Experimental Studies on the Effect of Leading-Edge Tubercles on Laminar Separation Bubble

An experimental investigation was carried out to study the influence of incorporating undulations (tubercles) at the leading edge of a NACA 4415 airfoil at a low Reynolds number of 120,000. Measurements were carried out at angles of attack of 6, 12, and 18 deg, which encompasses prestall (6 and 12 deg) and poststall (18 deg) regimes of the baseline, respectively. The aerodynamic performance of the NACA 4415 airfoil with leading-edge tubercles was compared against an airfoil without tubercles (baseline) through pressure measurements. Additionally, surface oil-flow visualization and two-dimensional (2-D) particle image velocimetry (PIV) were carried out to obtain insight into the on-surface flow topology and off-body flowfield of the modified and baseline airfoils. At the lower angle of attack (alpha=6 deg), the extent of the laminar separation bubble (in both length and height), which was the dominant flow feature over the baseline airfoil, was significantly altered by the presence of tubercles at the leading edge. The addition of tubercles resulted in the formation of pockets of smaller separation bubbles instead of one single long bubble spread along the span observed in the baseline airfoil. The 2-D PIV and oil-flow visualization results at an angle of attack of 18 deg prove that the tubercles are very much effective beyond the stall conditions of the baseline airfoil. The modified airfoil maintained attached flow until 50% of the chord, instead of complete separation starting from the leading edge, as noticed for the baseline case. The size of the recirculating zone downstream of the separation was also significantly reduced by the tubercles. All these factors contribute to the increased performance of the airfoil with leading-edge tubercles, especially at poststall angles of the baseline.