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A new approach to tag design in dolphin telemetry: Computer simulations to minimise deleterious effects
ARTICLE IN PRESS
Deep-Sea Research II 54 (2007) 404–414 www.elsevier.com/locate/dsr2
A new approach to tag design in dolphin telemetry: Computer simulations to minimise deleterious effects V.V. Pavlova,, R.P. Wilsonb, K. Luckec a
Crimean State Medical University, Lenin blvd 5/7, 95006 Simferopol, Ukraine Biological Sciences, University of Wales Swansea, Singleton Park, Swansea SA2 9PP, Wales, UK c Forschungs- und Technologiezentrum Westkueste, Universitaet Kiel, Hafentoern 1, 25761 Buesum, Germany b
Accepted 30 November 2006
Abstract Remote-sensors and transmitters are powerful devices for studying cetaceans at sea. However, despite substantial progress in microelectronics and miniaturisation of systems, dolphin tags are imperfectly designed; additional drag from tags increases swim costs, compromises swimming capacity and manoeuvrability, and leads to extra loads on the animal’s tissue. We propose a new approach to tag design, elaborating basic principles and incorporating design stages to minimise device effects by using computer-aided design. Initially, the operational conditions of the device are defined by quantifying the shape, hydrodynamics and range of the natural deformation of the dolphin body at the tag attachment site (such as close to the dorsal fin). Then, parametric models of both of the dorsal fin and a tag are created using the derived data. The link between parameters of the fin and a tag model allows redesign of tag models according to expected changes of fin geometry (difference in fin shape related with species, sex, and age peculiarities, simulation of the bend of the fin during manoeuvres). A final virtual modelling stage uses iterative improvement of a tag model in a computer fluid dynamics (CFD) environment to enhance tag performance. This new method is considered as a suitable tool of tag design before creation of the physical model of a tag and testing with conventional wind/water tunnel technique. Ultimately, tag materials are selected to conform to the conditions identified by the modelling process and thus help create a physical model of a tag, which should minimise its impact on the animal carrier and thus increase the reliability and quality of the data obtained. r 2007 Elsevier Ltd. All rights reserved. Keywords: Tag; Marine mammals; Modelling; Drag; Hydrodynamics
1. Introduction Loggers (see e.g., Naito, 2004) and radio-telemetry are both powerful methodologies for studying cetacean biology. Cooke et al. (2004) review some of Corresponding author. Tel.: +38 0652 483 122; fax: +38 0652 272 092. E-mail address: [email protected] (V.V. Pavlov).
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.11.010
the sensory technologies used in animal logging today, many of which are applicable to studies of cetacean biology. These include systems to study, seasonal and diurnal migrations, physiology (such as body temperature, rate of breathing, dive duration) and the physical parameters of the environment (such as depth, water temperature, etc.) (Fedak, 2004). In fact, to date, diving behaviour and movements studies have been carried out on
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representatives of at least eight different dolphin genera on species of greatly-differing size, exploiting open-sea and coastal zones, marine and freshwater areas and with widely differing swim speeds and dive depths, e.g., bottlenose dolphins (Tursiops truncatus), Atlantic spotted dolphins (Lagenorhynchus acutus), narwhals (Monodon monoceros), long-finned pilot whales (Globiocephala melas), Dall’s porpoises (Phocoenoides dalli), belugas (Delphinapterus leucas), killer whales (Orcinus orca), and Amazon River dolphins (Inia geoffrensis) (Mate et al., 1995; Baird and Dill, 1995; Davis et al., 1996; Hanson and Baird, 1998; Martin and da Silva, 1998; Baird et al., 2001; Dietz et al., 2001; Bloch et al., 2003; Litovka et al., 2004). Virtually all loggers used on dolphins include a pressure transducer, while some have speed sensors and a salt-water switch. More sophisticated tags may incorporate conductivity temperature depth loggers, such as those used on belugas to derive oceanographic data (Fedak, 2004). Tags may be broadly divided into satellite-linked recorders that transfer logged information via satellites to the researchers, and loggers that store saved information indefinitely. These latter are typically released from animals after certain period or recovered when the animals are recaptured. Currently, the main method of tag attachment uses bolts and pins drilled through the dorsal fin (Mate et al., 1995; Davis et al., 1996; Read and Westgate, 1997), although some researchers use suction cups (Hanson and Baird, 1998; Schneider et al., 1998; Baird et al., 2001) to attach units to different places on the body. In addition, the tag may be sewn to the animal’s back in finless species, such as the beluga (Litovka, personal communication). Thus, it would appear that, although cetacean tags have progressed remarkably with regard to microelectronics and miniaturisation, only a few have been attached with due consideration of drag or animal discomfort, particularly with regard to the extra pressure loads on dorsal fin tissue (Hanson, 2001). Extra drag increases the energy expenditure of swimming (Bannasch et al., 1994; Culik et al., 1994) and compromises swimming capacity and manoeuvrability. This has knock-on effects on the efficiency of foraging (see e.g., Wilson et al., 2004 for a simulation on penguins) and may have serious consequences for the evasion of predators. The extra loads on the dorsal fin tissues may result in necrosis and premature loss of tags due to tissue tearing (Geraci and Smith, 1990).
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In this work, we address the problem of inappropriately constructed tags and consider options to derive a system that is minimally small and maximally functional while causing minimal discomfort, negligible drag and not being subject to fouling problems. For this, we took measurements to define the 3D shape of the putative site of attachment (here the dorsal fin) and assumed that tags could be constructed with a specified volume, which can be essentially flexibly configured with regard to the outside dimensions of the units. Using a computer-aided design (CAD) approach, we produced a first tag design, which was then examined using software to determine how this unit affected water flow over the site. Areas of high drag were identified and the design modified in a series of iterative steps until the best configuration was achieved. In this work, we outline the basic principles in tag design considering the dolphin dorsal fin as the primary attachment site although the basic principles outlined here are applicable to other kinds of tags that differ in the place and method of attachment. 2. Methods 2.1. Measurements of the dorsal fin shape Measurements were made on the dorsal fins of two bottlenose dolphins (T. truncatus), five harbour porpoises (Phocoena phocoena), and three common dolphins (Delphinus delphis) by-caught or stranded on the Crimea coast between 1989 and 2003. Dorsal fins from all animals were in good condition, had no internal damage and corresponded to condition code #2 (Kuiten and Hartmann, 1991). The outline of the fin was photographed before being cut from the animals. The fin base was defined by the line between the point of maximum lateral curvature of the leading edge and the bottom of the fin’s trailing edge. Fin height was measured from the dorsal tip to the root chord of the fin. Fins were then removed from the body and fixed in 10% neutral formalin. For the study of fin geometry, a sampling scheme representing a two-dimensional mesh on the fin surface was used (Pavlov, 2003). Each data point was defined by two parameters (i and j). Eight crosssections of the fixed fin were made at equal intervals (i-parameter, i ¼ (1y8), Di ¼ fin span/8). Measurements were taken at 20 points located at equal intervals on the left side of the section (j-parameter, j ¼ (1y20), Dj ¼ chord/19). The section thickness
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was measured by using callipers at data points along lines orthogonal to the section chord. The precision of the reported values was 70.1 mm and based on three repeated measures. The bent part of fin crosssections near the trailing edge was straightened in some cases before section thickness measurements. 2.2. Dorsal fin mobility The estimation of the natural flexibility of the dorsal fin of different species was obtained by analysing film of the swimming and turning patterns of harbour porpoises and bottlenose dolphins in Harderwijk dolphinarium. The video analysis was made with Uleads VideoStudioTM software. Video frames of different appearance of the dorsal fin were extracted as 1.2 Mb Truecolor graphic files in bmp format and 748 564 pixels size. Selected files were processed by the morphological filters with ImagePros software by Media Cybernetics Inc. Extracted contours of dorsal fins were used for the estimation of both spanwise and chordwise deformation. The position of the fin tip as well as the trailing edge at the middle of the fin was marked on the images. The position of the fin base along the long axis of the body also was marked on the images. The angle formed by the inclined tip of the fin and vertical position of the fin base was measured. The inclination of the trailing edge in the middle of the fin from the long axis of the body was measured as well. Limited data obtained were used for the estimation of both spanwise and chordwise deformation of the dorsal fin. 2.3. Modelling of the dorsal fin and a tag On the basis of the measurements of fin crosssections thickness, outlines of the cross-sections were made using SolidWorkss software by SolidWorks Corp. Then, parametric solid models of fin using a 1:1 scale were created from the fin outline and the set of cross-section outlines. In model space, the X-axis corresponded to streamwise direction, the Y-axis corresponded to spanwise direction and Z-axis corresponded to the direction normal to the wing plane. Basic airfoil parameters, the leading edge radius (r), chord length (CL), maximum thickness (MT) and position of maximum thickness (PMT) with respect to leading edge were measured on the model cross-sections (Fig. 1). The aspect ratio was calculated as the fin span squared divided by the fin area (Webb, 1975).
