Abstract
Recent investigations on the aerodynamics of natural fliers have illuminated the significance of the leading-edge vortex (LEV) for lift generation in a variety of flight conditions. A well-documented example of an LEV is that generated by aircraft with highly swept, delta-shaped wings. While the wing aerodynamics of a manoeuvring aircraft, a bird gliding and a bird in flapping flight vary significantly, it is believed that this existing knowledge can serve to add understanding to the complex aerodynamics of natural fliers. In this investigation, a model non-slender delta-shaped wing with a sharp leading edge is tested at low Reynolds number, along with a delta wing of the same design, but with a modified trailing edge inspired by the wing of a common swift Apus apus. The effect of the tapering swift wing on LEV development and stability is compared with the flow structure over the unmodified delta wing model through particle image velocimetry. For the first time, a leading-edge vortex system consisting of a dual or triple LEV is recorded on a swift wing-shaped delta wing, where such a system is found across all tested conditions. It is shown that the spanwise location of LEV breakdown is governed by the local chord rather than Reynolds number or angle of attack. These findings suggest that the trailing-edge geometry of the swift wing alone does not prevent the common swift from generating an LEV system comparable with that of a delta-shaped wing.
Highlights
The leading-edge vortex (LEV) is a commonly found mechanism that, under the correct conditions, can significantly augment the lift generation of both manufactured and natural fliers [1,2,3,4]
As reported previously (e.g. [10,31,32]), the LEV system increases in size and strength with increased α, resulting in a reduced proximity to the leading edge
The probable lack of a vortex at low α, and that no dual vortex was recorded in any case, are both in contrast to the results presented in this paper
Summary
The leading-edge vortex (LEV) is a commonly found mechanism that, under the correct conditions, can significantly augment the lift generation of both manufactured and natural fliers [1,2,3,4]. The LEV is robust to kinematic change [1] and has been identified across a wide range of Reynolds numbers (Re) (table 1), from the laminar flow conditions (10 < Re < 104) of autorotating. Cl lift coefficient cr root chord of the swift wing-shaped delta wing (m). N number of grid points within the domain Π. P and M two grid points within Π.
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