The effects of tubercles on swept wing performance at pre-stall angles of attack

Abstract

Engineers are constantly seeking improvements in wing efficiency, as improved efficiency can translate into improved wing performance and reduced operating costs. A wing’s efficiency can be improved by either increasing its lift production or reducing its drag. Tubercles are protuberances on a wing’s leading edge that can improve an unswept wing’s efficiency at stall and post-stall angles of attack, AOAs. However, many wings typically operate at pre-stall AOAs, and the effects of tubercles on wing performance at these AOAs are largely unknown. In addition, incorporating wing sweep has become an increasingly popular choice during wing design. Therefore, this thesis describes an investigation into the effects of tubercles on swept wing performance at pre-stall AOAs. Particular attention was given to their effects on the components of drag, and on the effects that various tubercle geometric parameters have on wing performance. It was found that tubercles can increase a swept wing’s performance through increasing its lift-to-drag ratio and reducing its drag coefficient at pre-stall AOAs. This conclusion was found from force measurements taken on a NACA 0021 wing swept with a quarter-chord sweep angle of 35°. For this particular wing, below 8° AOA, tubercles were found to reduce the lift and drag coefficients by 4-6% and 7-9.5%, respectively, and as a result, the wing’s lift-to-drag ratio increased by 2-6%. Above 8° AOA, premature flow separation behind the tubercle troughs resulted in the tubercles reducing the lift coefficient, while dramatically increasing the drag coefficient. As a result, the lift-to-drag ratio was reduced. In addition, a Laminar Separation Bubble, LSB, formed over the smooth wing, which resulted in an increased lift-curve slope. Force measurements, flow visualisation, and a numerical model demonstrated that the tubercled wing affected the LSB formation and, as a result, reduced the augmented lift-curve slope. Wake surveys showed that the majority of the tubercled wing’s drag coefficient reduction below 8° AOA was due to a reduced profile drag coefficient. Below 8° AOA, the tubercles had little effect on the induced drag coefficient. Above 8° AOA, the premature flow separation over the tubercled wing resulted in an increased profile drag coefficient and a reduced induced drag coefficient. Furthermore, it was found that the tubercles modulate the profile and induced drag coefficients along the span of the wing, with local minima occurring behind the peaks and troughs, respectively. Conversely, local maxima in the profile and induced drag coefficients arise behind the troughs and peaks, respectively. The induced drag coefficient increases behind the peaks as the augmented circulation further tilts the augmented lift vector into the freestream velocity direction. Conversely, behind the troughs, the reduced circulation tilts the lift vector into the freestream velocity direction to a lesser extent, thereby reducing the induced drag coefficient. From the results presented in this thesis, it is apparent that the reasons for the modulation of the profile drag coefficient are extremely complex, involving boundary layer formation, LSB formation, and other observed flow patterns. Therefore, it is concluded that further investigation is required in order to fully understand the effects of tubercles on the profile drag coefficient. While an unswept tubercled wing produces pairs of equally strong, and oppositely signed, vortices, sweeping a tubercled wing results in the outboard vortex of each tubercle becoming stronger than its paired inboard vortex. A new geometric parameter, the phase, has been introduced in this thesis to describe the point along a tubercle at which a wing terminates. A parametric analysis investigating the effects of the tubercle amplitude, wavelength, and phase on the wing’s lift coefficient, induced drag coefficient, and lift-to-induced-drag ratio at pre-stall AOAs showed that the phase typically has the greatest effect on these wing performance parameters, while the wavelength has the least. The phase also polarises the effects of tubercles on these performance parameters, whereby termination on a trough results in reduced lift and induced drag coefficients, and an increased lift-to-induced-drag ratio. Conversely, termination on a peak results in increased lift and induced drag coefficients, and a reduced lift-to-induced-drag ratio. A genetic algorithm was developed to optimise the tubercle’s amplitude, wavelength, phase, location, and number to achieve the greatest lift-to-induced-drag ratio; the result being a single trough located at the wingtip, which increased the lift-to-induced-drag ratio by up to 4.3%. A final experimental campaign showed that a single tubercle terminating at a wingtip typically yields smaller performance benefits than tubercles along an entire leading edge. As a result of this research, a framework now exists to design a tubercle’s geometry to maximise a wing’s lift coefficient, lift-to-drag ratio, or lift-to-induced-drag ratio, or to minimise its induced drag coefficient or total drag coefficient at pre-stall AOAs, given the operating conditions.