**2. Fluidic Oscillator Model and Computational Domain**

The fluidic oscillator model used in this study was proposed by Melton et al. [24] and has two feedback channels. They reported that this fluidic oscillator model exhibited excellent flow separation control when mounted on a NACA0015 Airfoil. The geometrical parameters of this model are summarized in Table 1.

**Table 1.** Geometrical parameters of the fluidic oscillator tested.


In the present work, the effect of a bent outlet nozzle on the fluidic oscillator performance was investigated with and without external flow, and two different cases were considered: the internal flow of a single fluidic oscillator and the interaction between the internal and external flows of fluidic oscillators mounted on a NACA0015 airfoil with a simple hinge flap. Figure 1 shows the computational domain for the internal flow of the fluidic oscillator. Figure 1a,b show the domains for the standard and bent oscillators, respectively. In the case of the bent oscillator, the bending occurs at the throat of the outlet nozzle with a bending angle (β), as shown in Figure 1b. The bending angle (β) varies in the range of 0–40◦.

**Figure 1.** Flow configuration and fluidic oscillator model: (**a**) Reference model, (**b**) Model with bent outlet nozzle.

Figure 2 shows the computational domain for the interaction between the internal and external flows of the fluidic oscillators on the airfoil with a flap. This computational domain consists of the internal and external domains of the fluidic oscillators. Using the periodic conditions, only three oscillators are included in the domain. The reason for selecting this oscillator number was presented in the previous work of Kim and Kim [29]. Melton's experimental work [30] was referenced for the external flow configuration. The angle of attack (α) is fixed at 8◦, and the flap deflection angle (*δf*) is 40◦. When *δ<sup>f</sup>* = 0◦, the chord length (c) is 305 mm, and the span (S) is 99 mm. The flap is located at 70% chord from the leading edge of the airfoil, and fluidic oscillators are installed just in front of the flap hinge.

**Figure 2.** Computational domain for the external flow over NACA0015 airfoil with a flap and fluidic oscillators [29].

For external flow, the fluidic oscillator's mounting conditions are shown in Figure 3, which are the same as in the study of Kim and Kim [29]. The main body of the fluidic oscillator was fixed so that it was always parallel to the airfoil's cord line. Since the pitch and bending angles of the fluidic oscillator coincide, both are expressed as β, and the test range of this angle was determined as β = 0–40◦ considering the results of the previous work [29]. In the case of β = 0◦ (reference model), the exit surface of the fluidic oscillator is mounted close to point A' on plane A-A', and this plane is perpendicular to the airfoil chord line. However, as β increased, the exit plane was inevitably moved to plane B-B' to avoid interference with the flap surface. The B-B' plane is translated 7 mm downstream from the A-A' plane and rotated by the angle β around point B.

**Figure 3.** Installation conditions of fluidic oscillators.