Fig. 1. The basic airfoil parameters applied to the cross-section of the dorsal fin (r ¼ leading edge radius, CL ¼ chord length, MT ¼ maximum thickness, PMT ¼ position of maximum thickness).
Fig. 2. Combined model of the simplified dolphin body and the dorsal fin.
Additionally, a simplified model of the dolphin body 1.5 m length was constructed (Fig. 2). The model presents the upper half of the body without flippers, flukes and complex details (eyes, blowhole). The models of dorsal fin were combined with simplified body model in order to simulate natural conditions of the fin flow. The parametric model of a tag was created by means of a suite of solid modelling methods with SolidWorkss software by SolidWorks Corp (Fig. 3). A tag was constructed to be located on the middle of the fin, taking into account the basic airfoil parameters at this location, thus providing the best fit to the fin geometry. Tag shapes were fitted to the individual fin models by means of the links between the derived parameters of the fin and the models. 2.4. Computer fluid dynamics (CFD) test Hydrodynamic tests of the dorsal fin and tag models were carried out with the CFDCOSMOSFloWorksTM software.
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Compressibility effects: off Default wall conditions: adiabatic Fluid: water Thermodynamic parameters Pressure: 101325 Pa Temperature: 20.05 1C
Fig. 3. Parametric model of a tag. The left fore-part is shown semi-transparent to illustrate some tag components: electronic box, antenna and bolts.
Two test modifications were applied in this study. First was the test of the 3D models of the fin combined with the simplified model of the dolphin body. The goals of the test were to get a general pattern of the flow around dorsal fins among three species of dolphins and to estimate the distribution of pressure and velocity on the dorsal fin surface. Calculations of the distribution of the pressure and velocity on the dorsal fin were carried out at zero yaw and pitch angles and Reynolds number 1.13107. Second was the test of 2D cross-sections of the dorsal fin of a bottlenose dolphin alone and the dorsal fin with different tag configurations. The goals were to estimate the additional drag caused by the tag as well as to select the best tag configuration in terms of hydrodynamic performance. Two-dimensional calculations of the drag force and drag coefficient were carried out at zero yaw and pitch angles and Reynolds numbers that ranged between 2.63105 and 1.09106. Reynolds number was calculated as UL , v where U mean fluid velocity, L characteristic length, v 1.06106 m2 s1, the kinematic viscosity for sea water. To calculate the drag force as well as distribution of pressure and velocity on the dorsal fin the following settings were used: Re ¼
Analysis type: external Result resolution: high (level 6) Geometry resolution: 0.001 m Physical features Heat transfer in solid: off Time settings: off Gravitation settings: off
Velocity parameters X component of velocity: 2; 4; 6; 8 m s1 Y component of velocity: 0 m s1 Z component of velocity: 0 m s1 Turbulence parameters Turbulent intensity: 0.1% Turbulent length: 0.002 m Dimensionless drag coefficient Cd of the fin and tag cross-sections was calculated as Cd ¼
2F x , rU 2 S
where Fx is the X component of total drag force, r (1028 kg m3), the density of sea water at the specified temperature of 20 1C, U is velocity and S is cross-sectional area. 3. Results and discussion 3.1. Stage I. Study of the operational conditions The quickest way to gain a basic understanding of problems in tag design is to look into the operational conditions of a tag. It helps to establish what forces influence a tag, their range and spatiotemporal characteristics. It also allows an estimate of the negative effect of a tag on dolphin well being. Due to the peculiarities of dolphin skin with its high proliferative activity of the epidermis (Brown et al., 1983) resulting in a continual flaking-off of the superficial layer of epidermal cells (Sokolov et al., 1969), it would appear that nothing can be glued to the skin surface for a long time. Noninvasive suction cups attachment is usually used for short-term data acquisition (Hanson and Baird, 1998; Baird et al., 2001), whereas longer studies, such as those of migrations, require tags attached by bolts and pins drilled through the dorsal fin (Mate et al., 1995; Davis et al., 1996). Both tag types are influenced by the highly variable flow pattern of water round the dolphin’s body (Pershin, 1988; Romanenko, 1997; Fish and Rohr, 1999). Unlike
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machines, dolphins display non-steady swimming with accelerations interspaced by gliding. Dolphins have a relatively high turning performance (Fish, 1997) although little is known about the hydrodynamics of the manoeuvring dolphin. In tagged animals it is very likely that tag loads increase when dolphins glide, accelerate, and make cusp turns. In addition, re-entry to the water after the leaps (Hanson, 2001) as well as attacks by other dolphins may lead to particularly high loads on tags. Under these conditions a streamlined shape of a tag becomes particularly important. High drag can lead to premature loss of suction cups tags or to tissue reaction to pressure in the dorsal fin where bolts/ pins are used for attachment (Geraci and Smith, 1990; Geertsen et al., 2004). In addition, a wellstreamlined shape reduces the risk of fouling, in particular seaweed attachment to the lugs of a tag (Geertsen et al., 2004). In this paper, we are specifically focused on bolton tags although the basic principles of tag design are common for other attachment modalities. The material properties of a tag with bolt/pin attachment to the dorsal fins also must be matched to the dorsal fin tissue (Hanson, 2001). Thus, any appropriate dolphin dorsal fin tag also must show some bending to reduce both additional drag and extra load on the tissue around pins. It thus differs significantly from a tag attached to a rigid structure such as a sea turtle carapace (Watson and Granger, 1998). The optimal arrangement of the pins on the dorsal fin should be found based on the data of the fin structure (Pabst, 1996; Hanson, 2001; Pavlov, 2003) and fin flexure in order to avoid the extra loads that may appear during the cusp turn. 3.1.1. Dorsal fin shape The dorsal fin of dolphins is a multifunctional organ: it resists yawing and rolling motion and acts to prevent side slip during manoeuvres (Fish and Rohr, 1999). In addition, it acts as a ‘‘thermal window’’ to prevent overheating (Scholander and Schevill, 1955; Williams et al., 1999). The form of the dorsal fin outline depends on species, sex, age, and even varies between monosex, equal age individuals from one species. Despite this, general trends are apparent in fin outline representatives of different genera (Fig. 4). Among the species studied, the aspect ratio varied from 0.8 to 0.9 in harbour porpoises to 1.1–1.3 in bottlenose and common dolphins.
The cross-sections of the dorsal fin could be analysed by considering the basic airfoil parameters: chord length, maximum thickness of section, position of maximum thickness, and leading edge radius (Fig. 1). Unlike the fin outline, the shape of the cross-section is conservative, revealing only subtle variability. Indeed, a similar pattern of thickness distribution was found in all the species examined in this study (Fig. 5). Any interspecific difference in the pattern of the spanwise distribution of airfoil parameters was found to be related to fin size and outline. The wing-like shape of the dorsal fin (Lang, 1966; Pershin, 1975) is a result of adaptation to relatively high-speed swimming in seawater, which may explain the consistency of cross-section parameters on which there is substantial selection pressure not to exceed the strict morphological limits which lead to effective hydrodynamic function of the dorsal fin. 3.1.2. Dorsal fin mobility During a manoeuvre the dorsal fin can bend in chordwise and spanwise directions. The central core structure appears to decrease rigidity of the fin in these directions. The ligamentous layer appears to function to prevent excessive bending (Pavlov, 2003). Structurally limited bending of the fin during manoeuvre is apparently a natural limiter of lift and related induced drag (Fish, 1998). Although, we have few data on dolphin swimming in captivity, the films do allow for conceptualisation of the degree of natural deformation of the fin during manoeuvres. Our video analyses showed that dolphin dorsal fins can bend in both spanwise and chordwise directions. The upper part of the fin showed a greater spanwise
Fig. 4. The outline of the dorsal fin among the representatives of different genera of small cetaceans. The dash-and-dot, dot, dash, and solid line represent harbour porpoise, common dolphin, bottlenose dolphin and male pilot whale respectively. The pilot whale is added in order to show the trend in the outline of the dorsal fin.
