Airfoil camber decreases the strength of the LEV, but increases the total bound circulation at the same time, due to an increase of the 'conventional' bound circulation at the inner half of the wing. The vortex is stronger on thin wings and enhances the total circulation. A strong LEV was found on all wing types at high St. The effect of several Strouhal numbers (0.2 < St < 0.4), camber and thickness on the flow morphology and on the circulation was analysed. To analyse the influence of wing morphology in slow-speed bird flight, the time-resolved three-dimensional flow field around different flapping wing models in translational motion at a Reynolds number of 22 000 < Re < 26 000 was studied. Existing studies have focused on force measurements, which do not provide sufficient insight into the dominant flow features. Therefore, it is expected that airfoil design parameters affect flapping wing aerodynamics differently. In flapping flight, very high angles of attack and partly separated flow are common features. But this knowledge cannot be readily applied to the flapping flight in insects and birds: flow visualizations and computational analyses of flapping flight have identified that in many cases, a leading-edge vortex (LEV) contributes substantially to the generation of aerodynamic force. The effect of airfoil design parameters, such as airfoil thickness and camber, are well understood in steady-state aerodynamics. Our results suggest that the size and shape of the alula can be explained in one allometric landscape defined by wing length and loading in these two closely related families of birds with similar wing shapes. We hypothesize that heavier species may benefit from having longer alula if they perform flights with higher attack angles than lighter species, as longer alula would better suppress flow separation at higher attack angles. Combined, these results suggest that the species with high loading potential and long wings exhibit long alula. The aspect ratio of the alula was greater in the species that are relatively heavier in the Sternidae but not in the Laridae. In the Laridae, the aspect ratio of the alula was smaller in the species that have relatively longer wings, but the pattern was opposite in the Sternidae. The two families differed in the relationships between body size or wing length and the size or shape of the alula. Laridae birds have generally longer wings and greater loadings than Sternidae birds. In this study, we investigated the relationship between the size and shape of the alula and the features of the wing in the Laridae and Sternidae. Alula size vary among birds, but how this variation is associated with the function of the alula remains unclear. The alula is a small structure present on the leading edge of bird wings and is known to enhance lift by creating a small vortex at its tip.
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