**3.7. Gurney flaps**

Gurney flap is a small boundary layer passive control method and it can be easily mounted at the trailing edge of an airfoil. The Gurney flap with 2% of the chord length of the airfoil can affect the aerodynamic performance by increasing the lift coefficient by 0.4. Moreover, the lift-to-drag ratio of the airfoil can be nearly improved by 35% [54]. Flow progress and

**Figure 9.** Whole and enlarged view of mesh domain for a leading-edge microcylinder [51].

mechanism of lift increment by the Gurney flap were explained by Liebeck [55]. As depicted in **Figure 10** [55], a pair of counter-rotating vortices composed at downstream of the Gurney flap creates a low-pressure zone within. This low-pressure region makes the flow to increase over the suction surface, resulting in rising to an increase in the suction pressure. On the other hand, the flow velocity is reduced at upstream of Gurney flap by anticlockwise vortices and pressure at the pressure surface is increased. Consequently, the variation of pressure distribution between surfaces leads to an increment of lift force. The unsteady flow characteristics, especially at low Reynolds numbers, may be mitigated and suppressed with the use of Gurney flap. Based on the transient two-dimensional numerical simulations performed by Zhu et al. [56], the adaptive Gurney flap was compared with the fixed Gurney flap and the greater energy harvesting efficiency was obtained when the adaptive Gurney flap was selected for the oscillating wing. Shukla and Kaviti [57] numerically investigated four symmetric NACA airfoils in conjunction with a dimple, Gurney flap and combination of both dimple and Gurney flap at Reynolds number of 3.6 × 10<sup>5</sup> . Their results indicated that better aerodynamic performance was obtained at NACA0021 airfoil with a combination of both dimple and Gurney flap when the angle of attack was 12°.