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motion close to the water surface the trailing edge of the fin showed low-range oscillations, while the leading edge and the tip of the fin remained still. The most extensive dorsal fin deformation occurred when a cusp turn was made after slow swimming. In this case, both the tip of the fin and its trailing edge could bend, although, generally, the angle of inclination of the tip of the fin from the fin base did not exceed 601. Chordwise flexibility was most apparent in the middle of the fin. The angle of inclination of the trailing edge from the leading edge did not exceed 451. 3.1.3. Hydrodynamics of the dorsal fin model The dorsal fin of dolphins has pattern of flow close to that of an aircraft wing. Modelling the fin showed that the velocity has a steep positive gradient at the leading edge, which then decreases to the trailing edge. Furthermore, there is a difference in velocity distribution in the upper and lower parts of the fin and the velocity in the chordwise direction of the fin but more abruptly over the lower part. All the models of dorsal fin were shown to have an unstable flow near the trailing edge at the base of the fin. Not surprisingly, the leading edge of the fin was exposed to the highest pressure, with maximum values occurring at the base of the fin. The steep negative pressure gradient near the leading edge reverses in the low-pressure region; then pressure grows again to the trailing edge (Fig. 6). Over the upper part of the fin the pressure grows rapidly to the trailing edge, while the lower part has a more elongated low-pressure region.
Fig. 5. Spanwise distribution of the basic airfoil parameters of fin cross-sections (means7SD). (A) Maximum thickness, (B) position of the maximum thickness, (C) leading edge radius. Lines with diamond, squares and delta symbols correspond to the harbour porpoise (Phocoena phocoena), bottlenose dolphin (Tursiops truncatus) and common dolphin (Delphinus delphis), respectively.
flexibility in contrast to the base of the fin. The leading edge of the fins was rigid and flexibility appeared to increase from the position of maximum thickness to the trailing edge of the fin. The range of the natural deformation of the dorsal fin varied considerably and depended on the species, speed of swimming and turning radius. During fast rectilinear
Fig. 6. Chordwise distribution of the pressure. Cross-sections are taken at the middle of the dorsal fin of selected animals. Lines with diamond, squares and delta symbols correspond to the harbour porpoise, bottlenose dolphin and common dolphin respectively. Data are normalised from 0 to 1.
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3.2. Stage II. Parametric modelling of the dorsal fin and a tag The parametric model of the fin was created using the defined regularities in the fin shape. It included the pattern of spanwise distribution of the crosssection parameters: leading edge radius, maximum thickness, and position of the maximum thickness. Low variability of the pattern in the cross-section parameters between the species studied led us to assume interspecific consistency. Parametric representation of the outline and fin cross-sections of the fin was made. Links between the outline and basic cross-section parameters were provided, as was the fixed pattern of spanwise thickness distribution. It enabled the model shape to change automatically according to a new outline while maintaining the requisite pattern of thickness distribution. Such a feature allowed a new fin model to be constructed from the basic shape according to naturally differing situations, e.g., between species or sexual dimorphism. In an attempt to model the natural conditions, the parametric model of the fin can simulate the fin deformation during a manoeuvre. The shape of the parametric model can be changed according to our observed range of the fin flexibility both in spanwise and chordwise directions. The limited results we have do not yet allow precise analysis of the range of fin bend, and in order to improve tag performance, the relationship between natural deformation of the fin and speed of swimming and turning radius (incorporating differing species, sex, and age) should be studied in more detail. It was supposed that hydrodynamic performance of a tag can be optimised by the convergence of tag design parameters with parameters of fin crosssections. Following this supposition two tag models having the shape close to the fin crosssection were constructed at the initial phase of tag design. The first tag had paired side-mount design, while the second one had connected paired side-mount design, i.e. U-shape leaned on the leading edge of the fin (Fig. 7). Both tags had equal chord length and maximum thickness while they differed in position of maximum thickness and leading edge radius. Each design had three configurations (Table 1). At constant maximum thickness and chord length the position of maximum thickness varied from 26% chord length to 44% chord length in Tag I and from 18% chord length to 35% chord length in Tag II. Besides, leading edge
Fig. 7. Cross-sections of Tag I and Tag II models (r ¼ leading edge radius, CL ¼ chord length, MT ¼ maximum thickness, PMT ¼ position of maximum thickness).
Table 1 Parameters of Tag I and Tag II configurations
Tag Tag Tag Tag Tag Tag
I config 1 I config 2 I config 3 II config 1 II config 2 II config 3
CL (m)
MT (% Cl)
PMT (% Cl)
r (% CL)
Area (m2)
0.114 0.114 0.114 0.114 0.114 0.114
29.9 29.9 29.9 29.9 29.9 29.9
26.3 35.1 43.9 17.6 26.3 35.1
8.4 6.3 5.1 6.3 4.2 3.2
0.0014 0.0014 0.0014 0.0013 0.0013 0.0013
r ¼ leading edge radius, CL ¼ chord length, MT ¼ maximum thickness, PMT ¼ position of maximum thickness.
radius varied from 5.1% chord length to 8.4% chord length in Tag I and from 3.2% chord length to 6.3% chord length in Tag II. The alteration of leading edge radius and position of maximum thickness of the tags was made in CAD–CFD cycle in order to examine the difference in Tag I and II hydrodynamic performance as well as to reveal the best tag configuration possessing the lowest additional drag. The final parametric model of the tag as example of a simple satellite-linked tag was constructed. The model based on Tag I configuration 3, which had the best performance in terms of additional drag. In keeping with conventional studies, it included a set of sensors, a radio transmitter, batteries and an antenna. The fore-part of a tag was constructed to be rigid while the rear part is flexible.
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The place of attachment of the fore-part of a tag by the bolts to the fore-part of the fin was chosen taking into account the assumed lowest flexibility of the fin at this area. The attachment of the inflexible fore-part of a tag by the bolts at this area should prevent from appearance of differential forces on the bolts causing them to migrate or fail early. The fore-part of a tag is intended to contain the electronic components of a tag, while the rear part is assigned to simulate the trailing edge behaviour to minimise the induced drag during the manoeuvre. Links between the tag and fin model parameters provided the best fit of a tag to the dorsal fin shape as well as control of the changeable shape of a tag. The tag model was able to change its shape according to the bend of the fin model simulating the natural flexibility (Fig. 8). The estimated range of the deformation of a tag model can be used in the process of selection of properties of the composite material for the tag. It is worth noting here that there is a huge range of inert, stable silastic materials, which can be moulded with a precise degree of softness, according to the addition of specific softeners, which would lend themselves as a suitable substrate for a fin tag. Therefore, the mechanical properties of the flexible rear part of the tag can be selected to reach the same degree of the deformation under the water flow as the trailing edge of the dorsal fin of dolphin (Fig. 8). The perfect tag should possess minimal impact on the animal behaviour as well as phenomenon studied. Moreover, longer tag functioning duration has become important for data collection. Apparently
Fig. 8. Simulation of the dorsal fin bend during the manoeuver, top view. Note the bend of the rear end of a tag according to the bent trailing edge of the fin.
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conventional tag design should be developed as a compromise between these conflicting requirements. Small cetaceans have a variety of behaviour patterns that imply different swimming modes, with a range of static and dynamic loads on the dorsal fin and the attached tag. Consequently, the optimisation of tag design should be not only for the certain range of the velocity inherent for the selected species but also for the scenarios that lead to the extra loads on the tag. The practice of dolphin tagging shows the examples of the operational conditions that increase the risk of premature loss of a tag. As found in exploitation of the pin-attached tags for dolphin telemetry, many tags have the vertical migration component that is most likely from frequent re-entry loading through the airwater interface as the animal dives after breathing (Hanson, 2001). 3.3. Stage III. Step-by-step improvement of a tag model As it is difficult to produce an optimal tag design in one attempt, tag performance was improved iteratively. The tag geometry constructed in a CAD environment was tested in the CFD environment in order to reveal the imperfections in tag construction. The CFD test revealed those features of tag design that were responsible for highest drag. Subsequently, those parts were redesigned and the procedure of the CFD test was repeated. The cycle CAD construction–CFD test was repeated until an acceptable hydrodynamics performance of a tag model was achieved. The criterion of hydrodynamic performance of the model was the level of additional drag caused by a tag. Attachment of a tag to the fin led to an increase in drag force that ranged between 0.29 and 6.46 N over all simulated velocities (Fig. 9). It was found that at constant length and thickness of the tag, the lowest additional drag can be reached by altering the leading edge radius and position of maximum thickness parameters. Generally, the Cd decreased from low to high Reynolds number (Fig. 10). For the assumed cruising speed of swimming 2 m s1 the combined decrease in leading edge radius and increase in position of maximum thickness lead to decreasing of Cd from 0.097 to 0.044 for Tag I and from 0.075 to 0.053 for Tag II. For the assumed burst speed of swimming 8 m s1 the same changes in tag configuration decreased Cd from 0.051 to 0.041 for Tag I and from 0.063 to 0.049 for Tag II.
