# Numerical Investigation of Jet Angle Effect on Airfoil Stall Control
## 4 Conclusions  
The effect of the jet angle on flow separation control is investigated for the rotorcraft OA209 airfoil using steady-blowing jets. Two jet angles are used: \(\theta_{jet}=22^{\circ}\) for tangential jet and \(\theta_{jet}=80^{\circ}\) for the chord-normal jet. Other jet parameters, including the jet location \(x_{jet}/c=0.1\) and the jet momentum \(c_{\mu}=0.06\), remain the same between the two steady-blowing jets.  
The tangential jet can provide additional momentum into the boundary layer directly, reducing the shape factor of the suction side boundary layer. The aerodynamic lift and moment are enhanced beyond the static stall angle for the static airfoil. In contrast, the chord-normal jet deteriorates the baseline aerodynamic performance, severing the external flow.  
The tangential jet helps to increase the aerodynamic performance of the OA209 airfoil for the case of deep dynamic stall. The dynamic stall onset is delayed, and, as a result, a higher aerodynamic lift is obtained with the tangential jet, compared to the baseline. The overall increase in the lift-to-drag ratio from the baseline is almost 100%. As with the static stall, the chord-normal jet is not effective in suppressing the dynamic stall.  
Although the tangential jet is promising for the suppression of both the static and dynamic stalls, it struggles in reducing the peak moment of the dynamic stall. It can be interpreted that the delayed formation of the dynamic stall vortex does not yield any major modification of the stall vortex strength.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Case** & \(W_{jet}\) [kW/m] & \(W_{d}\) [kW/m] & \(W_{l}\) [kW/m] & \(W_{net}\) [kW/m] & \(\Delta W_{net}\) [kW/m] \\ \hline Baseline & 0 & 3.9 & 105.6 & 101.7 & 0 \\ Tangential jet & 17.2 & 6.7 & 137.6 & 113.7 & 12.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Estimate of power for jet generation, power to overcome the drag, power from the lift, and the net power during the pitch period.  
Either active variation of the jet angle or multiple jet actuation with various jet angles would attenuate the severe moment stall associated with deep dynamic stall.  
Author contributions are given as follows: conceptualization, J.K. and S.J.; data curation, J.K., Y.M.P., J.L., T.K., M.K. and J.L.; formal analysis, J.K., Y.M.P., J.L. and T.K.; funding acquisition, S.J.; investigation, J.K., Y.M.P. and J.L.; methodology, J.K., T.K. and S.J.; project administration, S.J.; resources, S.J.; software, J.K. and T.K.; supervision, S.J.; validation, J.K.; visualization, J.K., Y.M.P. and J.L.; writing--original draft, J.K. and S.J.; writing--review and editing, J.K. and S.J.  
This work is supported by the Active Flow Control Actuators for Next-Generation Rotorcraft Main-Rotor Blades project (No. 17-113-305-016) funded by Defense Acquisition Program Administration (DAPA) and Agency for Defense Development (ADD) in Korea.  
The authors declare no conflict of interest. The funder had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.