Experimental Flow Visualization Study of Flow Separation Control With High-Frequency Translational Surface Actuation

Flow separation causes aircraft to experience an increase in drag degrading their aviation performance. The current study aims to delay flow separation on an airfoil by embedding a high-frequency translational piezoelectric actuator along the surface of the airfoil. This study focuses on investigating the extent to which the piezoelectric actuator displaces the flow separation downstream of the airfoil or prevents it altogether utilizing a fog-based flow visualization experiment. The actuators with two actuation surfaces were embedded on the suction surface of an Eppler 862 airfoil model and placed in a low-speed wind tunnel. The actuation surfaces were located at the maximum thickness point of the airfoil in the streamwise direction. A fog generator was used to inject dry ice fog into the wind tunnel which was illuminated by a continuous laser in order to visualize the flow. Consecutive pictures of the flow field around the airfoil were taken using a high-speed camera in order to observe the flow separation phenomenon before and after turning on the high-frequency translational surface actuation. The effects of the actuation on the flow separation were observed at various actuation displacements, angles of attack, and free stream velocities. The vibrational characteristics of the actuation were investigated using a single-point laser Doppler vibrometer. The operating frequency of the surface actuation was 565 Hz. The measured actuation displacement ranged up to 0.12 mm at the maximum applied voltage of 150 V. The angle of attack of the airfoil varied from 6° to 24°. The maximum free stream velocity was increased up to 12.7 m/s, which corresponded to the chord Reynolds number of around 26,000. The translational surface actuation consistently and significantly suppressed the flow separation at all the conditions tested. It was confirmed that the actuation had a very strong influence on the flow separation even at a very small displacement of 0.024 mm remaining significantly reduced separation bubble compare to the one before activating the actuators at 4.3 m/s of velocity and 14° of the angle of attack. The flow separation was completely suppressed when the actuation displacement reached around 0.06 mm under the same conditions of flow velocity and angle of attack. This implied that the actuation should generate a strong enough momentum relative to the free stream in order to completely suppress the flow separation. It was also observed that there clearly existed the location effects of actuation relative to the separation point when the angle of attack and free stream velocity were varied under the fixed actuation displacement. In summary, the study confirmed that the employed high-frequency translational surface actuation had the obvious control authority on delaying or suppressing the flow separation over the airfoil depending on the parameters changed.