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Fig. 9. Drag force plotted against simulated velocity for a fin cross-section as well as all Tag I and II configurations.
placement of tag components. Besides, the load produced by the bolts on the dorsal fin tissue during horizontal swimming may be decreased by the redistribution of the part of the load to the leading edge of the fin. Only rectilinear swimming patterns with a range of constant velocities at zero yaw and pitch angle were chosen for the CFD test of the dorsal fin and attached tag. These onerous conditions are likely to be most detrimental to the swimming dolphin and it is here where effects are most obvious. Ideally, though, CFD tests should be performed using the full range of dolphin operational conditions, i.e. dolphin manoeuvres, accelerations, the tag reentering the water after the dolphin surfaces to breathe, etc. The initial tag design was examined across CAD–CFD cycle and an optimal tag configuration was chosen as a basis for the following development of tag design. Unlike trial methods, this procedure offers a suitable tool for the verification of engineering ideas at the initial stages of tag design before wind/water tunnel testing. The results obtained can help in the choice of how to compromise between the demands of placement of a tag components and hydrodynamic performance of a tag. It should be noted, nevertheless, that the conventional wind/water tunnel technique provides the exact values of additional drag caused by the tag. It is reasonable therefore to carry out the final wind/water tunnel testing of a tag model to get real data of a tag performance. 3.4. The outline of a new approach to tag design in dolphin telemetry
Fig. 10. Coefficient of drag (Cd) plotted against Reynolds number for (A) the fin cross-section and Tag I configurations and (B) the fin cross-section and Tag II configurations.
The basic steps of the new approach to tag design in dolphin telemetry are:
Configuration 3 of Tag I had best performance in terms of additional drag. The backward-shifted position of maximum thickness (43.9% of chord length) and small leading edge radius (5.1% of chord length) provided the best performance for the selected range of velocities. The value of position of maximum thickness E 44% chord length is close to that of the ‘‘laminar’’ airfoils although this parameter recalculated with respect to the total length of fin cross-section plus tag is 34.6%. Generally, the U-shaped Tag I model looks preferable due to low level of additional drag and increased space for the
1. Study of the operational conditions of a tag. Quantification of the parameters of the fin shape, hydrodynamics and mobility. 2. Parametric modelling of the fin shape and a tag using the parameters obtained for the operational conditions. 3. Fitting a parametric model of the fin to the expected fin shape according to species. 4. Optimising a tag model to the expected fin shape. 5. Iterative improvement of the performance of a species-specific tag model in a CFD environment. The primary criterion for tag performance is minimised additional drag.
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6. Selection of a tag materials and creation of the physical model of a tag optimised to the selected species. 7. Wind/water tunnel test of the physical models of a tag. 8. Producing of a species-specific tag with high hydrodynamic performance. The known restrictions need to be taken into account in tag design. These are a volume-based drag factor, the risk of impact, the ability to dump heat in tags covering a large proportion of the dorsal fin, as well as the problems related with stability of tag attachment, namely tissue reaction on tag pins as well as static and dynamic loads generated by the tag (Geraci and Smith, 1990; Hanson, 2001). Considering the dorsal fin as usual place of tag attachment, it is preferable to outline the desired properties of the tag: the hydrodynamic design of a tag should affect the natural pattern of water flow over the fin minimally. A tag should produce minimal loads on the fin tissue within the range of swimming patterns, i.e. ranging, diving, accelerations, manoeuvring, as well as the tag reentering the water after the dolphin surfaces. To this end, the shape of a tag should differ minimally from the cross-sectional shape of the dorsal fin. Additionally, the mechanical properties of a tag should correspond well with those of the fin and not prevent any natural bending. Along with this, the U-shape of a tag, with the tag leaning on the leading edge, should minimise load on the fin tissues caused by bolts or pins if used. Overall, minimising the negative effect of attached tags on dolphin swimming should increase the reliability and quality of the data obtained. Acknowledgements The authors thank Harderwijk Dolphinarium, The Netherlands, for the video of swimming dolphins. They also thank Brad Hanson and an anonymous reviewer for the valuable comments to the manuscript.
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Baird, R.W., Ligon, A.D., Hooker, S.K., Gorgone, A.M., 2001. Subsurface and nighttime behaviour of pantropical spotted dolphins in Hawaii. Canadian Journal of Zoology 79, 988–996. Bannasch, R., Wilson, R.P., Culik, B., 1994. Hydrodynamic aspects of design and attachment of a back-mounted device in penguins. Journal of Experimental Biology 194, 83–96. Bloch, D., Heide-Jørgensen, M.P., Stefansson, E., Mikkelsen, B., Ofstad, L.H., Dietz, R., Andersen, L.W., 2003. Short-term movements of long-finned pilot whales Globiocephala melas around the Faroe Islands. Wildlife Biology 9, 47–58. Brown, W.R., Geraci, J.R., Hicks, B.D., St. Aubin, D.J., Schroeder, J.P., 1983. Epidermal cell proliferation in the bottlenose dolphin (Tursiops truncatus). Canadian Journal of Zoology 61, 1587–1590. Cooke, S.J., Hinch, S.G., Wikelski, M., Andrews, R.D., Kuchel, L.J., Wolcott, T.G., Butler, P.J., 2004. Biotelemetry: a mechanistic approach to ecology. Trends in Ecology & Evolution 19, 334–343. Culik, B.M., Bannasch, R., Wilson, R.P., 1994. External devices on penguins: how important is shape? Marine Biology 118, 353–357. Davis, R.W., Worthy, G.A., Wu¨rsig, B., Lynn, S.K., 1996. Diving behavior and at-sea movements of an Atlantic spotted dolphin in the Gulf of Mexico. Marine Mammal Science 12 (4), 569–581. Dietz, R., Heide-Jørgensen, M.P., Richard, P., Acquarone, M., 2001. Summer and fall movements of narwhals (Monodon monoceros) from northeastern Baffin Island towards Northern Davis Strait. Arctic 54 (3), 244–261. Fedak, M., 2004. Marine animals as platforms for oceanographic sampling: a ‘‘win/win’’ situation for biology and operational oceanography. Memoirs of the National Institute for Polar Research, Special Issue (58), 133–147. Fish, F.E., 1997. Biological Designs for enhanced maneuverability: analysis of marine mammal performance. In: Proceedings of the Special Session on Bio-Engineering Research Related to Autonomous Underwater Vehicles. Autonomous Undersea Systems Institute Lee, NH, USA, pp. 109–117. Fish, F.E., 1998. Biomechanical perspective on the origin of cetacean flukes. In: Thewissen, J.G.M. (Ed.), The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. Plenum Press, New York pp. 303–324. Fish, F.E, Rohr, J.J., 1999. Review of dolphin hydrodynamics and swimming performance. SPAWARS System Center Technical Report 1801, San Diego, CA, pp. 1–193. Geertsen, B.M., Teilmann, J., Kastelein, R.A., Vlemmix, H.N.J., Miller, L.A., 2004. Behaviour and physiological effects of transmitter attachments on a captive harbour porpoise (Phocoena phocoena). Journal of Cetacean Research and Management 6 (2), 139–146. Geraci, J.R., Smith, G.J.D., 1990. Cutaneous response to implants, tags, and marks in Beluga Whales, Delphinapterus leucas, and bottlenose dolphins, Tursiops truncatus. In: Smith, T.G., St. Aubin, D.J., Geraci J.R. (Eds.), Advances in the Research in the Beluga Whale, Delphinapterus leucas. Canadian Bulletin of Fisheries and Aquatic Sciences, vol. 224, pp. 81–95. Hanson, M.B., 2001. An evaluation of the relationship between small cetacean tag design and attachment durations: a bioengineering approach. Ph.D. Thesis, University of Washington, Seattle, Washington, 208pp.
