}\) to the smallest size of turbulent eddies. Figure 13 shows the obtained data in three different axial locations at 10 m/s of freestream speed. Based on the provided information, it seems that riblet surfaces have been observed to exhibit lower values of the wall pressure spectrum compared to smooth surfaces in both low and high-frequency regions. While the wall pressure spectrum shows almost the same decay rate in comparison to the smooth surface, the high-frequency zone represents a higher level of frequency decay in \(f^{-5}\). The analyses from [PERSON] [25; 62], [PERSON] et al. [34] in pressure fluctuations, and [PERSON] et al. [39] in velocity fields indicated that riblets have the potential for modifications in low and high frequencies by affecting large- and small-scale turbulent structures. As these structures have a direct relation to sublayer low-speed streaks, force alignment of these vortices by riblet valleys would significantly reduce the signature of turbulent energies [65]. Although a decrease in turbulent structures was observed in the low and high frequencies, the mid-frequency becomes slightly increased.  
While a decrease in turbulent structures was observed in the low and high frequencies on riblet surfaces, there appears to be a slight increase in the mid-frequency range. This increase is believed to be a result of modifications to the boundary layer through momentum exchanges between the inner and outer layers, which is facilitated by the riblets. The reduced boundary layer thickness caused by the presence of riblets limits the energy in the mid frequencies compared to smooth surfaces, resulting in a slight increase in the wall pressure spectrum [62].