Structure of the vortex wake in hovering Anna's hummingbirds (
            Calypte anna
            )

Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna's hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna's hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8–26% of weight support. Lift production in Anna's hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds.

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Evolution and breakup of vortex rings in straining and shearing flows

Journal of Fluid Mechanics ◽

10.1017/s0022112094001941 ◽

1994 ◽

Vol 273 ◽

pp. 285-312 ◽

Cited By ~ 6

Author(s):

J. S. Marshall ◽

J. R. Grant

Keyword(s):

Linear Theory ◽

Vortex Pair ◽

Small Strain ◽

Oscillation Period ◽

Numerical Calculations ◽

Vortex Rings ◽

Single Vortex ◽

Discrete Vortex ◽

Initial Plane ◽

Shear Rates

A study of the effect of external straining and shearing flows on the evolution and form of breakup of vortex rings has been performed. Two orientations each of straining and shearing flows are considered. A theoretical analysis of the ring motion for small strain and shear rates is performed, and it is found that for shearing and straining flows in the plane of the ring, the ring may oscillate periodically. For a straining flow with compression normal to the initial plane of the ring, the linear theory predicts that the ring radius will expand with time. For shearing flow normal to the initial plane of the ring, the linear theory predicts tilting of the ring in the direction of the shear flow rotation.Numerical calculations are performed with both single vortex filaments and with a three-dimensional discrete vortex element method. The numerical calculations confirm the predictions of the linear theory for values of strain and shear rates below a certain critical value (which depends on the ratio R/σ0 of initial ring to core radii), whereas for strain and shear rates above this value the ring becomes very elongated with time and eventually pinches off. Three distinct regimes of long-time behaviour of the ring have been identified. Regime selection depends on initial ring geometry and orientation and on values of strain and shear rates. These regimes include (i) periodic oscillations with no pinching off, (ii) pinching off at the ring centre, and (iii) development of an elongated vortex pair at the ring centre and wider ‘heads’ near the ends (with pinching off just behind the heads). The boundaries of these regimes and theoretical reasons for the vortex behaviour in each case are described. It is also shown that the breakup of stretched vortex rings exhibits a self-similar behaviour, in which the number and size of ‘offspring’ vortices, at the point of pinching-off the ring, may be scaled by the product of the strain rate e (or shear rate s) and the oscillation period τ of a slightly elliptical ring with mean radius R.

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Momentum and Energy in the Wake of a Pigeon (Columba Livia) in Slow Flight

Journal of Experimental Biology ◽

10.1242/jeb.111.1.81 ◽

1984 ◽

Vol 111 (1) ◽

pp. 81-102 ◽

Cited By ~ 38

Author(s):

G. R. SPEDDING ◽

J. M.V. RAYNER ◽

C. J. PENNYCUICK

Keyword(s):

Theoretical Models ◽

Columba Livia ◽

Vortex Rings ◽

Vortex Wake ◽

Bird Flight ◽

Helium Bubbles ◽

Level Flight ◽

Weight Support ◽

A Chain ◽

Neutrally Buoyant

A technique is described whereby the vortex wake of birds in slow forward flight may be investigated with a view towards testing some of the assumptions and predictions of existing theoretical models of bird flight. Multiflash stereophotogrammetry was used to analyse the wake as a pigeon passed through a cloud of neutrally-buoyant helium bubbles. All photographs obtained support the hypothesis that the wake is composed of a chain of discrete, small-cored vortex rings. This being the case, velocity profiles taken from sections through the wake allow us to estimate the momentum in the wake as represented by vortex rings. The momentum in the wake appears to be approximately half that required for weight support in unaccelerated, level flight. The possible causes and consequences of this paradoxical result are discussed.

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Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers

Journal of The Royal Society Interface ◽

10.1098/rsif.2011.0238 ◽

2011 ◽

Vol 9 (67) ◽

pp. 292-303 ◽

Cited By ~ 42

Author(s):

Florian T. Muijres ◽

Melissa S. Bowlin ◽

L. Christoffer Johansson ◽

Anders Hedenström

Keyword(s):

Aerodynamic Performance ◽

The Body ◽

Double Loop ◽

Vortex Wake ◽

Vortex Loop ◽

Single Vortex ◽

Time Resolved ◽

Cluttered Environments ◽

Weight Support ◽

Flight Mode

Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a ‘feathered upstroke’ during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called ‘normal hovering’ as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body–tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability.

