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PubMed2002

Omnicarbon 21 mm aortic valve prosthesis: in vitro hydrodynamic and echo-Doppler study.

The aim of this study was to perform combined hydrodynamic and Doppler echocardiographic tests of the Omnicarbon 21 mm cardiac valve in aortic position in our Pulse Duplicator System for simultaneous assessment of valve performance and valve leakage data. During forward flow conditions, measured mean pressure gradients are between 4.5 and 20.2 mmHg (11.9 +/- 4.4 mmHg) for cardiac outputs between 3.6 and 5.3 /min (4.5 +/- 0.4 /min). Doppler-derived mean pressure gradients are between 2.0 and 170 mmHg (9.3 +/- 3.9 mmHg) for the same flow conditions. Effective Orifice area is 1.31 +/- 0.08 cm2 and the performance index is 0.74 +/- 0.04, using the actual geometric orifice area, and 0.38 +/- 0.02, using the tissue annulus diameter, for a cardiac output of 4.5 +/- 0.4 l/min. Regurgitation volumes are below 3 ml. There is a trend to an effect of valve orientation on hemodynamics.

Aortic Valve↗
PubMed2002

In vitro hydrodynamics of a decellularized pulmonary porcine valve, compared with a glutaraldehyde and polyurethane heart valve.

BACKGROUND: Hydrodynamic performance of a decellularized pulmonary porcine valve was evaluated with a computer versatile pulse duplicator and compared to glutaraldehyde fixated stentless porcine bioprosthesis and a polyurethane heart valve. METHODS: Decellularized pulmonary porcine matrices (Group I, n = 5) were treated chemically to become cell-free collagen matrices. The findings of this heart valve were compared with aortic glutaraldehyde treated porcine prostheses (Group II, n = 5) and polyurethane three leaflet valve prostheses (Group III, n = 1). Measurements were performed in 0.9% saline test fluid at room temperature. Measurements compared were closing time, closing volume, systemic pressure difference and energy losses. Each valve was measured 6 times with 70 beats/minute, a stroke volume of 70 ml corresponds to a cardiac output of 4.9 L/minute. RESULTS: Group I and group III showed no significant differences between parameters. The measured closing time was significantly different (p < 0.001) between group I and II, respectively 24.333 and 53.600 ms and group II and III respectively 53.600 and 24.000. Difference in closing volume was significant (p < 0.05) between groups II and I respectively 3.67 and 0.68 ms and group II and III respectively 3.67 and 0.71. Systolic mean pressure gradient was 18.25 +/- 1.04 mm Hg in group II which was significantly different (p < 0.001) from groups I and III, respectively 10.65 +/- 0.29 mm Hg and 7.70 +/- 0.30 mm Hg. CONCLUSIONS: Decellularized pulmonary porcine valves showed the same excellent performance as polyurethane valve prosthesis, which are superior to the investigated glutaraldehyde fixed xenograft.

Animals↗
PubMedDate not supplied

Branched hybrid vessel: in vitro loaded hydrodynamic forces influence the tissue architecture.

This study was conducted to investigate how a continuous load of hydrodynamic stresses influences the tissue architecture of a branched hybrid vessel in vitro. Tubular hybrid medial tissue of small (3 mm) and large (6 mm) diameters, prepared by thermal gelation of a cold mixed solution of bovine smooth muscle cells (SMCs) and type I collagen in glass molds, was assembled into a branched hybrid medial tissue by end-to-side anastomosis. After a 2-week culture period, bovine endothelial cells (ECs) were seeded onto the luminal surface. The branched hybrid vessel was connected to a mock circulatory loop system and tested for two modes of flow: 1) low flow rate for 24 h, 2) high flow rate for 24 or 72 h. After exposure to a low flow rate for 24 h, cobblestone appearance of the ECs was dominant. After exposure to a high flow rate, EC alignment in the direction of flow was observed in the branch region, except at the region of predicted flow separation where ECs retained their polygonal configuration. Elongation of SMCs with no preferential orientation was observed in the case of vessels exposed to a high flow rate for 24 h, and circumferential orientation was prominent in those exposed to a high flow rate for 72 h. On the other hand, collagen fibrils exhibited no preferential orientation in either case. After injection of Evans blue-albumin conjugate into the circulating medium, the luminal surface of the hybrid vessel exposed to a high flow rate for 24 h was examined by confocal laser scanning microscopy. The fluorescence intensity was low at the high shear zone in the branch region, while at the flow separation region it was very high, indicating the increased albumin permeability at the latter region. These findings reflect region-specific tissue architecture in the branch region, in response to the local flow pattern, and may provide an in vitro atherosclerosis model as well as a fundamental basis for the development of functional branched hybrid grafts.

