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- Hedenström, Anders, et al.
(författare)
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Bat flight generates complex aerodynamic tracks
- 2007
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 316:5826, s. 894-897
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Tidskriftsartikel (refereegranskat)abstract
- The flapping flight of animals generates an aerodynamic footprint as a time-varying vortex wake in which the rate of momentum change represents the aerodynamic force. We showed that the wakes of a small bat species differ from those of birds in some important respects. In our bats, each wing generated its own vortex loop. Also, at moderate and high flight speeds, the circulation on the outer ( hand) wing and the arm wing differed in sign during the upstroke, resulting in negative lift on the hand wing and positive lift on the arm wing. Our interpretations of the unsteady aerodynamic performance and function of membranous-winged, flapping flight should change modeling strategies for the study of equivalent natural and engineered flying devices.
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- Hedenström, Anders, et al.
(författare)
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High-speed stereo DPIV measurement of wakes of two bat species flying freely in a wind tunnel
- 2009
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 1432-1114 .- 0723-4864. ; 46:5, s. 923-932
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies on wake flow visualization of live animals using DPIV have typically used low repetition rate lasers and 2D imaging. Repetition rates of around 10 Hz allow similar to 1 image per wingbeat in small birds and bats, and even fewer in insects. To accumulate data representing an entire wingbeat therefore requires the stitching-together of images captured from different wingbeats, and at different locations along the wing span for 3D-construction of wake topologies. A 200 Hz stereo DPIV system has recently been installed in the Lund University wind tunnel facility and the high-frame rate can be used to calculate all three velocity components in a cube, whose third dimension is constructed using the Taylor hypothesis. We studied two bat species differing in body size, Glossophaga soricina and Leptonycteris curasoa. Both species shed a tip vortex during the downstroke that was present well into the upstroke, and a vortex of opposite sign to the tip vortex was shed from the wing root. At the transition between upstroke/downstroke, a vortex loop was shed from each wing, inducing an upwash. Vorticity iso-surfaces confirmed the overall wake topology derived in a previous study. The measured dimensionless circulation, I"/Uc, which is proportional to a wing section lift coefficient, suggests that unsteady phenomena play a role in the aerodynamics of both species.
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- Hedenström, Anders, et al.
(författare)
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Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel
- 2006
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Ingår i: Journal of the Royal Society Interface. - : The Royal Society. - 1742-5662 .- 1742-5689. ; 3:7, s. 263-276
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Tidskriftsartikel (refereegranskat)abstract
- The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments.
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- Hedenström, Anders, et al.
(författare)
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Vortex wakes of birds: recent developments using digital particle image velocimetry in a wind tunnel
- 2006
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Ingår i: Animal Biology. - : Brill. - 1570-7555 .- 1570-7563. ; 56:4, s. 535-549
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Tidskriftsartikel (refereegranskat)abstract
- A flying animal generates a trail of wake vortices that contain information about the time history and magnitude of aerodynamic forces developed on the wings and body. Methods for visualising and recording wake vortices have been developed, allowing quantitative measurements by digital particle image velocimetry (DPIV). Results from DPIV experiments in a wind tunnel are presented for four passerine species of differing size and morphology. The normalised vorticity and its integrated quantity, circulation (Gamma) both decline gradually with increasing flight speed. The measured circulations are successfully explained by a simple aerodynamic model where a normalised circulation, Gamma/Uc, represents half the time-averaged lift coefficient, which is > 2 at 4 m s(-1) for a thrush nightingale.
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- Muijres, Florian, et al.
(författare)
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Actuator disk model and span efficiency of flapping flight in bats based on time-resolved PIV measurements
- 2011
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 1432-1114 .- 0723-4864. ; 51, s. 511-525
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Tidskriftsartikel (refereegranskat)abstract
- All animals flap their wings in powered flight to provide both lift and thrust, yet few human-engineered designs do so. When combined with flexible wing surfaces, the resulting unsteady fluid flows and interactions in flapping flight can be complex to describe, understand, and model. Here, a simple modified actuator disk is used in a quasi-steady description of the net aerodynamic lift forces on several species of bat whose wakes are measured with time-resolved PIV. The model appears to capture the time-averaged and instantaneous lift forces on the wings and body, and could be used as basis for comparing flapping flight efficiency of different animal species and micro air vehicle designs.
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- Muijres, Florian, et al.
(författare)
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Leading edge vortices lift in bat flight
- 2008
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Ingår i: Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology. - : Elsevier BV. - 1095-6433. ; 150:3, s. 66-66
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Konferensbidrag (refereegranskat)
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- Rosén, Mikael, et al.
(författare)
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The relationship between wingbeat kinematics and vortex wake of a thrush nightingale
- 2004
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Ingår i: Journal of Experimental Biology. - : The Company of Biologists. - 1477-9145 .- 0022-0949. ; 207:24, s. 4255-4268
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Tidskriftsartikel (refereegranskat)abstract
- The wingbeat kinematics of a thrush nightingale Luscinia luscinia were measured for steady flight in a wind tunnel over a range of flight speeds (5-10 m s(-1)), and the results are interpreted and discussed in the context of a detailed, previously published, wake analysis of the same bird. Neither the wingbeat frequency nor wingbeat amplitude change significantly over the investigated speed range and consequently dimensionless measures that compare timescales of flapping vs. timescales due to the mean flow vary in direct proportion to the mean flow itself, with no constant or slowly varying intervals. The only significant kinematic variations come from changes in the upstroke timing (downstroke fraction) and the upstroke wing folding (span ratio), consistent with the gradual variations, primarily in the upstroke wake, previously reported. The relationship between measured wake geometry and wingbeat kinematics can be qualitatively explained by presumed self-induced convection and deformation of the wake between its initial formation and later measurement, and varies in a predictable way with flight speed. Although coarse details of the wake geometry accord well with the kinematic measurements, there is no simple explanation based on these observed kinematics alone that accounts for the measured asymmetries of circulation magnitude in starting and stopping vortex structures. More complex interactions between the wake and wings and/or body are implied.
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