# Numerical Investigation of Jet Angle Effect on Airfoil Stall Control
## 3 Results and Discussion
### Pitching Airfoil Simulations
#### 3.2.2 Flow Control
Similar to the static stall, the chord-normal jet disrupts the external flow, forming a virtual wall with the strong jet with the high jet angle with respect to the freestream direction. It can be postulated that a high-velocity jet with such a large jet angle distributed uniformly in the span is not preferable for the stall control. It should be, however, noted that a discrete distribution of the high-angle jet in the span can avoid the negative disruption on the external flow. As [PERSON] et al. (2017) investigated, a discrete jet can generate secondary vortices in the span which help the mixing of the low-momentum boundary layer flow with the high-momentum freestream flow, and eventually suppress both the static and dynamics stall. Numerical study of a high-angle jet discretely distributed in the span requires three dimensional computations, not pursued in the current investigation. The flow control case with the tangential jet is further investigated at several major events in the pitch cycle (see Figure 17). This analysis is similar to the baseline shown in Figure 1 in the introduction. As the airfoil pitches up beyond the static stall angle (from stage 1 to 2 in Figure 17), the lift increases continuously due to the well attached flow on the suction side. At stage 3, the initial DSV grows to almost its maximum size, causing the first moment stall while elevating the lift and the drag. From stage 3 to 4, the initial DSV separates from the airfoil, resulting in the first lift stall. In the downstroke, the secondary DSV forms and grows to its maximum size around stage 5, which leads to the second moment stall and the second lift peak. As the secondary DSV detaches from the airfoil, the second lift stall occurs at stage 6. The tangential jet interacts dynamically with the separating flow even at the relatively low angle of stage 7 in the downstroke. For the sufficiently small angle at stage 8, the suction-side flow reattaches on the airfoil. Several key parameters associated with the aerodynamic coefficients are computed with and without flow control, as shown in Figure 18. The cycle-averaged lift with no actuation is \(c_{l,avg}\simeq 0.97\). In the case of the tangential jet, \(c_{l,avg}\simeq 1.29\), about 33% increase from the baseline lift. The tangential jet also increases the average drag \(c_{d,avg}\simeq 0.17\), about 42% increase from the baseline drag. It should be mentioned that the lift-to-drag ratio during the cycle increases drastically with the tangential jet. This is because (1) the lift is enhanced at low angles of attack when the drag remains small and (2) the drag increase is accompanied with the lift enhancement at high angles in the pitch (see Figure 15). The chord-normal jet depreciates the average lift \(c_{l,avg}\simeq 0.6\). Therefore, the overall aerodynamic performance of the OA209 airfoil is improved with the tangential jet. A tangentially blowing jet is preferable for most of the pitch cycle, significantly increasing the aerodynamic lift and the lift-to-drag ratio. Despite the favorable aspect of the tangential jet, it minimally affects the magnitude of the stall vortex--the peak moment is not reduced with the tangential jet. Additional jet actuation may enhance the control authority further. The chord-normal jet can be effective in suppressing the moment stall, if the jet is generated briefly when the stall vortex breakdown is susceptible to this high-momentum jet. Furthermore, it can be speculated that active variation of the jet angle could enhance the lift and suppress the peak moment. Active control of the jet angle can directly suppress the growth of the DSV, breaking it into smaller and weaker vortices. Such elaborate control methods may also help to avoid the double dynamic stall events which occur in the current case of the tangential jet actuation. Figure 17: The OA209 airfoil with the tangential jet in the static and pitching conditions. \(Ma=0.3\), \(Re=1.15\) M, \(c_{\mu}=0.06\), \(k=0.05\) and \(\alpha=13^{\circ}\pm 7^{\circ}\).