Albumins↗
PubMed2003

Suspected pulmonary artery disruption after transvenous pulmonary embolectomy using a hydrodynamic thrombectomy device: clinical case and experimental study on porcine lung explants.

PURPOSE: To use porcine lung explants for reconstructing possible situations in which a vessel wall disruption might have occurred in a patient suffering fatal hemoptysis after pulmonary embolectomy with a hydrodynamic thrombectomy device. METHODS: A 76-year-old woman with massive pulmonary embolism underwent transvenous pulmonary embolectomy using a 6-F AngioJet Xpeedior catheter according to manufacturer's instructions. While activating the device in the middle lobe artery (approximately 8 mm diameter), massive and ultimately fatal arterial bleeding occurred through the tracheal tube. Because no autopsy was authorized, an experimental study was designed to examine possible causes for the vessel disruption. Five fresh porcine heart-lung preparations were examined inside a dedicated chest phantom. Access to the pulmonary vessels was provided through catheters inside the right and left ventricular outlets. A low-flow circulation was maintained with an external pump. The 6-F AngioJet thrombectomy device was activated at 42 sites inside vessels from 2 to 10 mm in diameter; in one lung, 8 activations were made after deliberately withdrawing the guidewire. RESULTS: Vessels >6 mm in diameter remained intact. Vessel wall disruption occurred in 4 of 7 vessels between 4 and 6 mm in diameter and in 13 of 14 segmental arteries <4 mm in diameter (regardless of whether or not a guidewire was used). The signs of vessel wall disruption included extravasation of contrast material, arteriovenous fistula, and laceration of distal airspaces with contrast inside the bronchus. CONCLUSIONS: The application of this system has to be considered potentially dangerous when activated inside vessels with diameters <6 mm. The use of this device appears to be safe only inside main branches of the lung vessels at this time. Additional experiments will be required to substantiate these initial results.

Aged↗
PubMed1988

Structural properties of the renal parathyroid hormone receptor: hydrodynamic analysis and protease sensitivity.

The highly specific ligand [125I]bovine (b) PTH-(1-34) and a chemical cross-linking technique were used to explore structural features of the canine renal cortical PTH receptor. Membranes isolated under conditions designed to inhibit endogenous proteolysis displayed a major 85K labeled PTH receptor moiety on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Cross-linked receptors were solubilized with Lubrol-PX and partially purified by affinity chromatography on wheat germ agglutinin-agarose, and their hydrodynamic properties were assessed [Stokes radius = 7.3 +/- 0.1 nm; sedimentation coefficient = 6.4 +/- 0.2S; partial specific volume = 0.758 +/- 0.01 ml/g; frictional coefficient = 1.68 +/- 0.04; mol wt (Mr) = 216,000 +/- 14,000]. Corrections for detergent binding and for the presence of carbohydrate yielded an estimated Mr of 166,000 +/- 11,000 for the solubilized PTH receptor. Thus, the renal PTH receptor is oligomeric, with a Mr approximating that expected of a homodimer of 85K subunits. Peptide-mapping experiments revealed the presence within the 85K PTH receptor subunit of at least two major regions sensitive to proteolytic attack. Both elastase and an endogenous renal protease(s) cleaved the PTH receptor to a 70K form that is fully functional with respect to high affinity, guanyl nucleotide-sensitive PTH binding. Cleavage in a second domain by elastase, S. aureus V8 protease, or chymotrypsin generated a 50K labeled PTH receptor fragment. Cleavage at this second site was prevented by prior occupancy of the receptor with [125I]bPTH-(1-34), suggesting that this domain may be functionally important. Reduction of receptor disulfide bonds with dithiothreitol and beta-mercaptoethanol released a low Mr (less than or equal to 14K) labeled PTH receptor component, similar treatment of renal membranes abolished specific PTH binding, indicating that an intact disulfide bond(s) is essential for receptor function. These results provide new insights into the structural basis of PTH receptor function.

Animals↗
PubMed2003

The hydrodynamics of locomotion at intermediate Reynolds numbers: undulatory swimming in ascidian larvae (Botrylloides sp.).

