Abstract
Insect wing shapes are diverse and a renowned source of inspiration for the new generation of autonomous flapping vehicles, yet the aerodynamic consequences of varying geometry is not well understood. One of the most defining and aerodynamically significant measures of wing shape is the aspect ratio, defined as the ratio of wing length (R) to mean wing chord (). We investigated the impact of aspect ratio, AR, on the induced flow field around a flapping wing using a robotic device. Rigid rectangular wings ranging from AR = 1.5 to 7.5 were flapped with insect-like kinematics in air with a constant Reynolds number (Re) of 1400, and a dimensionless stroke amplitude of (number of chords traversed by the wingtip). Pseudo-volumetric, ensemble-averaged, flow fields around the wings were captured using particle image velocimetry at 11 instances throughout simulated downstrokes. Results confirmed the presence of a high-lift, separated flow field with a leading-edge vortex (LEV), and revealed that the conical, primary LEV grows in size and strength with increasing AR. In each case, the LEV had an arch-shaped axis with its outboard end originating from a focus-sink singularity on the wing surface near the tip. LEV detachment was observed for around mid-stroke at span, and initiated sooner over higher aspect ratio wings. At the larger, stronger vortex persisted under the wing surface well into the next half-stroke leading to a reduction in lift. Circulatory lift attributable to the LEV increased with AR up to AR = 6. Higher aspect ratios generated proportionally less lift distally because of LEV breakdown, and also less lift closer to the wing root due to the previous LEV's continuing presence under the wing. In nature, insect wings go no higher than likely in part due to architectural and physiological constraints but also because of the reducing aerodynamic benefits of high AR wings.
Highlights
Insects are expert fliers capable of achieving remarkable amounts of lift for their size—often in excess of twice their body weight (Weis-Fogh 1964)—and feats of exceptional aerial agility and control in confined spaces
Skin friction lines produced by line integral convolution (LIC) are coloured by in-plane velocity magnitude normalized by vtip and streamlines are coloured with normalized
The effect of varying wing aspect ratio within, and beyond, the range found in nature was investigated experimentally using high spatial- and temporalresolution particle image velocimetry (PIV)
Summary
Insects are expert fliers capable of achieving remarkable amounts of lift for their size—often in excess of twice their body weight (Weis-Fogh 1964)—and feats of exceptional aerial agility and control in confined spaces. Flapping flight is efficient at low flight speeds and in hover (Woods et al 2001). Taken together, this flight mode is very attractive for applications to unmanned air vehicles (UAVs). Flappingwing UAVs take advantage of the unique benefits of insect-like flight and are envisaged for a broad range of applications (Żbikowski 1999) where important characteristics will include energy efficiency, a low audible signature, and the ability to hover and manoeuvre safely in confined and cluttered environments. A synthesis of the functional consequences of evolved morphologies is of importance to the biological community where biomechanics can be used to test hypotheses about adaptation and evolutionary radiation
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