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Hanson, M.B., Baird, R.W., 1998. Dall’s porpoise reactions to tagging attempts using a remotely-deployed suction-cup tag. Marine Technology Society Journal 32 (2), 18–23. Kuiten, T., Hartmann, M.G. (Eds.), 1991. Standard protocol for the basic postmortem examination and tissue sampling of small cetacean. In: Proceedings of the First ECS Workshop on Cetacean Pathology: Dissection Techniques and Tissue Sampling, Leiden, The Netherlands, 26pp. Lang, T.G., 1966. Hydrodynamic analysis of dolphin fin profiles. Nature 209, 1110–1111. Litovka, D.I., Hobbs, R.C., Laidre, K.L., O’Corry-Crowe, G.M., Orr, J.R., Richard, P.R., 2004. Studying of dive patterns of belugas (Delphinapterus leucas) in Anadyr-Navarin region using satellite telemetry. In: Proceedings of the III. Conference ‘‘Marine Mammals of the Holarctic’’, Koktebel, Ukraine, pp. 327–331. Martin, A.R., da Silva, V.M.F., 1998. Tracking aquatic vertebrates in dense tropical forest using VHF telemetry. Marine Technology Society Journal 32, 82–88. Mate, B.R., Rossbash, K.A., Nieukirk, S.L., 1995. Satellitemonitored movements and dive behavior of a bottlenose dolphin (Tursiops truncatus) in Tampa Bay, Florida. Marine Mammal Science 11 (4), 452–463. Naito, Y., 2004. Bio-logging science. Memoirs of the National Institute for Polar Research, Special Issue (58), 118–132. Pabst, D.A., 1996. Morphology of the subdermal connective tissue sheath of dolphins: a new fibre-wound, thin-walled, pressurized cylinder model for swimming vertebrates. Journal of Zoology, London 238, 35–52. Pavlov, V.V., 2003. Wing design and morphology of the harbour porpoise dorsal fin. Journal of Morphology 258, 284–295. Pershin, S.V., 1975. Hydrodynamic analysis of dolphin and whale fin profiles. Bionika 9, 26–32 (in Russian).
Pershin, S.V., 1988. Fundamentals of hydrobionics. Sudostroyeniye. Leningrad, in Russian. Read, A.J., Westgate, A.J., 1997. Monitoring the movements of harbour porpoises (Phocoena phocoena) with satellite telemetry. Marine Biology 130, 315–322. Romanenko, E.V., 1997. The black sea bottlenose dolphin: The hydrodynamics, In: Sokolov, V.E., Romanenko, E.V. (Eds.), The Black Sea Bottlenose Dolphin Tursiops truncatus ponticus: Morphology, Physiology, Acoustics, Hydrodynamics. Nauka, Moscow, pp. 621–650 (in Russian). Scholander, P.F., Schevill, W.E., 1955. Counter-current vascular heat exchange in the fins of whales. Journal of Applied Physiology 8, 279–282. Schneider, K., Baird, R.W., Dawson, S., Childerhouse, S., 1998. Reactions of bottlenose dolphins to tagging attempts using a remotely-deployed suction-cup tag. Marine Mammal Science 14 (2), 316–324. Sokolov, V., Bulina, I., Rodionov, V., 1969. Interaction of dolphin epidermis with flow boundary layer. Nature 222, 267–268. Watson, K.P., Granger, R.A., 1998. Hydrodynamic effects of a satellite transmitter on a juvenile green turtle (Chelonia mydas). Journal of Experimental Biology 201 (2), 497–505. Webb, P.W., 1975. Hydrodynamics and energetics of fish propulsion. Bulletin of the Fisheries Research Board of Canada 190, 1–158. Williams, T.M., Noren, D., Berry, P., Estes, J.A., Allison, C., Kirtland, J., 1999. The diving physiology of bottlenose dolphin (Tursiops truncatus) III. Thermoregulation at depth. Journal of Experimental Biology 202, 2763–2769. Wilson, R.P., Kreye, J.A., Lucke, K., Urquhart, H., 2004. Antennae on transmitters on penguins: balancing energy budgets on the high wire. Journal of Experimental Biology 207, 2649–2662.
Deep-Sea Research II 54 (2007) 404–414 www.elsevier.com/locate/dsr2
A new approach to tag design in dolphin telemetry: Computer simulations to minimise deleterious effects V.V. Pavlova,, R.P. Wilsonb, K. Luckec a
Crimean State Medical University, Lenin blvd 5/7, 95006 Simferopol, Ukraine Biological Sciences, University of Wales Swansea, Singleton Park, Swansea SA2 9PP, Wales, UK c Forschungs- und Technologiezentrum Westkueste, Universitaet Kiel, Hafentoern 1, 25761 Buesum, Germany b
Accepted 30 November 2006
Abstract Remote-sensors and transmitters are powerful devices for studying cetaceans at sea. However, despite substantial progress in microelectronics and miniaturisation of systems, dolphin tags are imperfectly designed; additional drag from tags increases swim costs, compromises swimming capacity and manoeuvrability, and leads to extra loads on the animal’s tissue. We propose a new approach to tag design, elaborating basic principles and incorporating design stages to minimise device effects by using computer-aided design. Initially, the operational conditions of the device are defined by quantifying the shape, hydrodynamics and range of the natural deformation of the dolphin body at the tag attachment site (such as close to the dorsal fin). Then, parametric models of both of the dorsal fin and a tag are created using the derived data. The link between parameters of the fin and a tag model allows redesign of tag models according to expected changes of fin geometry (difference in fin shape related with species, sex, and age peculiarities, simulation of the bend of the fin during manoeuvres). A final virtual modelling stage uses iterative improvement of a tag model in a computer fluid dynamics (CFD) environment to enhance tag performance. This new method is considered as a suitable tool of tag design before creation of the physical model of a tag and testing with conventional wind/water tunnel technique. Ultimately, tag materials are selected to conform to the conditions identified by the modelling process and thus help create a physical model of a tag, which should minimise its impact on the animal carrier and thus increase the reliability and quality of the data obtained. r 2007 Elsevier Ltd. All rights reserved. Keywords: Tag; Marine mammals; Modelling; Drag; Hydrodynamics
1. Introduction Loggers (see e.g., Naito, 2004) and radio-telemetry are both powerful methodologies for studying cetacean biology. Cooke et al. (2004) review some of Corresponding author. Tel.: +38 0652 483 122; fax: +38 0652 272 092. E-mail address: [email protected] (V.V. Pavlov).
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.11.010
the sensory technologies used in animal logging today, many of which are applicable to studies of cetacean biology. These include systems to study, seasonal and diurnal migrations, physiology (such as body temperature, rate of breathing, dive duration) and the physical parameters of the environment (such as depth, water temperature, etc.) (Fedak, 2004). In fact, to date, diving behaviour and movements studies have been carried out on
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representatives of at least eight different dolphin genera on species of greatly-differing size, exploiting open-sea and coastal zones, marine and freshwater areas and with widely differing swim speeds and dive depths, e.g., bottlenose dolphins (Tursiops truncatus), Atlantic spotted dolphins (Lagenorhynchus acutus), narwhals (Monodon monoceros), long-finned pilot whales (Globiocephala melas), Dall’s porpoises (Phocoenoides dalli), belugas (Delphinapterus leucas), killer whales (Orcinus orca), and Amazon River dolphins (Inia geoffrensis) (Mate et al., 1995; Baird and Dill, 1995; Davis et al., 1996; Hanson and Baird, 1998; Martin and da Silva, 1998; Baird et al., 2001; Dietz et al., 2001; Bloch et al., 2003; Litovka et al., 2004). Virtually all loggers used on dolphins include a pressure transducer, while some have speed sensors and a salt-water switch. More sophisticated tags may incorporate conductivity temperature depth loggers, such as those used on belugas to derive oceanographic data (Fedak, 2004). Tags may be broadly divided into satellite-linked recorders that transfer logged information via satellites to the researchers, and loggers that store saved information indefinitely. These latter are typically released from animals after certain period or recovered when the animals are recaptured. Currently, the main method of tag attachment uses bolts and pins drilled through the dorsal fin (Mate et al., 1995; Davis et al., 1996; Read and Westgate, 1997), although some researchers use suction cups (Hanson and Baird, 1998; Schneider et al., 1998; Baird et al., 2001) to attach units to different places on the body. In addition, the tag may be sewn to the animal’s back in finless species, such as the beluga (Litovka, personal communication). Thus, it would appear that, although cetacean tags have progressed remarkably with regard to microelectronics and miniaturisation, only a few have been attached with due consideration of drag or animal discomfort, particularly with regard to the extra pressure loads on dorsal fin tissue (Hanson, 2001). Extra drag increases the energy expenditure of swimming (Bannasch et al., 1994; Culik et al., 1994) and compromises swimming capacity and manoeuvrability. This has knock-on effects on the efficiency of foraging (see e.g., Wilson et al., 2004 for a simulation on penguins) and may have serious consequences for the evasion of predators. The extra loads on the dorsal fin tissues may result in necrosis and premature loss of tags due to tissue tearing (Geraci and Smith, 1990).