Understanding how the shape and motion of an aquatic animal affects the performance of swimming requires knowledge of the fluid forces that generate thrust and drag. These forces are poorly understood for the large diversity of animals that swim at Reynolds numbers (Re) between 10(0) and 10(2). We experimentally tested quasi-steady and unsteady blade-element models of the hydrodynamics of undulatory swimming in the larvae of the ascidian Botrylloides sp. by comparing the forces predicted by these models with measured forces generated by tethered larvae and by comparing the swimming speeds predicted with measurements of the speed of freely swimming larvae. Although both models predicted mean forces that were statistically indistinguishable from measurements, the quasi-steady model predicted the timing of force production and mean swimming speed more accurately than the unsteady model. This suggests that unsteady force (i.e. the acceleration reaction) does not play a role in the dynamics of steady undulatory swimming at Re approximately 10(2). We explored the relative contribution of viscous and inertial force to the generation of thrust and drag at 10(0) 10(2)) and low (<10(0)) Re, the fluid forces that generate thrust cannot be assumed to be the same as those that generate drag at intermediate Re.

Acceleration↗
PubMed2003

A hydrodynamic topographic map in the midbrain of goldfish Carassius auratus.

Sensory systems often consist of several parallel pathways. Within each pathway, sensory information may be processed in topographically arranged maps or in maps derived by neuronal computation. Parallel pathways have so far not been described in the central lateral line system of teleost fish at levels higher than the medulla, and evidence for midbrain lateral line maps in fish is still weak. We found two classes of units with different response patterns in the central lateral line nucleus in the torus semicircularis of the goldfish Carassius auratus. Units of one class responded to a passing sphere and to the wake caused by that sphere with excitation. Units of the second class also responded to the moving sphere. However, these units did not respond to the wake behind the sphere. Hydrodynamic information received by class two units was topographically organized in the torus semicircularis of goldfish in that anterior body areas projected to rostral midbrain and posterior body areas to caudal midbrain. Units that responded only to the passing sphere were on average located more ventrally in the lateral TS than the units that responded exclusively to a vibrating sphere.

Afferent Pathways↗
PubMed2004

Kinematics and hydrodynamics of swimming in the mayfly larva.

The kinematics and hydrodynamics of free-swimming mayfly larvae (Chloeon dipterum) were investigated with the aid of a simple wake visualisation technique (tracer dyes) and drag measurements on dead insects. The basic swimming movement consists of a high-amplitude dorso-ventral undulation and, during continuous swimming, this produces a wake of ring vortices shed alternately to the dorsal and ventral sides of the body. The ring vortices propagate laterally away from the body at an angle of approximately 80 degrees relative to dead aft of the swimming line. Thus, mayfly larvae, like damsel-fly larvae, resemble eels in producing a wake consisting of separate vortices that propagate laterally rather than the reverse von Karman vortex street characteristic of most caudal fin swimming fish. The thrust estimated from the momentum in the wake of swimming mayfly larvae was comparable with the drag measured on dead specimens. Possible sources of error in these estimates are discussed, but the conclusion is reached that even though only 14% of the total force generated by vortex production is directed forwards, it is still sufficient to account for the thrust required for steady locomotion.

Animals↗
PubMed2004

The hydrodynamics of eel swimming II. Effect of swimming speed.

Simultaneous swimming kinematics and hydrodynamics are presented for American eels, Anguilla rostrata, swimming at speeds from 0.5 to 2 L s(-1). Body outlines and particle image velocimetry (PIV) data were collected using two synchronized high-speed cameras, and an empirical relationship between swimming motions and fluid flow is described. Lateral impulse in the wake is estimated assuming that the flow field represents a slice through small core vortex rings and is shown to be significantly larger than forces estimated from the kinematics via elongated body theory (EBT) and via quasi-steady resistive drag forces. These simple kinematic models predict only 50% of the measured wake impulse, indicating that unsteady effects are important in undulatory force production. EBT does, however, correctly predict both the magnitude and time course of the power shed into the wake. Other wake flow structures are also examined relative to the swimming motions. At all speeds, the wake contains almost entirely lateral jets of fluid, separated by an unstable shear layer that rapidly breaks down into two vortices. The jet's mean velocity grows with swimming speed, but jet diameter varies only weakly with swimming speed. Instead, it follows the body wavelength, which changes more among individuals than at different speeds. Circulation of the stop-start vortex, shed each time the tail changes direction, can also be predicted at low speeds by the integral of squared tail velocity over half of a tail beat. At high speeds, these kinematics predict more circulation than is actually present in the stop-start vortex. Finally, the cost of producing the wake, one component of the total cost of transport, increases with swimming speed to the 1.48 power, lower than would be expected if the power coefficient remained constant over the speed range examined.

