Experimental Studies on the effect of Leading edge Tubercles on Laminar Separation Bubble

The effect of wavy leading ledge, in the manner seen in humpback whales, known as tubercles, to the leading edge of a NACA 4415 airfoil is investigated experimentally at low Reynolds number of 120,000. Comparisons are carried out against a baseline NACA-4415 airfoil is addressed through surface flow visualization, surface pressure and particle image velocimetry (2D-PIV) measurements. Experiments were carried out at angles of attack of 6° and 18° corresponding to pre-stall and post-stall conditions of the baseline airfoil with free stream velocity of 7.5 m/s. The oil-flow visualization studies reveal interesting complex flow features, repeating behind every tubercle at both angles of attack. At the lower angle of attack the extent of the laminar separation bubble, which is a dominant feature on the baseline airfoil, is significantly altered by the presence of the tubercles, which result in the formation of smaller separation bubbles spread along the span instead of one single bubble. The 2-D PIV and oil flow results angle of attack of 18° show that tubercles are very much effective beyond the stall conditions of the base line airfoil. They maintain attached flow till 50% of the cord at least in regions behind the tubercles, instead of complete separation at the leading edge as noticed for the baseline case. The size of recirculating zone downstream of the airfoil post-stall is also significantly reduced by the tubercles. All these factors contribute to the increased performance of the tubercles compared to the baseline NACA 4415 airfoil. However, post-stall the use of tubercles seems to impose a highly varying separation point and subsequently the associated wake width on the airfoil, the effects of which are yet to be explored in detail