Nathan Phillips

Effect of flapping kinematics on the mean lift of an insect like flapping wing

An experimental investigation of the effects of varying flapping kinematics on the mean lift produced by an insect-like flapping wing in hover is presented. This was performed with application to flapping-wing micro-air vehicles (FMAVs) in mind. Experiments were accomplished with a first-of-its-kind mechanical flapping-wing apparatus capable of reproducing a wide range of insect-like wing motions in air on the FMAV scale (~150 mm wingspan). This apparatus gives an insect-like wing the three controllable degrees of freedom required to produce the three separate motions necessary for mimicking an insect-like flapping-wing trajectory: sweeping (side to side), plunging (up and down), and pitching (angle of attack variation). Lift was measured via a force balance while the following kinematic parameters were varied: flapping frequency (f ), angle of attack at mid-stroke (αmid), timing of pitch reversal with stroke reversal (rotation phase), stroke amplitude (Φ), and plunge amplitude (Θ). Results revealed that mean lift scaled with f   1.5 and varied proportionally with Φ. A pitch reversal advanced by up to 5 per cent of the flapping period relative to stroke reversal was found to maximize mean lift, and delayed pitch reversals were detrimental to mean lift. Of the parameters tested, mean lift was also maximized for αmid = 45° and Θ = 8.6°.

Petiolate wings: effects on the leading-edge vortex in flapping flight

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.

Low Reynolds number acceleration of flat plate wings at high incidence (Invited)

© 2016, (publisher Name). All rights reserved. This paper discusses the force history and flow topology of accelerating flat plate wings. The work is a collaborative effort to study fundamental, unsteady low Reynolds number flows under the umbrella of the NATO AVT-202 task group. The motion kinematics are designed to be relevant to the Micro-Air Vehicle (MAV) flight regime. A combination of empirical and computational techniques are used to obtain data for comparison. There is a striking correlation of lift history data and flow topology from both experimental and computational datasets. In an effort to source inputs for a low-order model, a Leading Edge Vortex (LEV)/Trailing Edge Vortex (TEV) relative advection velocity of 0:5.U∞ has been estimated based on the data.

Spanwise Flow on an Impulsively-Started Rotating Wing at Low Reynolds Numbers

For a significant portion of the wingbeat cycle in insect-like flapping, the wing sweeps at more or less constant speed and angle of attack. The aerodynamics associated with this phase of insect-like wing kinematics are investigated by making 2D-2C particle image velocimetry measurements on a rotating wing immersed in a tank of seeded water. In an attempt to capture flow features pertaining to many insects and future flappingwing micro air vehicles, the experiments are carried out at two Reynolds numbers—500 and 15000—and various techniques, including vortex identification schemes, are used to analyse the results. The behaviour of the leading-edge vortex, especially in terms of its spanwise propagation, is studied here. Results show that, despite its unsteady nature, the structure of the leading-edge vortex is essentially stable once fully developed.

Experiments and computations on the lift of accelerating flat plates at incidence

R. J. Bomphrey and N. Phillips were supported by the Engineering and Physical Sciences Research Council (EP/H004025/1 to R. J. Bomphrey). R. J. Bomphrey and T. Nakata were supported by the Biotechnology and Biological Sciences Research Council (BB/J001244/1 to R. J. Bomphrey).