Boundary Layer and Near-Wake Measurements of NACA 0012 Airfoil at Low Reynolds Numbers

A study on the boundary layers and near-wake behavior of a NACA 0012 airfoil at low Reynolds numbers was conducted in order to gain knowledge about flow properties applicable to turbine blades and MAVs. Hot-wire anemometers were used to measure the boundary layers and near-wakes at angles-of-attack of α =0°, 3°, and 6°; and Reynolds Numbers of Rec=2.3×10, 3.3×10, and 4.8×10. The boundary layers were measured with small intervals along the direction normal to the airfoil surface by a 3-D traverse system. The near-wakes were measured at four downstream stations of x/C=0.1, 0.3, 0.5, and 1.0. The results show that the laminar separation of the boundary layers occurred at α =3°. The reattachment of the separated boundary layers was clearly observed for the case of Re=4.8×10 at α =6°, showing the formation of a long separation bubble. The properties of the long separation bubble were then observed and found to be different compared to the properties of short separation bubbles studied previously. The shear flow instability wave was investigated with respect to the formation of coherent vortical structures in the separated shear layer. It was found that the Reynolds number critically influenced the shear layer instability wave rather than the angle-of-attack. The near-wakes in the present paper were analyzed in detail from the boundary layer characteristics that directly affect nearwakes. In particular, the near-wake properties for Rec=4.8×10 and α =6° represented a clearly different evolutions compared to the other Reynolds number cases owing to reattachment.