Effect of leading edge tubercles on airfoil performance.

Abstract

This thesis provides a detailed account of an experimental investigation into the effects of leading edge sinusoidal protrusions (tubercles) on the performance of airfoils. The leading edge geometry was inspired by the morphology of the Humpback whale flipper, which is a highly acrobatic species. The aim of this study is to investigate the potential advantages and disadvantages of incorporating tubercles into the leading edge of an airfoil. Specific parameters have been varied to identify an optimum tubercle configuration in terms of improved lift performance with minimal drag penalties. The investigation has shown that for all tubercle arrangements investigated, increased lift performance in the post-stall regime comes at the expense of degraded lift performance in the pre-stall regime. However, it has also been noted that through optimizing the amplitude and wavelength of the tubercles, pre-stall lift performance approaches the values attained by the unmodified airfoil and post-stall performance is much improved. In general, the configuration which demonstrates the best performance in terms of maximum lift coefficient, maximum stall angle and minimum drag has the smallest amplitude and wavelength tubercles. A new alternative modification has also been explored, whereby sinusoidal surface waviness is incorporated into the airfoil, giving a spanwise variation in local attack angle. Results indicate that optimisation of this configuration leads to similar performance advantages as the best-performing tubercle configuration. It is believed that the flow mechanism responsible for performance variation is similar to tubercles. The deterioration in pre-stall performance for airfoils with tubercles in the current study has been explained in terms of Reynolds number effects and also the relatively weak spanwise flow in the boundary layer. In swept and tapered wings such as the Humpback whale flipper, spanwise flow occurs along the entire span, so the effect of tubercles can be expected to be much larger. Surface pressure measurements have indicated that the region of separation and reattachment for airfoils with tubercles is restricted to the trough between the tubercles rather than extending across the entire span. Hence, leading-edge separation is initiated at the troughs but occurs at a higher angle of attack for other locations, leading to a delayed overall stall for airfoils with tubercles. In addition, integration of the surface pressures along the airfoil chord has indicated that lift, and hence circulation, varies with spanwise position, providing suitable conditions for the formation of streamwise vorticity. A spanwise variation in circulation is also predicted for the wavy airfoil since the relative angle of attack varies along the span. Counter-rotating streamwise vortices have been identified in the troughs between tubercles using particle image velocimetry in a series of cross-streamwise, crosschordwise planes which have not been investigated previously using this technique. The associated peak primary vorticity and circulation have been found to increase with angle of attack for a given measurement plane. This provides an explanation for the effectiveness of tubercles post-stall since an increased primary vortex strength leads to a greater boundary layer momentum exchange. The results show that the magnitude of the circulation generally increases in the streamwise direction, except when there exist secondary vortex structures of opposite sign on the flow side of the primary vortices. A proposed mechanism for this increasing circulation of the primary vortices is the entrainment of secondary vorticity which is generated between the adjacent primary vortex and the airfoil surface. It is postulated that this process of entrainment alternates between the primary vortices in an unsteady fashion. Leading edge tubercles have also been found to mitigate tonal noise associated with the NACA 0021 and the NACA 65-021 at all angles of attack in a novel investigation. Elimination of the tonal noise occurred for the majority of modified airfoils and in many cases the broadband noise level was also reduced for certain frequency ranges. It is believed that tonal noise elimination is facilitated by the presence of the streamwise vortices and that the spanwise variation in separation location is also an important factor. Both characteristics modify the stability characteristics of the boundary layer, altering the frequency of velocity fluctuations in the shear layer near the trailing edge. This affects the coherence of the vortex generation downstream of the trailing edge, hence leading to a decrease in trailing edge noise generation.