Physics of Separated Flow over a NACA 0015 Airfoil and Detection of Flow Separation

This paper discusses an experimental study of flow separation over a 0.35 m chord NACA0015 airfoil at an incidence angle of 11o and a chord Reynolds number of 1 million. 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, angled fluidic vortex generators (FVG) are deployed through 44 orifices, spaced 15 mm apart and positioned in a single spanwise array located 0.3c downstream of the leading edge of the airfoil. One of the objectives of this work is to establish a statistical relationship between pressure and velocity signals during both the uncontrolled steady state and the transient processes of attachment/separation. After establishing the physics of the control, a separation criterion based on scale differentiation of coherent structures during the process of flow separation is proposed. The criterion is based on the rms of the pressure signals that have been band pass filtered around the energy containing range at the respective positions of the transducers. A threshold rms successfully predicts the onset of separation.