# Advances in Riblets Design
## 3 Experimental Investigation of Riblets Performance
### Wind Tunnel Testing  
Wind tunnel studies on riblet surfaces primarily focus on evaluating their effectiveness and potential for drag reduction. On this basis, the Reynolds number plays an important role in the measurements; at lower Reynolds numbers, where viscous forces dominate, and the flow near the surface tends to be laminar and smooth. In this regime, riblets have a relatively smaller impact on drag reduction since the flow is already less turbulent. However, as the Reynolds number increases, the flow transitions to a turbulent regime characterized by energetic flow with fluctuations and eddies near the surface. In this turbulent flow regime, riblets can be more effective in reducing drag by interacting with the turbulent flow and minimizing energy losses associated with turbulent boundary layers. In addition, riblets have been observed to reduce the occurrence of flow separation. They help to delay or prevent the detachment of the fluid flow from the surface, resulting in improved aerodynamic performance and reduced drag [56]. Table 1 summarizes experimental activities that are related to 2D riblet geometries tested in the wind tunnel. To measure drag reduction in turbulent flows, sensors have to obtain both the mean values and fluctuating components of the flow. Classification of direct and indirect measuring methods has been introduced by [PERSON] [87]. Direct measurements utilize a flush-mounted floating element that is able to displace tangentially by flow viscous force. Wall shear stress is calculated by the load cells that hold the floating surface. [PERSON] [23], [PERSON] [88], [PERSON] et al. [55], [PERSON] [67], [PERSON] et al. [89], and [PERSON] et al. [83] use direct measurement in their studies. [PERSON] [23] studied several riblet geometries in a closed-loop wind tunnel with a floating element at NASA Langley Research Center. In the optimum case, the author recorded around an 8% reduction in drag for triangular and semi-circular riblets. [PERSON] et al. [55] tested triangular riblets in both zero and adverse pressure gradients. They found different accuracies of measured data by the drag balance mechanism, which were between 1-3% for zero and adverse pressure gradients, respectively. Although the direct method determines viscous shear stress without any assumption of the flow properties, measurement uncertainties related to the displacements and the presence of gaps between the test plate and tunnel walls must be considered [50; 55]. On the other hand, indirect techniques for shear stress measurements in both momentum balance and correlation methods [87] have been implemented. These methods offer alternative approaches to estimating shear stress, providing additional options for researchers in the field. Due to the complex behavior of coherence structures, indirect methods are more popular for viscous shear stress measurements. In this classification, hot wire anemometry, which is used to measure mean and fluctuating components of the velocity, is a subdivision for momentum balance measurements. [PERSON] [23], [PERSON] [62], [PERSON] et al. [90], [PERSON] et al. [63], [PERSON] et al. [80], [PERSON] et al. [40], [PERSON] et al. [91], [PERSON] et al. [92], and [PERSON] et al. [51] investigated the drag reduction effects of ribletsby the use of momentum balance on flat plates and airfoils. [PERSON] et al. [63] proposed hot wire anemometry over the x/c = 0.964 of the NACA 0012 airfoil tested at four different angles of attack between \(\alpha=0^{\circ}-6^{\circ}\). The results showed more drag reduction for positive angles of attack rather than zero angles, with more drag reduction obtained for \(\alpha=6^{\circ}\). [PERSON] et al. [91] investigated the turbulent scales within the turbulent boundary layer by the use of hot wire to evaluate skin friction drag. Flow properties, such as turbulent scales and intensities, were evaluated by free stream velocity and hot-wire anemometry.