# Advances in Riblets Design
## 3 Experimental Investigation of Riblets Performance
### Wind Tunnel Testing
[PERSON] [25] evaluated wall pressure fluctuations by use of pinhole microphones in trapezoidal-bladed riblets (see Figure 1e), with appropriate spacing between the ribs. A similar study was carried out over an airfoil by [PERSON] [51] to evaluate the effect of riblet geometries in the near wall pressure fluctuations, especially focused on the trailing edge noise. As the riblets affect the structure of the turbulent boundary layer, a miniature loudspeaker was positioned in the middle of the strip to generate turbulent spots with specified square wave pulse signals in a defined frequency. Based on the instrument's geometrical limitations, the authors used trapezoidal riblets, which allowed the mounting of pinhole microphones between each protuberance in the longitudinal and lateral directions. In addition, a reference microphone and a loudspeaker were temporarily mounted to evaluate each pinhole microphone one by one. As the microphones were positioned at a distance from the test plate (see Figure 3), the response times of signals were calibrated by use of a reference microphone and loudspeaker [51; 62]. In the context of wall pressure fluctuation measurements, the size of the microphone diaphragm must be considered due to the size of the turbulent eddies within the boundary layer [65; 98]. Pressure sensors should be small enough to resolve small scales of turbulent structures. As an estimation of the sensor's size, these should be in the order of a tenth of the viscous lengths, which is roughly estimated by the value of Reynolds number based on the momentum thickness. As an example, increasing the Reynolds number by a factor of ten would change the Kolmogorov scales from millimeters to micrometers [98]. Flow visualization techniques made their way to characterize the effectiveness of riblets by measuring the three-dimensional properties of the flow within the turbulent boundary layer. In recent years, researchers developed several techniques for capturing pictures of turbulent flow structures [48; 62; 83; 92; 99]. [PERSON] et al. [100] investigated the effect of riblets in the structure of turbulent boundary layers by using dye and hydrogen bubbles. Results showed a burst rate reduction of about 20-25% for different values of \(h^{+}\) and \(s^{+}\). Particle Image Velocimetry (PIV), Particle Trace Velocimetry (PTV), and Tomographic Particle Image Velocimetry (TPIV) techniques were used in several test procedures to investigate the boundary layer on a flat plate [38; 77] or in the wake region produced by airfoils trailing edge [83; 99]. Smoke wire and sheets of laser light over the flat plate riblet  
Figure 3: Schematic of calibration mechanism for the microphones [51]. surface showed a significant change in turbulent coherence in the riblet surface area. Due to the complex structure of flow vortices, fluctuating terms of the flow, and synchronization between the instantaneous 2D anemometry and captured frames should be considered [62]. Flow visualization can also be carried out on the wake region of airfoil geometries. [PERSON] et al. [99] and [PERSON] et al. [83] indicated the effectiveness of triangular micro-riblets on the NACA 0012 by the use of a two-frame PIV film measurement technique and ensemble averaging in the wake region behind the airfoil geometry. [PERSON] et al. [53] conducted experimental evaluations on a wing by use of the oil-flow technique and titanium dioxide as a pigment for various angles of attack between 0\({}^{\circ}\) and 6\({}^{\circ}\). Results demonstrated deviations in streamlines for various incidence angles, indicating the presence and effects of adverse pressure gradients on the trailing edge region [68]. Using synchronized smoke wire, [PERSON] et al. [92] investigated the positive effect of riblets in lifting the flow vortices over semi-circular riblet tips, leading to the reduction in wetted area. The authors reported the effect of riblets on the reduction of velocity fluctuations and turbulent kinetic energy inside the viscous sublayer compared to the flat plate.