Browsing by Author "Phillips, Nathan"

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    The effect of aspect ratio on the leading-edge vortex over an insect-like flapping wing
    (IOP, 2015-10-12) Phillips, Nathan; Knowles, Kevin; Bomphrey, Richard J.
    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.
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    Experimental investigation of some aspects of insect-like flapping flight aerodynamics for application to micro air vehicles
    (Springer Science Business Media, 2009-05-31T00:00:00Z) Ansari, S. A.; Phillips, Nathan; Stabler, G.; Wilkins, P. C.; Zbikowski, Rafal; Knowles, Kevin
    Insect-like flapping flight offers a power-efficient and highly manoeuvrable basis for micro air vehicles for indoor applications. Some aspects of the aerodynamics associated with the sweeping phase of insect wing kinematics are examined by making particle image velocimetry measurements on a rotating wing immersed in a tank of seeded water. The work is motivated by the paucity of data with quantified error on insect-like flapping flight, and aims to fill this gap by providing a detailed description of the experimental setup, quantifying the uncertainties in the measurements and explaining the results. The experiments are carried out at two Reynolds numbers-500 and 15,000-accounting for scales pertaining to many insects and future flapping-wing micro air vehicles, respectively. The results from the experiments are used to describe prominent flow features, and Reynolds number-related differences are highlighted. In particular, the behaviour of the leading-edge vortex at these Reynolds numbers is studied and the presence of Kelvin-Helmholtz instability observed at the higher Reynolds number in computational fluid dynamics calculations is also verified.
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    Experimental unsteady aerodynamics relevant to insect-inspired flapping-wing micro air vehicles
    (2011-09-07) Phillips, Nathan; Knowles, Kevin
    Small hand-held micro air vehicles (MAVs) can serve many functions unsuitable for a manned vehicle, and can be inexpensive and easily deployed. MAVs for indoor applications are underdeveloped due to their demanding requirements. Indoor requirements are best met by a flapping-wing micro air vehicle (FMAV) based on insect-like flapping-wing flight, which offers abilities of sustained hover, aerial agility, and energy efficiency. FMAV development is hampered by a lack of understanding of insect-like flapping-wing aerodynamics, particularly at the FMAV scale. An experimental programme at the FMAV scale (Reynolds number on the order of 104) was undertaken, investigating: leading-edge vortex (LEV) stability, flapping kinematic effects on lift and the flowfield, and wing planform shape effects on the flowfield. For these experiments, an apparatus employing a novel flapping mechanism was developed, which achieved variable three-degreeof- freedom insect-like wing motions (flapping kinematics) with a high degree of repeatability in air up to a 20Hz flapping frequency. Mean lift measurements and spatially dense volumetric flowfield measurements using stereoscopic particle image velocimetry (PIV) were performed while various flapping kinematic parameters and wing planform were altered, to observe their effects. Three-dimensional vortex axis trajectories were reconstructed, revealing vortex characteristics such as axial velocity and vorticity, and flow evolution patterns. The first key result was the observation of a stable LEV at the FMAV scale which contributed to half of the mean lift. The LEV exhibited vortex breakdown, but still augmented lift as Reynolds number was increased indicating that FMAVs can exploit this lifting mechanism. The second key result was the identification of the trends of mean lift versus the tested kinematic parameters at the FMAV scale, and appropriate values for FMAV design. Appropriate values for lift generation, while taking mechanical practicalities into account, included a flat wingtip trajectory with zero plunge amplitude, angle of attack at mid-stroke of 45 degrees , rotation phase of +5:5%, and maximum flapping frequency and stroke amplitude.
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    Formation of the leading-edge vortex and spanwise flow on an insect-like flapping-wing throughout a flapping half cycle
    (Royal Aeronautical Society, 2013-05-16T00:00:00Z) Phillips, Nathan; Knowles, Kevin
    This paper presents an experimental investigation of the evolution of the leading-edge vortex and spanwise flow generated by an insect-like flapping-wing at a Reynolds number relevant to flapping-wing micro air vehicles (FMAVs) (Re = ~15,000). Experiments were accomplished with a first-of-its-kind flapping-wing apparatus. Dense pseudo-volumetric particle image velocimetry (PIV) measurements from 18% - 117% span were taken at twelve azimuthal positions throughout a flapping half cycle. Results revealed the formation of a primary leading-edge vortex (LEV) which saw an increase in size and spanwise flow (towards the tip) through its core as the wing swept from rest to the mid-stroke position where signs of vortex breakdown were observed. Beyond mid-stroke, spanwise flow decreased and the tip vortex grew in size and exhibited a reversal in its axial direction. At the end of the flapping half cycle, the primary LEV was still present over the wing surface, suggesting that the LEV remains attached to the wing throughout the entire flapping half cycle.
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    Petiolate wings: effects on the leading-edge vortex in flapping flight
    (Royal Society, 2017-02-06) Phillips, Nathan; Knowles, Kevin; Bomphrey, R. J.
    The wings of many insect species including crane flies and damselflies are petiolate (on stalks), with the wing planform beginning some distance away from the wing hinge, rather than at the hinge. The aerodynamic impact of flapping petiolate wings is relatively unknown, particularly on the formation of the lift-augmenting leading-edge vortex (LEV): a key flow structure exploited by many insects, birds and bats to enhance their lift coefficient. We investigated the aerodynamic implications of petiolation P using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of P = 1–3. The wings were driven using a mechanical device, the ‘Flapperatus’, to produce highly repeatable insect-like kinematics. The wings maintained a constant Reynolds number of 1400 and dimensionless stroke amplitude Λ* (number of chords traversed by the wingtip) of 6.5 across all test cases. Our results showed that for more petiolate wings the LEV is generally larger, stronger in circulation, and covers a greater area of the wing surface, particularly at the mid-span and inboard locations early in the wing stroke cycle. In each case, the LEV was initially arch-like in form with its outboard end terminating in a focus-sink on the wing surface, before transitioning to become continuous with the tip vortex thereafter. In the second half of the wing stroke, more petiolate wings exhibit a more detached LEV, with detachment initiating at approximately 70% and 50% span for P = 1 and 3, respectively. As a consequence, lift coefficients based on the LEV are higher in the first half of the wing stroke for petiolate wings, but more comparable in the second half. Time-averaged LEV lift coefficients show a general rise with petiolation over the range tested.