**3.9. Airfoils with cavity**

One of the passive control techniques is the application of a cavity mounted on a thick airfoils' suction surface that is taken from original Kasper's wings [62]. The principle of this concept is to create a convenient pressure gradient when two counter-rotating vortices inside the cavity are trapped. Furthermore, these trapped vortices over the suction surface not only ensure an extra low-pressure region but also cause a lower drag to produce. Thus, this method has recently gained interests among aerodynamic researchers. Olsman and Colonius [63] investigated an airfoil with a cavity at Reynolds number of 2 × 104 and different angles of attack ranging from 0° to 15° as seen in **Figure 12**. Their results revealed that stall phenomenon was delayed by means of counter-rotating separated flows, resulted in reduced flow separation region. A detailed numerical study regarding the aeroacoustics of NACA 0018 cavitied airfoil was reported by Lam and Leung [64] at Reynolds number of 2 × 104 and Mach number (Ma) of 0.2.

**Figure 11.** Illustration of a movable flap at a high angle of attack: seagull at landing (top picture) and sketch in principle (bottom picture) [58].

**Figure 12.** The vorticity contour plot over the airfoil with a cavity [63].

The presence of cavity caused the lift-to-drag ratio to increase. Moreover, cavitied airfoil produced less acoustic power, making it a noiseless airfoil design at low Reynolds number regimes.