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A numerical study of vortex interaction

Journal of Fluid Mechanics ◽

10.1017/s0022112084001890 ◽

1984 ◽

Vol 146 ◽

pp. 331-345 ◽

Cited By ~ 5

Author(s):

I. G. Bromilow ◽

R. R. Clements

Keyword(s):

Flow Visualization ◽

Numerical Study ◽

Experimental Studies ◽

Shear Layers ◽

Controlled Study ◽

Vortex Interaction ◽

Flow Conditions ◽

Discrete Vortex ◽

Vortex Model ◽

Discrete Vortex Model

Flow visualization has shown that the interaction of line vortices is a combination of tearing, elongation and rotation, the extent of each depending upon the flow conditions. A discrete-vortex model is used to study the interaction of two and three growing line vortices of different strengths and to assess the suitability of the method for such simulation.Many of the features observed in experimental studies of shear layers are reproduced. The controlled study shows the importance and rapidity of the tearing process under certain conditions.

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Simulation of particle dispersion in an axisymmetric jet

Journal of Fluid Mechanics ◽

10.1017/s0022112088000102 ◽

1988 ◽

Vol 186 ◽

pp. 199-222 ◽

Cited By ~ 172

Author(s):

J. N. Chung ◽

T. R. Troutt

Keyword(s):

Mixing Layer ◽

Jet Flow ◽

Particle Dispersion ◽

Vortex Rings ◽

Local Flow ◽

Discrete Vortex ◽

Turbulent Mixing Layer ◽

Axisymmetric Jet ◽

Free Jet ◽

Global And Local

Particle dispersion in an axisymmetric jet is analysed numerically by following particle trajectories in a jet flow simulated by discrete vortex rings. Important global and local flow quantities reported in experimental measurements are successfully simulated by this method.The particle dispersion results demonstrate that the extent of particle dispersion depends strongly on γτ, the ratio of particle aerodynamic response time to the characteristic time of the jet flow. Particles with relatively small γτ values are dispersed at approximately the fluid dispersion rate. Particles with large γτ values are dispersed less than the fluid. Particles at intermediate values of γτ may be dispersed faster than the fluid and actually be flung outside the fluid mixing region of the jet. This result is in agreement with some previous experimental observations. As a consequence of this analysis, it is suggested that there exists a specific range of intermediate γτ at which optimal dispersion of particles in the turbulent mixing layer of a free jet may be achieved.

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Vortex patch with wall interaction as new benchmark test

Open Engineering ◽

10.2478/s13531-011-0037-2 ◽

2011 ◽

Vol 1 (4) ◽

Author(s):

Ziemowit Malecha

Keyword(s):

Fluid Dynamics ◽

Boundary Layer ◽

Reynolds Number ◽

Vortex Structure ◽

Wall Region ◽

Threshold Values ◽

Single Vortex ◽

Benchmark Test ◽

Vortex Patch ◽

Wall Interaction

AbstractIn this paper, a new computational benchmark test for fluid dynamics is presented. The new benchmark is based on the interaction of a single vortex structure (vortex patch) with a wall. It will be shown that it is possible to distinguish two critical or threshold values of the Reynolds number in the considered flow. The increase of the Reynolds number causes the appearance of the vortex bubble in the near-wall region first, and then next, the eruption of the boundary layer phenomenon. Further increase of the Reynolds number causes the flow to be more complex. The eruption phenomenon becomes more intense and also shows its regenerative nature.

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Experimental Analysis of Opposing Flow in Mixed Convection in a Channel With an Open Cavity Below

Heat Transfer: Volume 1 ◽

10.1115/ht2005-72526 ◽

2005 ◽

Author(s):

Oronzio Manca ◽

Sergio Nardini ◽

Kambiz Vafai

Keyword(s):

Reynolds Number ◽

Mixed Convection ◽

Flow Visualization ◽

Vortex Structure ◽

Open Cavity ◽

Forced Flow ◽

Heated Wall ◽

Heated Plate ◽

Thermal Plume ◽

Recirculation Flow

In this paper mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The cavity has the heated wall on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization for Reynolds number (Re) from 100 to 2000 and Richardson number (Ri) in the range 4.3–6400; the ratio between the length and the height of cavity (L/D) is in the range 0.5–2.0 and the ratio between the channel and cavity height (H/D) is equal to 1.0. The present results show that at the lowest investigated Reynolds number the surface temperatures are lower than the corresponding surface temperature for Re = 2000, at same the ohmic heat flux. The flow visualization points out that for Re = 1000 there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100 the effect of a stronger buoyancy determines a penetration of thermal plume from the heated plate wall into the upper channel. Moreover, the flow visualization points out that for lower Reynolds numbers the forced motion penetrates inside the cavity and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds number the vortex structure has a larger extension at same L/D value.

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