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In this work, we address the problem of inappropriately constructed tags and consider options to derive a system that is minimally small and maximally functional while causing minimal discomfort, negligible drag and not being subject to fouling problems. For this, we took measurements to define the 3D shape of the putative site of attachment (here the dorsal fin) and assumed that tags could be constructed with a specified volume, which can be essentially flexibly configured with regard to the outside dimensions of the units. Using a computer-aided design (CAD) approach, we produced a first tag design, which was then examined using software to determine how this unit affected water flow over the site. Areas of high drag were identified and the design modified in a series of iterative steps until the best configuration was achieved. In this work, we outline the basic principles in tag design considering the dolphin dorsal fin as the primary attachment site although the basic principles outlined here are applicable to other kinds of tags that differ in the place and method of attachment. 2. Methods 2.1. Measurements of the dorsal fin shape Measurements were made on the dorsal fins of two bottlenose dolphins (T. truncatus), five harbour porpoises (Phocoena phocoena), and three common dolphins (Delphinus delphis) by-caught or stranded on the Crimea coast between 1989 and 2003. Dorsal fins from all animals were in good condition, had no internal damage and corresponded to condition code #2 (Kuiten and Hartmann, 1991). The outline of the fin was photographed before being cut from the animals. The fin base was defined by the line between the point of maximum lateral curvature of the leading edge and the bottom of the fin’s trailing edge. Fin height was measured from the dorsal tip to the root chord of the fin. Fins were then removed from the body and fixed in 10% neutral formalin. For the study of fin geometry, a sampling scheme representing a two-dimensional mesh on the fin surface was used (Pavlov, 2003). Each data point was defined by two parameters (i and j). Eight crosssections of the fixed fin were made at equal intervals (i-parameter, i ¼ (1y8), Di ¼ fin span/8). Measurements were taken at 20 points located at equal intervals on the left side of the section (j-parameter, j ¼ (1y20), Dj ¼ chord/19). The section thickness
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was measured by using callipers at data points along lines orthogonal to the section chord. The precision of the reported values was 70.1 mm and based on three repeated measures. The bent part of fin crosssections near the trailing edge was straightened in some cases before section thickness measurements. 2.2. Dorsal fin mobility The estimation of the natural flexibility of the dorsal fin of different species was obtained by analysing film of the swimming and turning patterns of harbour porpoises and bottlenose dolphins in Harderwijk dolphinarium. The video analysis was made with Uleads VideoStudioTM software. Video frames of different appearance of the dorsal fin were extracted as 1.2 Mb Truecolor graphic files in bmp format and 748 564 pixels size. Selected files were processed by the morphological filters with ImagePros software by Media Cybernetics Inc. Extracted contours of dorsal fins were used for the estimation of both spanwise and chordwise deformation. The position of the fin tip as well as the trailing edge at the middle of the fin was marked on the images. The position of the fin base along the long axis of the body also was marked on the images. The angle formed by the inclined tip of the fin and vertical position of the fin base was measured. The inclination of the trailing edge in the middle of the fin from the long axis of the body was measured as well. Limited data obtained were used for the estimation of both spanwise and chordwise deformation of the dorsal fin. 2.3. Modelling of the dorsal fin and a tag On the basis of the measurements of fin crosssections thickness, outlines of the cross-sections were made using SolidWorkss software by SolidWorks Corp. Then, parametric solid models of fin using a 1:1 scale were created from the fin outline and the set of cross-section outlines. In model space, the X-axis corresponded to streamwise direction, the Y-axis corresponded to spanwise direction and Z-axis corresponded to the direction normal to the wing plane. Basic airfoil parameters, the leading edge radius (r), chord length (CL), maximum thickness (MT) and position of maximum thickness (PMT) with respect to leading edge were measured on the model cross-sections (Fig. 1). The aspect ratio was calculated as the fin span squared divided by the fin area (Webb, 1975).
Fig. 1. The basic airfoil parameters applied to the cross-section of the dorsal fin (r ¼ leading edge radius, CL ¼ chord length, MT ¼ maximum thickness, PMT ¼ position of maximum thickness).
Fig. 2. Combined model of the simplified dolphin body and the dorsal fin.
Additionally, a simplified model of the dolphin body 1.5 m length was constructed (Fig. 2). The model presents the upper half of the body without flippers, flukes and complex details (eyes, blowhole). The models of dorsal fin were combined with simplified body model in order to simulate natural conditions of the fin flow. The parametric model of a tag was created by means of a suite of solid modelling methods with SolidWorkss software by SolidWorks Corp (Fig. 3). A tag was constructed to be located on the middle of the fin, taking into account the basic airfoil parameters at this location, thus providing the best fit to the fin geometry. Tag shapes were fitted to the individual fin models by means of the links between the derived parameters of the fin and the models. 2.4. Computer fluid dynamics (CFD) test Hydrodynamic tests of the dorsal fin and tag models were carried out with the CFDCOSMOSFloWorksTM software.
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Compressibility effects: off Default wall conditions: adiabatic Fluid: water Thermodynamic parameters Pressure: 101325 Pa Temperature: 20.05 1C
Fig. 3. Parametric model of a tag. The left fore-part is shown semi-transparent to illustrate some tag components: electronic box, antenna and bolts.
Two test modifications were applied in this study. First was the test of the 3D models of the fin combined with the simplified model of the dolphin body. The goals of the test were to get a general pattern of the flow around dorsal fins among three species of dolphins and to estimate the distribution of pressure and velocity on the dorsal fin surface. Calculations of the distribution of the pressure and velocity on the dorsal fin were carried out at zero yaw and pitch angles and Reynolds number 1.13107. Second was the test of 2D cross-sections of the dorsal fin of a bottlenose dolphin alone and the dorsal fin with different tag configurations. The goals were to estimate the additional drag caused by the tag as well as to select the best tag configuration in terms of hydrodynamic performance. Two-dimensional calculations of the drag force and drag coefficient were carried out at zero yaw and pitch angles and Reynolds numbers that ranged between 2.63105 and 1.09106. Reynolds number was calculated as UL , v where U mean fluid velocity, L characteristic length, v 1.06106 m2 s1, the kinematic viscosity for sea water. To calculate the drag force as well as distribution of pressure and velocity on the dorsal fin the following settings were used: Re ¼
Analysis type: external Result resolution: high (level 6) Geometry resolution: 0.001 m Physical features Heat transfer in solid: off Time settings: off Gravitation settings: off
Velocity parameters X component of velocity: 2; 4; 6; 8 m s1 Y component of velocity: 0 m s1 Z component of velocity: 0 m s1 Turbulence parameters Turbulent intensity: 0.1% Turbulent length: 0.002 m Dimensionless drag coefficient Cd of the fin and tag cross-sections was calculated as Cd ¼
2F x , rU 2 S
where Fx is the X component of total drag force, r (1028 kg m3), the density of sea water at the specified temperature of 20 1C, U is velocity and S is cross-sectional area. 3. Results and discussion 3.1. Stage I. Study of the operational conditions The quickest way to gain a basic understanding of problems in tag design is to look into the operational conditions of a tag. It helps to establish what forces influence a tag, their range and spatiotemporal characteristics. It also allows an estimate of the negative effect of a tag on dolphin well being. Due to the peculiarities of dolphin skin with its high proliferative activity of the epidermis (Brown et al., 1983) resulting in a continual flaking-off of the superficial layer of epidermal cells (Sokolov et al., 1969), it would appear that nothing can be glued to the skin surface for a long time. Noninvasive suction cups attachment is usually used for short-term data acquisition (Hanson and Baird, 1998; Baird et al., 2001), whereas longer studies, such as those of migrations, require tags attached by bolts and pins drilled through the dorsal fin (Mate et al., 1995; Davis et al., 1996). Both tag types are influenced by the highly variable flow pattern of water round the dolphin’s body (Pershin, 1988; Romanenko, 1997; Fish and Rohr, 1999). Unlike
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machines, dolphins display non-steady swimming with accelerations interspaced by gliding. Dolphins have a relatively high turning performance (Fish, 1997) although little is known about the hydrodynamics of the manoeuvring dolphin. In tagged animals it is very likely that tag loads increase when dolphins glide, accelerate, and make cusp turns. In addition, re-entry to the water after the leaps (Hanson, 2001) as well as attacks by other dolphins may lead to particularly high loads on tags. Under these conditions a streamlined shape of a tag becomes particularly important. High drag can lead to premature loss of suction cups tags or to tissue reaction to pressure in the dorsal fin where bolts/ pins are used for attachment (Geraci and Smith, 1990; Geertsen et al., 2004). In addition, a wellstreamlined shape reduces the risk of fouling, in particular seaweed attachment to the lugs of a tag (Geertsen et al., 2004). In this paper, we are specifically focused on bolton tags although the basic principles of tag design are common for other attachment modalities. The material properties of a tag with bolt/pin attachment to the dorsal fins also must be matched to the dorsal fin tissue (Hanson, 2001). Thus, any appropriate dolphin dorsal fin tag also must show some bending to reduce both additional drag and extra load on the tissue around pins. It thus differs significantly from a tag attached to a rigid structure such as a sea turtle carapace (Watson and Granger, 1998). The optimal arrangement of the pins on the dorsal fin should be found based on the data of the fin structure (Pabst, 1996; Hanson, 2001; Pavlov, 2003) and fin flexure in order to avoid the extra loads that may appear during the cusp turn. 3.1.1. Dorsal fin shape The dorsal fin of dolphins is a multifunctional organ: it resists yawing and rolling motion and acts to prevent side slip during manoeuvres (Fish and Rohr, 1999). In addition, it acts as a ‘‘thermal window’’ to prevent overheating (Scholander and Schevill, 1955; Williams et al., 1999). The form of the dorsal fin outline depends on species, sex, age, and even varies between monosex, equal age individuals from one species. Despite this, general trends are apparent in fin outline representatives of different genera (Fig. 4). Among the species studied, the aspect ratio varied from 0.8 to 0.9 in harbour porpoises to 1.1–1.3 in bottlenose and common dolphins.
