Near- and far-field aerodynamics in insect hovering flight: an integrated computational study

100

SUMMARYMost hovering insects flap their wings in a horizontal plane (body having a large angle from the horizontal), called 'normal hovering'. But some of the best hoverers, e.g. true hoverflies, hover with an inclined stroke plane (body being approximately horizontal). In the present paper, wing and body kinematics of four freely hovering true hoverflies were measured using threedimensional high-speed video. The measured wing kinematics was used in a Navier-Stokes solver to compute the aerodynamic forces of the insects. The stroke amplitude of the hoverflies was relatively small, ranging from 65 to 85deg, compared with that of normal hovering. The angle of attack in the downstroke (~50deg) was much larger that in the upstroke (~20deg), unlike normalhovering insects, whose downstroke and upstroke angles of attack are not very different. The major part of the weight-supporting force (approximately 86%) was produced in the downstroke and it was contributed by both the lift and the drag of the wing, unlike the normal-hovering case in which the weight-supporting force is approximately equally contributed by the two half-strokes and the lift principle is mainly used to produce the force. The mass-specific power was 38.59-46.3 and 27.5-35.4Wkg -1 in the cases of 0 and 100% elastic energy storage, respectively. Comparisons with previously published results of a normal-hovering true hoverfly and with results obtained by artificially making the insects' stroke planes horizontal show that for the true hoverflies, the power requirement for inclined stroke-plane hover is only a little (<10%) larger than that of normal hovering. Supplementary material available online at

Section: Computation Of Aerodynamic Forces and Power Requirementsmentioning

confidence: 54%

Wing motion measurement and aerodynamics of hovering true hoverflies

Mou

Liu

Sun

2011

100

“…Most likely spanwise flow due to a pressure gradient is primarily confined to the core of the LEV. Aono and co-workers (Aono et al, 2008) further show that little spanwise flow and pressure gradient exists in the LEV of a fruit fly at Re=134, while they do find significant spanwise flow behind the LEV. For a hawkmoth at Re=6300 they did find strong spanwise flow and pressure gradient in the LEV.…”

Section: Lentink and M H Dickinsonmentioning

confidence: 93%

Rotational accelerations stabilize leading edge vortices on revolving fly wings

Lentink

Dickinson

2009

463

474

SUMMARYThe aerodynamic performance of hovering insects is largely explained by the presence of a stably attached leading edge vortex (LEV) on top of their wings. Although LEVs have been visualized on real, physically modeled, and simulated insects, the physical mechanisms responsible for their stability are poorly understood. To gain fundamental insight into LEV stability on flapping fly wings we expressed the Navier-Stokes equations in a rotating frame of reference attached to the wing's surface. Using these equations we show that LEV dynamics on flapping wings are governed by three terms: angular, centripetal and Coriolis acceleration. Our analysis for hovering conditions shows that angular acceleration is proportional to the inverse of dimensionless stroke amplitude, whereas Coriolis and centripetal acceleration are proportional to the inverse of the Rossby number. Using a dynamically scaled robot model of a flapping fruit fly wing to systematically vary these dimensionless numbers, we determined which of the three accelerations mediate LEV stability. Our force measurements and flow visualizations indicate that the LEV is stabilized by the 'quasi-steady' centripetal and Coriolis accelerations that are present at low Rossby number and result from the propeller-like sweep of the wing. In contrast, the unsteady angular acceleration that results from the back and forth motion of a flapping wing does not appear to play a role in the stable attachment of the LEV. Angular acceleration is, however, critical for LEV integrity as we found it can mediate LEV spiral bursting, a high Reynolds number effect. Our analysis and experiments further suggest that the mechanism responsible for LEV stability is not dependent on Reynolds number, at least over the range most relevant for insect flight (100

“…This section- by-section, quasi-two-dimensional concept of wing circulation makes the formation of two separate rings behind flapping wing pairs quite a likely outcome, and many such complicated wake structures have been reported in the insect flight literature (e.g. Aono et al, 2008;Brodsky, 1991;Grodnitsky and Morozov, 1993). However, the shedding and subsequent roll-up of shed vorticity is not easy to predict and may be very different from a quasi-two-dimensional process.…”

Section: One Ring or Two?mentioning

confidence: 93%

The near and far wake of Pallas' long tongued bat (Glossophaga soricina)

Johansson

Wolf

Busse

et al. 2008

SUMMARYThe wake structures of a bat in flight have a number of characteristics not associated with any of the bird species studied to this point. Unique features include discrete vortex rings generating negative lift at the end of the upstroke at medium and high speeds, each wing generating its own vortex loop, and a systematic variation in the circulation of the start and stop vortices along the wingspan, with increasing strength towards the wing tips. Here we analyse in further detail some previously published data from quantitative measurements of the wake behind a small bat species flying at speeds ranging from 1.5 to 7 m s -1 in a wind tunnel. The data are extended to include both near-and far-wake measurements. The near-/far-wake comparisons show that although the measured peak vorticity of the start and stop vortices decreases with increasing downstream distance from the wing, the total circulation remains approximately constant. As the wake evolves, the diffuse stop vortex shed at the inner wing forms a more concentrated vortex in the far wake. Taken together, the results show that studying the far wake, which has been the standard procedure, nevertheless risks missing details of the wake. Although study of the far wake alone can lead to the misinterpretation of the wake topology, the net, overall circulation of the main wake vortices can be preserved so that approximate momentum balance calculations are not unreasonable within the inevitably large experimental uncertainties. Supplementary material available online at