**3.1. Vortex generators**

The vortex generators (VGs) as depicted in **Figure 3** [19] are the most effective and simplest passive flow control devices that are widely preferred and utilized on wind turbine blades by aerodynamic researchers in order to prohibit and suppress flow separation caused by APGs. VG examples are not limited to airfoil [20], and they can also utilize the devices such as bluff bodies [21], noise reduction [22], wind turbines [23], swept wings [24], and heat exchangers [25, 26], just to name a few. VGs, which were first investigated by Taylor [27], are generally small plates having rectangular or triangular shapes. They can be mounted on the surface where desired to flow control at an angle of the incoming flow. They are used to decrease (to even suppress) the boundary layer separation, which is caused by APGs and turbulence effects [28]. The slower moving boundary layer is energized by VGs in conjunction with high momentum fluid in the outer part of boundary layer and in the free stream [29], resulting in reducing the drag force [30] and increasing the lift force [31, 32].

Regarding the optimization of VGs, many aerodynamic researchers have investigated the VG's parameters to obtain optimal impacts on fluid flow. As shown in **Figure 4** [33], it can be said that the important parameters are height (*h*), cropped edge length (*b*), vane length (*L*),

**Figure 3.** A sketch of vortex generator rows [19].

type, shape, pattern, long (*D*) and short (*d*) gaps among the vanes, size, location, and inflow angle (*β*). In addition to these important parameters, VG configurations termed as counter rotational and corotational also play critical roles in terms of rotational directions of vortices formed by VG pairs [34].
