Dynamic Stall on Airfoils with Leading-Edge Tubercles

Abstract

Inspired by the leading-edge protuberances found on the flippers of humpback whales, tubercles have drawn significant interest for improving airfoil aerodynamic performance. In static conditions, airfoils with tubercles exhibit a softer onset of stall and increased lift in post-stall regime, though with a reduced maximum lift coefficient. However, their impact under dynamic conditions is less understood, particularly how they affect the formation and convection of the dynamic stall vortex (DSV), which is crucial to the dynamic stall process. Therefore, this study aims to investigate how tubercles affect the development and behavior of the DSV during dynamic stall conditions. To deliberately trigger the formation of the DSV and thereby force the airfoil into dynamic stall conditions, this study employed a pitch-up and hold motion starting at a zero angle of attack and increasing to final angles of 30 and 55 degrees. Two pitching rates of k=0.05 and 0.1 were investigated, providing a comprehensive analysis of how these conditions influence the aerodynamic characteristics of the airfoils across a wide range of dynamic stall scenarios.

To assess the impact of leading-edge tubercles on airfoil aerodynamics under dynamic stall conditions, a series of wind tunnel experiments was conducted, involving two tubercled airfoils and a smooth leading- edge airfoil. These experiments were performed at a Reynolds number Re=3.3x10⁴, determined by the chord length and free-stream velocity. Employing particle tracking velocimetry (PTV), the velocity field on the suction side of each airfoil was measured, while ensuring precise airfoil positioning within the flow-field through the use of white tracking markers. Detailed monitoring of the DSV core location and circulation was conducted using the normalized angular momentum (NAM) criterion, also referred to as Γ₁ method. This approach provided quantitative insights into the influence of the tubercles on the DSV. Due to the experimental setup limitations in directly measuring aerodynamic forces, computational fluid dynamics (CFD) simulations were conducted using OpenFOAM. These simulations employed a sliding mesh technique, which facilitated the consistent application of the same computational framework for various tubercle geometries and pitching conditions. The accuracy and reliability of the simulations were rigorously validated against the experimental data.

The results reveal that leading-edge tubercles significantly influence the aerodynamic performance of airfoils under dynamic stall conditions. It has been found that tubercles modify the onset and severity of dynamic stall by reducing the strength of the DSV and shifting its formation closer to the trailing-edge. These changes result in a weaker and shorter lift overshoot, facilitating a quicker transition to the deep stall regime where tubercles enhance the lift provided by the airfoil. This alteration in the dynamic stall process has been consistently observed across all tested pitching motions, with the effects of the tubercles found to be proportional to their amplitude. These findings suggest that tubercles serve as dynamic stall mitigation devices, potentially benefiting applications where dynamic stall frequently occurs and can compromise structural integrity.