**3.5. Slotted airfoils**

Slots (generally known as a narrow rectangular channel along spanwise of a wind turbine blade) are one of passive flow control methods and flow control is ensured by changing the flow velocity over the airfoil. The principle of a slotted airfoil is that flow velocity increases at the slot exit after interior flow passes within the airfoil. This increment of flow velocity at the slot exit causes the streamlines to disrupt, resulting in creating the flow separation. The flow separation occurred over the airfoil means velocity reduction. This reduction in flow velocity enables the local pressure underneath the airfoil to increase, resulting in producing more lift. **Figure 8** illustrates the slot geometric characteristics performed by Belamadi et al. [49].

**Figure 7.** Schematic view of a wind turbine blade with modified models via serrations [43, 44].

**Figure 8.** Schematic demonstration of a slotted airfoil [49].

Symbols of *c*, *X*, *γ*, and *ψ* mean chord of the airfoil, the slot position, the slot width, and angle between slot axis and chord normal, respectively. Numerical results conducted by Belamadi et al. [49] indicated that stall phenomenon on S809 airfoil at an angle of attack of 20° was completely eliminated by creating a nozzle effect over the airfoil, ensuring the extra kinetic energy (inherently extra momentum) to suction surface. Based on experimental and numerical results obtained by Beyhaghi and Amano [50], an increment of the lift coefficient by 30% was ensured without conceding any drag force.