Anguilla↗
PubMed2004

Hydrodynamics of surface swimming in leopard frogs (Rana pipiens).

The kinematics of swimming frogs have been studied extensively in the past and, based on these results, hypotheses regarding the hydrodynamics of frog swimming can be generated. To test these hypotheses we used digital particle image velocimetry (DPIV) to quantify the flow structure of the wake produced by the feet during the propulsion phase of the kick of surface swimming frogs (Rana pipiens). These frogs use two different gaits, asynchronous and synchronous kicking, and the magnitude of the thrust produced by the feet differs between asynchronous (34+/-5.4 mN foot(-1)) and synchronous kicking (71+/-13.3 mN foot(-1)), as does maximum swimming speed, with higher swimming speed and forces produced during the synchronous kicks. Previous studies have suggested that an interaction between the feet, resulting in a single posteriorly directed fluid jet, as the feet come together at the end of synchronous kicks, may augment force production. Our results show, however, that each foot produces its own distinct vortex ring, in both asynchronous and synchronous kicking of the feet. There is no evidence of a central jet being produced even during powerful synchronous kicks (maximum thrust calculated was 264 mN foot(-1)). An alternative mechanism of force production could be the lift-based paddling recently suggested for delta-shaped feet of swimming birds. However, the orientation of the vortex rings generated by the feet is almost perpendicular to the swimming direction for both gaits and there is only a slight asynchrony of the shedding of the distal (start) and proximal (stop) vortex rings, which is different from what would be expected by a dominantly lift-based mechanism. Thus, our results do not support lift as a major mechanism contributing to thrust. Instead, our data support the hypothesis that propulsion is based on drag and acceleration reaction forces where the thrust is generated by separated, but attached, vortex rings on the suction side of the feet, resulting in vortices that are shed behind the frogs during both asynchronous and synchronous kicking.

Acceleration↗
PubMed2006

The hydrodynamic effects of shape and size change during reconfiguration of a flexible macroalga.

Rocky intertidal organisms experience large hydrodynamic forces due to high water velocities created by breaking waves. Flexible organisms, like macroalgae, often experience lower drag than rigid organisms because their shape and size change as velocity increases. This phenomenon, known as reconfiguration, has been previously quantified as Vogel's E, a measure of the relationship between velocity and drag. While this method is very useful for comparing reconfiguration among organisms it does not address the mechanisms of reconfiguration, and its application to predicting drag is problematic. The purpose of this study was twofold: (1) to examine the mechanisms of reconfiguration by quantifying the change in shape and size of a macroalga in flow and (2) to build a mechanistic model of drag for reconfiguring organisms. Drag, frontal area and shape of the intertidal alga Chondrus crispus were measured simultaneously in a recirculating flume at water velocities from 0 to approximately 2 m s(-1). Reconfiguration was due to two separate mechanisms: whole-alga realignment (deflection of the stipe) at low velocities (<0.2 m s(-1)) and compaction of the crown (reduction in frontal area and change in shape) at higher velocities. Change in frontal area contributed more to drag reduction than change in drag coefficient. Drag coefficient and frontal area both decrease exponentially with increasing water velocity, and a mechanistic model of drag was developed with explicit functions to describe these changes. The model not only provides mechanistic parameters with which to compare reconfiguration among individuals and species, but also allows for more reliable predictions of drag at high, ecologically relevant water velocities.

Adaptation, Physiological↗
PubMed1994

HYDRODYNAMIC ASPECTS OF DESIGN AND ATTACHMENT OF A BACK-MOUNTED DEVICE IN PENGUINS

Wind tunnel and water tank experiments were carried out on a penguin model in order to optimise the shape and attachment of a back-mounted datalogger. Device-induced turbulence was minimised when the unit was placed in the most caudal position. Drag was further reduced by shaping the device to match the body contour. The hydrodynamic resistance of the package could be reduced by 65 % compared with an earlier unit. These results are discussed together with results from new studies on kinematics and energetics of underwater swimming of live instrumented penguins.

