**3.2. Leading-edge slats**

The leading-edge slats, which were known as a passive flow controller by delaying the flow separation, were initially presented by Handley Page [35] in Great Britain and it was first utilized for an aircraft [36]. As illustrated in **Figure 5** [19, 37], the flow in space between the main body and slat is augmented and accelerated with either large vortices or multiple smaller vortices. Large vortices moving from slat's midspan to its edge can occur at lower Reynolds

**Figure 4.** (a) Isometric perspective and (b) planar sketch of the VG row [33].

**Figure 5.** (a) Configuration of leading-edge slat [19] and (b and c) its planar view [37].

number, whereas smaller vortices can be observed at higher Reynolds number. Accelerating flow with leading-edge slats gains kinetic energy and momentum to the boundary layer, resulting in delaying of stall phenomenon [38]. In the literature, there are three types of slats: (i) fixed slat [39], (ii) retractable slat [40], and (iii) Kruger flap [41]. Recently, Genç et al. [42] have investigated NACA2415 airfoil with NACA22 leading-edge slat experimentally and computationally. Their computational results indicated that experimentally stated LSB was correctly estimated. Moreover, delaying of the stall phenomenon was obtained by means of experimental investigation, resulting in providing the maximum lift coefficient of 1.3.
