# Advances in Riblets Design
## 3 Experimental Investigation of Riblets Performance
### Wind Tunnel Testing
Their results indicated the effect of riblets on 8.3% turbulent intensity reduction, increasing about 30% in integral scales, and a 2.8% skin friction drag reduction for the hot-wire position at 67% downstream of the flat plate from the leading edge. [PERSON] [62] used hot film/wire anemometry for mean and turbulence measurements. The hot film sensor head with a 1.5 mm diameter was flush mounted within the valleys of riblets, aiming to record low-frequency response by heat losses. The boundary-type hot wire sensor, used with an accurate positioning traverse mechanism, was used to calculate the friction coefficient inside the viscous sublayer [93]. [PERSON] et al. [90] tested serrate-semi-circular riblet surfaces by focusing on the interaction between the riblet edges and the turbulent boundary layer. The authors used a hot-wire and a 3D displacement traverse system with a displacement accuracy of 0.01 mm. They investigated the effect of riblets with modified tips in the ejection of turbulent flows. To evaluate the friction coefficient in each Mach number, they proposed multi-curves of friction coefficient based on u/U\({}_{\rm e}\) and (yU\({}_{\rm e}\))/v of the Clauser [94] chart to find out local skin friction on both flat and riblet surfaces. By comparing the interpolated values, a 7% reduction in drag was observed. Pittol measurements by rake or a traverse mechanism constitute another momentum integration method used for the indirect calculation of shear stress, which has been implemented by several research studies [63; 80; 88; 90; 91; 95; 96]. A significant number of research studies were facilitated by using both hot-wire and pitot pressure measurements aiming to validate the quality of gathered data. [PERSON] et al. [63] used pitot measurements in the wake location after the NACA 0012 airfoil. They assessed the two-dimensionality of the flow by use of airfoil trailing edge wake analyses at three constant stations (x/c = 2.0, 2.2, 2.5). Results showed that variation of \(C_{D}\) in three different stations have less than \(\pm 1\%\) deviation from each other. [PERSON] et al. [97] measured the total skin friction drag of a supercritical ADA-S1 airfoil by determining stagnation pressure positioned 1.2 chords downstream of the trailing edge. Their measurements were performed at Mach numbers range of 0.6-0.76 and angles of attack from \(-0.5^{\circ}\) to \(1^{\circ}\). The drag coefficient was obtained by use of pitot measurements, as follows:  
\[C_{D}=\frac{2}{\gamma P_{0}M_{\rm os}^{2}c}\int\Delta P_{0}dy \tag{1}\]  
where \(P_{0}\) is the total pressure stagnation, and \(\Delta P_{0}\) is total pressure loss. This formulation is valid for a 10% variation of Mach value on both sides of the airfoil compared to the free-stream Mach number. Results showed a drag reduction in the range of 3-6% for the riblet with h = 0.018 m, while drag was increased for h = 0.033 mm. [PERSON] et al. [91] also used hot-wire anemometry and pitot rake to measure the riblet surface drag reduction. They indicated the uncertainty of hot-wire velocity measurements by traversing under a turbulent boundary layer, in which the pitot rake was used to calibrate the measured U-velocity component of the hot-wire. [PERSON] et al. [96] carried out boundary layer pitot rake measurements to evaluate 2D momentum integral balance and local skin friction measurements. In addition, wall pressure fluctuations under the turbulent boundary layer are measured by flush-mounted microphones in both streamwise and spanwise directions. The selection of a proper microphone and method of mounting is critical for optimal accuracy of the measurements. The frequency of data gathering is directly related to the flow regime and the frequency of near-wall pressure fluctuations. In very small sizes, such as those from riblet geometries, and due to the limited space for positioning, pinhole microphonesare used.