Journal Article↗
PubMed1996

Why the urchin lost its spines: hydrodynamic forces and survivorship in three echinoids

Two species of sea urchins (Colobocentrotus atratus and Echinometra mathaei) commonly co-occur on wave-swept intertidal shores in the Indo West Pacific. E. mathaei is a typical spiny urchin and is confined to cavities in the rock. In contrast, C. atratus has an unusual morphology, in which the spines are much reduced, and is found on substrata fully exposed to wave-induced velocities and accelerations. Previous researchers have suggested that spine reduction may therefore be a morphological adaptation to hydrodynamic forces. However, measurement of the drag, lift and accelerational forces on sea urchins show that the adaptive significance of spine reduction is less straightforward than it initially appears. The reduction in drag in C. atratus as compared with that in E. mathaei is to a large extent offset by an increase in lift. Instead, the 'streamlined' morphology of C. atratus seems best adapted to provide a reduction in the force imposed by water acceleration, thereby making it feasible for C. atratus to venture safely into the tumultuous flows of the surf zone.

Journal Article↗
PubMed1997

The three-dimensional hydrodynamics of tadpole locomotion.

Tadpoles are unusual among vertebrates in having a globose body with a laterally compressed tail abruptly appended to it. Compared with most teleost fishes, tadpoles swim awkwardly, with waves of relatively high amplitude at both the snout and tail tip. In the present study, we analyze tadpole propulsion using a three-dimensional (3D) computational fluid dynamic (CFD) model of undulatory locomotion that simulates viscous and unsteady flow around an oscillating body of arbitrary 3D geometry. We first confirm results from a previous two-dimensional (2D) study, which suggested that the characteristic shape of tadpoles was closely matched to their unusual kinematics. Specifically, our 3D results reveal that the shape and kinematics of tadpoles collectively produce a small 'dead water' zone between the head-body and tail during swimming precisely where tadpoles can and do grow hind limbs--without those limbs obstructing flow. We next use our CFD model to show that 3D hydrodynamic effects (cross flows) are largely constrained to a small region along the edge of the tail fin. Although this 3D study confirms most of the results of the 2D study, it shows that propulsive (Froude) efficiency for tadpoles is overall lower than predicted from a 2D analysis. This low efficiency is not, however, a result of the high-amplitude undulations of the tadpole. This was demonstrated by forcing our 'virtual' tadpole to swim with fish-like kinematics, i.e. with lower-amplitude propulsive waves. That particular simulation yielded a much lower Froude efficiency, confirming that the large-amplitude lateral oscillations of the tadpole do, indeed, provide positive thrust. This, we believe, is the first time that the unsteady flow generated by an undulating vertebrate has been realistically modelled in three dimensions. Our study demonstrates the feasibility of using 3D CFD methods to model the locomotion of other undulatory organisms.

Animals↗
PubMed1998

Burrow ventilation in the tube-dwelling shimp Callianassa subterranea (Decapoda: thalassinidea). III. Hydrodynamic modelling and the energetics of pleopod pumping.

The process of flow generation with metachronally beating pleopods in a tubiform burrow was studied by designing a hydrodynamic model based on a thrust-drag force balance. The drag of the tube (including the shrimp) comprises components for accelerating the water into the tube entrance, for adjusting a parabolic velocity profile, for accelerating the flow into a constriction due to the shrimp's body and another constriction due to the extended tail-fan, for shear due to separation and for the viscous resistance of all tube parts. The thrust produced by the beating pleopods comprises components for the drag-based thrust and for the added-mass-based thrust. The beating pleopods are approximated by oscillating flat plates with a different area and camber during the power stroke and the recovery stroke and with a phase shift between adjacent pleopod pairs. The added mass is shed during the second half of the power stroke and is minimized during the recovery stroke. A force balance between the pleopod thrust and the tube drag is effected by calculating the mean thrust during one beat cycle at a certain flow velocity in the tube and comparing it with the drag of the tube at that flow velocity. The energetics of the tube and the pump are derived from the forces, and the mechanical efficiency of the system is the ratio of these two. Adjusted to standard Callianassa subterranea values, the model predicts a mean flow velocity in the tube of 1.8 mm s-1. The mean thrust force, equalling the drag, is 36. 8 microN, the work done by the pleopod pump per beat cycle is 0.91 microJ and the energy dissipated by the tube system is 0.066 microJ per cycle. The mechanical efficiency is therefore 7.3 %. Pump characteristics that may be varied by the shrimp are the beat frequency, the phase shift, the amplitude and the difference in pleopod area between the power and recovery strokes. These parameters are varied in the model to evaluate their effects. Furthermore, the moment of added mass shedding, the distance between adjacent pleopods, the number of pleopods and the total tube drag were also varied to evaluate their effects.

Animals↗
PubMed1999

The aerodynamics and hydrodynamics of plants.

