Transient phenomena in separation control over a NACA 0015 airfoil

We present the transient phenomena occurring during the impulsive control of flow separation over a NACA0015 airfoil at an incidence angle of 11° and a chord Reynolds number of 1 million. Actuation is performed via pneumatic vortex generators, impulsively activated in order to analyze the transient phenomena corresponding to the attachment process and, conversely, to transient re-separation occurring when the actuators are switched off. Measurements are performed using a linear array of unsteady pressure transducers and a single traversing crosswire. The pressure transducers are positioned in the separated region of the airfoil, which extends ～ 0.3c upstream of the trailing edge at the above flow condition. To control the flow, the angled fluidic vortex generators are positioned in a single spanwise array located 0.3c downstream of the leading edge of the airfoil. We establish a statistical relationship between pressure and velocity signals during both the uncontrolled steady state and the transient processes of attachment and separation. The unsteady behavior of the attachment process is also qualitatively analyzed via a 0.3 million Reynold number visualizations. The emission of a “starting vortex” is evidenced. This corresponds to a transient increase of drag.