The cross-sections of the dorsal fin could be analysed by considering the basic airfoil parameters: chord length, maximum thickness of section, position of maximum thickness, and leading edge radius (Fig. 1). Unlike the fin outline, the shape of the cross-section is conservative, revealing only subtle variability. Indeed, a similar pattern of thickness distribution was found in all the species examined in this study (Fig. 5). Any interspecific difference in the pattern of the spanwise distribution of airfoil parameters was found to be related to fin size and outline. The wing-like shape of the dorsal fin (Lang, 1966; Pershin, 1975) is a result of adaptation to relatively high-speed swimming in seawater, which may explain the consistency of cross-section parameters on which there is substantial selection pressure not to exceed the strict morphological limits which lead to effective hydrodynamic function of the dorsal fin. 3.1.2. Dorsal fin mobility During a manoeuvre the dorsal fin can bend in chordwise and spanwise directions. The central core structure appears to decrease rigidity of the fin in these directions. The ligamentous layer appears to function to prevent excessive bending (Pavlov, 2003). Structurally limited bending of the fin during manoeuvre is apparently a natural limiter of lift and related induced drag (Fish, 1998). Although, we have few data on dolphin swimming in captivity, the films do allow for conceptualisation of the degree of natural deformation of the fin during manoeuvres. Our video analyses showed that dolphin dorsal fins can bend in both spanwise and chordwise directions. The upper part of the fin showed a greater spanwise
Fig. 4. The outline of the dorsal fin among the representatives of different genera of small cetaceans. The dash-and-dot, dot, dash, and solid line represent harbour porpoise, common dolphin, bottlenose dolphin and male pilot whale respectively. The pilot whale is added in order to show the trend in the outline of the dorsal fin.
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motion close to the water surface the trailing edge of the fin showed low-range oscillations, while the leading edge and the tip of the fin remained still. The most extensive dorsal fin deformation occurred when a cusp turn was made after slow swimming. In this case, both the tip of the fin and its trailing edge could bend, although, generally, the angle of inclination of the tip of the fin from the fin base did not exceed 601. Chordwise flexibility was most apparent in the middle of the fin. The angle of inclination of the trailing edge from the leading edge did not exceed 451. 3.1.3. Hydrodynamics of the dorsal fin model The dorsal fin of dolphins has pattern of flow close to that of an aircraft wing. Modelling the fin showed that the velocity has a steep positive gradient at the leading edge, which then decreases to the trailing edge. Furthermore, there is a difference in velocity distribution in the upper and lower parts of the fin and the velocity in the chordwise direction of the fin but more abruptly over the lower part. All the models of dorsal fin were shown to have an unstable flow near the trailing edge at the base of the fin. Not surprisingly, the leading edge of the fin was exposed to the highest pressure, with maximum values occurring at the base of the fin. The steep negative pressure gradient near the leading edge reverses in the low-pressure region; then pressure grows again to the trailing edge (Fig. 6). Over the upper part of the fin the pressure grows rapidly to the trailing edge, while the lower part has a more elongated low-pressure region.
Fig. 5. Spanwise distribution of the basic airfoil parameters of fin cross-sections (means7SD). (A) Maximum thickness, (B) position of the maximum thickness, (C) leading edge radius. Lines with diamond, squares and delta symbols correspond to the harbour porpoise (Phocoena phocoena), bottlenose dolphin (Tursiops truncatus) and common dolphin (Delphinus delphis), respectively.
flexibility in contrast to the base of the fin. The leading edge of the fins was rigid and flexibility appeared to increase from the position of maximum thickness to the trailing edge of the fin. The range of the natural deformation of the dorsal fin varied considerably and depended on the species, speed of swimming and turning radius. During fast rectilinear
Fig. 6. Chordwise distribution of the pressure. Cross-sections are taken at the middle of the dorsal fin of selected animals. Lines with diamond, squares and delta symbols correspond to the harbour porpoise, bottlenose dolphin and common dolphin respectively. Data are normalised from 0 to 1.
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3.2. Stage II. Parametric modelling of the dorsal fin and a tag The parametric model of the fin was created using the defined regularities in the fin shape. It included the pattern of spanwise distribution of the crosssection parameters: leading edge radius, maximum thickness, and position of the maximum thickness. Low variability of the pattern in the cross-section parameters between the species studied led us to assume interspecific consistency. Parametric representation of the outline and fin cross-sections of the fin was made. Links between the outline and basic cross-section parameters were provided, as was the fixed pattern of spanwise thickness distribution. It enabled the model shape to change automatically according to a new outline while maintaining the requisite pattern of thickness distribution. Such a feature allowed a new fin model to be constructed from the basic shape according to naturally differing situations, e.g., between species or sexual dimorphism. In an attempt to model the natural conditions, the parametric model of the fin can simulate the fin deformation during a manoeuvre. The shape of the parametric model can be changed according to our observed range of the fin flexibility both in spanwise and chordwise directions. The limited results we have do not yet allow precise analysis of the range of fin bend, and in order to improve tag performance, the relationship between natural deformation of the fin and speed of swimming and turning radius (incorporating differing species, sex, and age) should be studied in more detail. It was supposed that hydrodynamic performance of a tag can be optimised by the convergence of tag design parameters with parameters of fin crosssections. Following this supposition two tag models having the shape close to the fin crosssection were constructed at the initial phase of tag design. The first tag had paired side-mount design, while the second one had connected paired side-mount design, i.e. U-shape leaned on the leading edge of the fin (Fig. 7). Both tags had equal chord length and maximum thickness while they differed in position of maximum thickness and leading edge radius. Each design had three configurations (Table 1). At constant maximum thickness and chord length the position of maximum thickness varied from 26% chord length to 44% chord length in Tag I and from 18% chord length to 35% chord length in Tag II. Besides, leading edge
Fig. 7. Cross-sections of Tag I and Tag II models (r ¼ leading edge radius, CL ¼ chord length, MT ¼ maximum thickness, PMT ¼ position of maximum thickness).