Because they grow away from their substratum to compete for light, plants have to withstand hydrodynamic or aerodynamic drag. Both water and land plants reconfigure in response to this drag, and this is presumed to reduce the risk of mechanical failure. However, there is little unequivocal evidence of drag reduction in large trees as a result of reconfiguration. Land plants must also transport water internally to their tissues, and many have developed xylem tracheids and vessels that help speed up this process. Recent evidence that tree height is limited by water supply suggests that water transport efficiency must be a crucial element in tree design. However, the resistance of many parts of the xylem is still unknown. More focused work is urgently required to shed light on the evolution and ecology of plants in relation to the flow of fluids.

Air Movements↗
PubMed2000

The mechanics of locomotion in the squid Loligo pealei: locomotory function and unsteady hydrodynamics of the jet and intramantle pressure.

High-speed, high-resolution digital video recordings of swimming squid (Loligo pealei) were acquired. These recordings were used to determine very accurate swimming kinematics, body deformations and mantle cavity volume. The time-varying squid profile was digitized automatically from the acquired swimming sequences. Mantle cavity volume flow rates were determined under the assumption of axisymmetry and the condition of incompressibility. The data were then used to calculate jet velocity, jet thrust and intramantle pressure, including unsteady effects. Because of the accurate measurements of volume flow rate, the standard use of estimated discharge coefficients was avoided. Equations for jet and whole-cycle propulsive efficiency were developed, including a general equation incorporating unsteady effects. Squid were observed to eject up to 94 % of their intramantle working fluid at relatively high swimming speeds. As a result, the standard use of the so-called large-reservoir approximation in the determination of intramantle pressure by the Bernoulli equation leads to significant errors in calculating intramantle pressure from jet velocity and vice versa. The failure of this approximation in squid locomotion also implies that pressure variation throughout the mantle cannot be ignored. In addition, the unsteady terms of the Bernoulli equation and the momentum equation proved to be significant to the determination of intramantle pressure and jet thrust. Equations of propulsive efficiency derived for squid did not resemble Froude efficiency. Instead, they resembled the equation of rocket motor propulsive efficiency. The Froude equation was found to underestimate the propulsive efficiency of the jet period of the squid locomotory cycle and to overestimate whole-cycle propulsive efficiency when compared with efficiencies calculated from equations derived with the squid locomotory apparatus in mind. The equations for squid propulsive efficiency reveal that the refill period of squid plays a greater role, and the jet period a lesser role, in the low whole-cycle efficiencies predicted in squid and similar jet-propelled organisms. These findings offer new perspectives on locomotory hydrodynamics, intramantle pressure measurements and functional morphology with regard to squid and other jet-propelled organisms.

Animals↗
PubMed2001

Hydrodynamic drag of diving birds: effects of body size, body shape and feathers at steady speeds.

For birds diving to depths where pressure has mostly reduced the buoyancy of air spaces, hydrodynamic drag is the main mechanical cost of steady swimming. Drag is strongly affected by body size and shape, so such differences among species should affect energy costs. Because flow around the body is complicated by the roughness and vibration of feathers, feathers must be considered in evaluating the effects of size and shape on drag. We investigated the effects of size, shape and feathers on the drag of avian divers ranging from wing-propelled auklets weighing 75 g to foot-propelled eiders weighing up to 2060 g. Laser scanning of body surfaces yielded digitized shapes that were averaged over several specimens per species and then used by a milling machine to cut foam models. These models were fitted with casts of the bill area, and their drag was compared with that of frozen specimens. Because of the roughness and vibration of the feathers, the drag of the frozen birds was 2-6 times that of the models. Plots of drag coefficient (C(D)) versus Reynolds number (Re) differed between the model and the frozen birds, with the pattern of difference varying with body shape. Thus, the drag of cast models or similar featherless shapes can differ both quantitatively and qualitatively from that of real birds. On the basis of a new towing method with no posts or stings that alter flow or angles of attack, the dimensionless C(D)/Re curves differed among a size gradient of five auklet species (75-100g) with similar shapes. Thus, extrapolation of C(D)/Re curves among related species must be performed with caution. At lower speeds, the C(D) at a given Re was generally higher for long-necked birds that swim with their neck extended (cormorants, grebes, some ducks) than for birds that swim with their head retracted (penguins, alcids), but this trend was reversed at high speeds. Because swimming birds actually travel at a range of instantaneous speeds during oscillatory strokes, species variations in drag at different speeds must be considered in the context of accelerational stroking.

Animals↗