**3.12. Partially flexible airfoil**

The last major control device, which is the objective of the chapter, among passive flow controllers is the flexible membrane used on the surface of the airfoil. This type of airfoil is called as a segmented or partially flexible airfoil. Since it is a new concept of flow control method, a detailed investigation of a partially flexible membrane is rarely studied in the aerodynamic literature. A pioneered computational fluid dynamics (CFD) analysis was performed using

flexible membrane material on the airfoil surface by using ANSYS software [95]. The fluidstructure interaction (FSI) method was used for numerical modeling to investigate interactions between fluid and membrane. The segmented airfoil is seen in **Figure 23** [95]. The flexible

**Figure 24.** Streamline of rigid and flexible airfoils, α = 13°: (a) rigid; (b) one-segment; (c) two-segment; (d) three-segment; and (e) four-segment [95].

**Figure 25.** Configuration of the partially flexible airfoil [99].

**Figure 26.** Integrated sketch of the flow visualization and standard deviation of the deformation at α = 8° for (a) Re = 2.5 × 104 and (b) Re = 5 × 104 [99].

membrane material was used on the suction side of the airfoil. They numerically modeled four different cases on which the upper surface of the airfoil was flexible. In this numerical model, the effect of flexibility on aerodynamic performance in various regions on the airfoil was investigated for a Reynolds number of 1.35 x 105 . **Figure 24** [95] gives information about flow over the uncontrolled airfoil and the segmented flexible airfoils. It has been observed that the interaction between flow and the segmented airfoil decreases flow separations at high angles of attack. It has been found that the airfoil with three separate flexible zones shows the best aerodynamic performance and increased the lift coefficient by 39% compared to the rigid airfoil around the stall angle.

Apart from numerical study, first detailed experimental investigations on a partially flexible airfoil at low Reynolds numbers were carried out by Açıkel and Genç [99]. They modified the rigid NACA 4412 airfoil by using a membrane material that was located on the upper side of the airfoil as denoted in **Figure 25** [99]. The location of the membrane was between x/c = 0.2 and x/c = 0.7. In this study, different experimental methods such as force measurement, velocity measurement, deformation measurement, and smoke wire visualization were used to investigate flow control on partially flexible membrane airfoil.

According to the experimental results, flow control with flexibility is more effective at low angles of attack. **Figure 26** [99] demonstrates a combined sketch of the membrane standard deviation and smoke wire visualization for α = 8°. This sketch showed that the membrane vibration modes were increased with increasing Reynolds number.