Table 1 Parameters of Tag I and Tag II configurations
Tag Tag Tag Tag Tag Tag
I config 1 I config 2 I config 3 II config 1 II config 2 II config 3
CL (m)
MT (% Cl)
PMT (% Cl)
r (% CL)
Area (m2)
0.114 0.114 0.114 0.114 0.114 0.114
29.9 29.9 29.9 29.9 29.9 29.9
26.3 35.1 43.9 17.6 26.3 35.1
8.4 6.3 5.1 6.3 4.2 3.2
0.0014 0.0014 0.0014 0.0013 0.0013 0.0013
r ¼ leading edge radius, CL ¼ chord length, MT ¼ maximum thickness, PMT ¼ position of maximum thickness.
radius varied from 5.1% chord length to 8.4% chord length in Tag I and from 3.2% chord length to 6.3% chord length in Tag II. The alteration of leading edge radius and position of maximum thickness of the tags was made in CAD–CFD cycle in order to examine the difference in Tag I and II hydrodynamic performance as well as to reveal the best tag configuration possessing the lowest additional drag. The final parametric model of the tag as example of a simple satellite-linked tag was constructed. The model based on Tag I configuration 3, which had the best performance in terms of additional drag. In keeping with conventional studies, it included a set of sensors, a radio transmitter, batteries and an antenna. The fore-part of a tag was constructed to be rigid while the rear part is flexible.
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The place of attachment of the fore-part of a tag by the bolts to the fore-part of the fin was chosen taking into account the assumed lowest flexibility of the fin at this area. The attachment of the inflexible fore-part of a tag by the bolts at this area should prevent from appearance of differential forces on the bolts causing them to migrate or fail early. The fore-part of a tag is intended to contain the electronic components of a tag, while the rear part is assigned to simulate the trailing edge behaviour to minimise the induced drag during the manoeuvre. Links between the tag and fin model parameters provided the best fit of a tag to the dorsal fin shape as well as control of the changeable shape of a tag. The tag model was able to change its shape according to the bend of the fin model simulating the natural flexibility (Fig. 8). The estimated range of the deformation of a tag model can be used in the process of selection of properties of the composite material for the tag. It is worth noting here that there is a huge range of inert, stable silastic materials, which can be moulded with a precise degree of softness, according to the addition of specific softeners, which would lend themselves as a suitable substrate for a fin tag. Therefore, the mechanical properties of the flexible rear part of the tag can be selected to reach the same degree of the deformation under the water flow as the trailing edge of the dorsal fin of dolphin (Fig. 8). The perfect tag should possess minimal impact on the animal behaviour as well as phenomenon studied. Moreover, longer tag functioning duration has become important for data collection. Apparently
Fig. 8. Simulation of the dorsal fin bend during the manoeuver, top view. Note the bend of the rear end of a tag according to the bent trailing edge of the fin.
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conventional tag design should be developed as a compromise between these conflicting requirements. Small cetaceans have a variety of behaviour patterns that imply different swimming modes, with a range of static and dynamic loads on the dorsal fin and the attached tag. Consequently, the optimisation of tag design should be not only for the certain range of the velocity inherent for the selected species but also for the scenarios that lead to the extra loads on the tag. The practice of dolphin tagging shows the examples of the operational conditions that increase the risk of premature loss of a tag. As found in exploitation of the pin-attached tags for dolphin telemetry, many tags have the vertical migration component that is most likely from frequent re-entry loading through the airwater interface as the animal dives after breathing (Hanson, 2001). 3.3. Stage III. Step-by-step improvement of a tag model As it is difficult to produce an optimal tag design in one attempt, tag performance was improved iteratively. The tag geometry constructed in a CAD environment was tested in the CFD environment in order to reveal the imperfections in tag construction. The CFD test revealed those features of tag design that were responsible for highest drag. Subsequently, those parts were redesigned and the procedure of the CFD test was repeated. The cycle CAD construction–CFD test was repeated until an acceptable hydrodynamics performance of a tag model was achieved. The criterion of hydrodynamic performance of the model was the level of additional drag caused by a tag. Attachment of a tag to the fin led to an increase in drag force that ranged between 0.29 and 6.46 N over all simulated velocities (Fig. 9). It was found that at constant length and thickness of the tag, the lowest additional drag can be reached by altering the leading edge radius and position of maximum thickness parameters. Generally, the Cd decreased from low to high Reynolds number (Fig. 10). For the assumed cruising speed of swimming 2 m s1 the combined decrease in leading edge radius and increase in position of maximum thickness lead to decreasing of Cd from 0.097 to 0.044 for Tag I and from 0.075 to 0.053 for Tag II. For the assumed burst speed of swimming 8 m s1 the same changes in tag configuration decreased Cd from 0.051 to 0.041 for Tag I and from 0.063 to 0.049 for Tag II.
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Fig. 9. Drag force plotted against simulated velocity for a fin cross-section as well as all Tag I and II configurations.
placement of tag components. Besides, the load produced by the bolts on the dorsal fin tissue during horizontal swimming may be decreased by the redistribution of the part of the load to the leading edge of the fin. Only rectilinear swimming patterns with a range of constant velocities at zero yaw and pitch angle were chosen for the CFD test of the dorsal fin and attached tag. These onerous conditions are likely to be most detrimental to the swimming dolphin and it is here where effects are most obvious. Ideally, though, CFD tests should be performed using the full range of dolphin operational conditions, i.e. dolphin manoeuvres, accelerations, the tag reentering the water after the dolphin surfaces to breathe, etc. The initial tag design was examined across CAD–CFD cycle and an optimal tag configuration was chosen as a basis for the following development of tag design. Unlike trial methods, this procedure offers a suitable tool for the verification of engineering ideas at the initial stages of tag design before wind/water tunnel testing. The results obtained can help in the choice of how to compromise between the demands of placement of a tag components and hydrodynamic performance of a tag. It should be noted, nevertheless, that the conventional wind/water tunnel technique provides the exact values of additional drag caused by the tag. It is reasonable therefore to carry out the final wind/water tunnel testing of a tag model to get real data of a tag performance. 3.4. The outline of a new approach to tag design in dolphin telemetry
Fig. 10. Coefficient of drag (Cd) plotted against Reynolds number for (A) the fin cross-section and Tag I configurations and (B) the fin cross-section and Tag II configurations.
The basic steps of the new approach to tag design in dolphin telemetry are:
Configuration 3 of Tag I had best performance in terms of additional drag. The backward-shifted position of maximum thickness (43.9% of chord length) and small leading edge radius (5.1% of chord length) provided the best performance for the selected range of velocities. The value of position of maximum thickness E 44% chord length is close to that of the ‘‘laminar’’ airfoils although this parameter recalculated with respect to the total length of fin cross-section plus tag is 34.6%. Generally, the U-shaped Tag I model looks preferable due to low level of additional drag and increased space for the
1. Study of the operational conditions of a tag. Quantification of the parameters of the fin shape, hydrodynamics and mobility. 2. Parametric modelling of the fin shape and a tag using the parameters obtained for the operational conditions. 3. Fitting a parametric model of the fin to the expected fin shape according to species. 4. Optimising a tag model to the expected fin shape. 5. Iterative improvement of the performance of a species-specific tag model in a CFD environment. The primary criterion for tag performance is minimised additional drag.
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6. Selection of a tag materials and creation of the physical model of a tag optimised to the selected species. 7. Wind/water tunnel test of the physical models of a tag. 8. Producing of a species-specific tag with high hydrodynamic performance. The known restrictions need to be taken into account in tag design. These are a volume-based drag factor, the risk of impact, the ability to dump heat in tags covering a large proportion of the dorsal fin, as well as the problems related with stability of tag attachment, namely tissue reaction on tag pins as well as static and dynamic loads generated by the tag (Geraci and Smith, 1990; Hanson, 2001). Considering the dorsal fin as usual place of tag attachment, it is preferable to outline the desired properties of the tag: the hydrodynamic design of a tag should affect the natural pattern of water flow over the fin minimally. A tag should produce minimal loads on the fin tissue within the range of swimming patterns, i.e. ranging, diving, accelerations, manoeuvring, as well as the tag reentering the water after the dolphin surfaces. To this end, the shape of a tag should differ minimally from the cross-sectional shape of the dorsal fin. Additionally, the mechanical properties of a tag should correspond well with those of the fin and not prevent any natural bending. Along with this, the U-shape of a tag, with the tag leaning on the leading edge, should minimise load on the fin tissues caused by bolts or pins if used. Overall, minimising the negative effect of attached tags on dolphin swimming should increase the reliability and quality of the data obtained. Acknowledgements The authors thank Harderwijk Dolphinarium, The Netherlands, for the video of swimming dolphins. They also thank Brad Hanson and an anonymous reviewer for the valuable comments to the manuscript.
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