Annexes - 194 - Figure 20 Vector velocity field during the reattachment process for quasi-steady actuation. The background presents the Γ1-criterion value highlighting the core of each vortex. 4. Conclusion The present study concerns the analysis of a controlled airflow over an axisymm<strong>et</strong>ric NACA 0015 airfoil. A single DBD actuator is used to interact with the surrounding airflow (U0=20 m/s) in order to control the d<strong>et</strong>ached region above the suction side of the airfoil model. To enhance the understanding of the interactions b<strong>et</strong>ween the ionized surface and the surrounding airflow, velocity vectors are measured by a high-speed PIV system. the airfoil is placed in post stall regime (angle of attack of 16°). The rep<strong>et</strong>ition rate of the acquisition system is high enough to obtain time-resolved informations for the baseline and forced flows. A single <strong>plasma</strong> actuator is mounted at the leading edge of the airfoil and the electrode configuration results in a co-flow actuation. Quasi-steady and unsteady actuations are performed while the effects of the excitation frequency and the used duty-cycle are described. The baseline flow is consistent with a post-stall regime as the flow is fully d<strong>et</strong>ached from the suction side with the presence of a vortex stre<strong>et</strong> initiated at the separation point. The computation of the temporal correlation
Annexes function reveals that a natural frequency of approximately 90 Hz drives the vortex shedding. The quasi-steady actuation results in a partial flow reattachment over 70% of the chord, and the flow remains attached along the acquisition sequence. The analysis of the unsteady actuation performed at different frequencies (with a low dutycycle value resulting in trains of single sine pulse) demonstrates that the unsteady mode is not more efficient than the steady one to control the separated airflow. The <strong>plasma</strong> effects depend strongly on the applied frequency, and the most effective actuation is performed with a reduced frequency of F + =1.5. The present results also demonstrate that a stationary flow reattachment is realized for duty-cycle value s<strong>et</strong> to 50% while reducing the duty-cycle has d<strong>et</strong>rimental effects on the control performance. The last part of this study is dedicated to the temporal characterization of the reattachment. The global analysis of the acquisitions for quasi-steady and unsteady actuations reveals a similar reattachment process. It appears that the electric wind is not able to produce beneficial vorticity by itself. However the actuation can promote a merging of the natural vortices. This energization of a large scale flow structure by the electric wind modifies the topology of the succeeding vortex shedding. In particular, few milliseconds after the beginning of the actuation, the trajectory of the vortex is changed and a small vortex rolls along the suction side with a sense of rotation promoting a momentum transfer toward the boundary layer. Through the observation of the time-resolved PIV fields, it seems that the vortex merging initiates the control process while the small vortex rolling along the airfoil surface seems responsible of the flow reattachment. 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