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2003 Hydrodynamic Stability of Swimming in Ostraciid Fishes: Role of the Carapace in the Smooth Trunkfish Triqueter (Teleostei : ) Ian K. Bartol Old Dominion University, [email protected]

Morteza Gharib

Daniel Weihs

Paul W. Webb

Jay R. Hove

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Repository Citation Bartol, Ian K.; Gharib, Morteza; Weihs, Daniel; Webb, Paul W.; Hove, Jay R.; and Gordon, Malcolm S., "Hydrodynamic Stability of Swimming in Ostraciid Fishes: Role of the Carapace in the Smooth Trunkfish Lactophrys Triqueter (Teleostei : Ostraciidae)" (2003). Biological Sciences Faculty Publications. 206. https://digitalcommons.odu.edu/biology_fac_pubs/206 Original Publication Citation Bartol, I. K., Gharib, M., Weihs, D., Webb, P. W., Hove, J. R., & Gordon, M. S. (2003). Hydrodynamic stability of swimming in ostraciid fishes: Role of the carapace in the smooth trunkfish Lactophrys triqueter (Teleostei : Ostraciidae). Journal of Experimental Biology, 206(4), 725-744. doi:10.1242/jeb.00137

This Article is brought to you for free and open access by the Biological Sciences at ODU Digital Commons. It has been accepted for inclusion in Biological Sciences Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contact [email protected]. Authors Ian K. Bartol, Morteza Gharib, Daniel Weihs, Paul W. Webb, Jay R. Hove, and Malcolm S. Gordon

This article is available at ODU Digital Commons: https://digitalcommons.odu.edu/biology_fac_pubs/206 The Journal of Experimental Biology 206, 725-744 725 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00137

Hydrodynamic stability of swimming in ostraciid fishes: role of the carapace in the smooth trunkfish Lactophrys triqueter (Teleostei: Ostraciidae) Ian K. Bartol1,*, Morteza Gharib2, Daniel Weihs3, Paul W. Webb4, Jay R. Hove2 and Malcolm S. Gordon1 1Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, CA 91606, USA, 2Options of Bioengineering and Aeronautics, California Institute of Technology, Pasadena, CA 91125, USA, 3Department of Aerospace Engineering, Technion, Haifa, 3200, Israel and 4School of Natural Resources and Department of Biology, University of Michigan, Ann Arbor, MI 48109, USA *Author for correspondence (e-mail: [email protected]) Accepted 11 November 2002

Summary The hydrodynamic bases for the stability of locomotory yawing angles of attack resulted in prominent dorsal and motions in fishes are poorly understood, even for those ventral vortices developing at far-field locations of the fishes, such as the rigid-bodied smooth trunkfish carapace; far-field vortices intensified posteriorly and as Lactophrys triqueter, that exhibit unusually small angles of attack deviated from 0°. Pressure distribution amplitude recoil movements during rectilinear swimming. results were consistent with the DPIV findings, with areas We have studied the role played by the bony carapace of of low pressure correlating well with regions of attached, the smooth trunkfish in generating trimming forces that concentrated vorticity. Lift coefficients of boxfish models self-correct for instabilities. The flow patterns, forces and were similar to lift coefficients of delta wings, devices that moments on and around anatomically exact, smooth also generate lift through vortex generation. Furthermore, trunkfish models positioned at both pitching and yawing nose-down and nose-up pitching moments about the center angles of attack were investigated using three methods: of mass were detected at positive and negative pitching digital particle image velocimetry (DPIV), pressure angles of attack, respectively. The three complementary distribution measurements, and force balance experimental approaches all indicate that the carapace of measurements. Models positioned at various pitching the smooth trunkfish effectively generates self-correcting angles of attack within a flow tunnel produced well- forces for pitching and yawing motions – a characteristic developed counter-rotating vortices along the ventro- that is advantageous for the highly variable velocity fields lateral keels. The vortices developed first at the anterior experienced by trunkfish in their complex aquatic edges of the ventro-lateral keels, grew posteriorly along environment. All important morphological features of the the carapace, and reached maximum circulation at the carapace contribute to producing the hydrodynamic posterior edge of the carapace. The vortical flow increased stability of swimming trajectories in this . in strength as pitching angles of attack deviated from 0°, and was located above the keels at positive angles of attack Key words: boxfish, Lactophrys triqueter, stability, hydrodynamics, and below them at negative angles of attack. Variation of swimming, pressure, particle image velocimetry, moment.

Introduction Hydrodynamic stability, as applied to moving, aquatic boulders) in origin. Self-generated perturbations may involve systems, may be best defined as the control and buoyancy forces, gill ventilation and locomotor movements. regulation of trajectories during steady and intentional Hydrodynamic stability control arises from flow over the body unsteady motions of maneuvers, such that the body resists and appendages and is achieved by trimming and powered movement from an original state, returns readily to an original correction systems. In trimming correction systems, control state, or maintains a desired path after and/or while being surfaces (effectors) move with the body and are positioned to affected by external and self-generated perturbations (Weihs, self-correct a disturbance and/or passively damp the rate of 1993, 2002; Webb, 2000, 2002). In the water column, a growth of perturbations (e.g. paired fins extended at a positive common external perturbation is turbulence, which may be dihedral angle to self-correct for rolling perturbations) biotic (e.g. wakes of other fishes) or abiotic (e.g. flow at (Aleyev, 1977; Weihs, 1989, 1993; Webb, 2002). Powered interfaces and over topographic features like coral and correction systems, as the name suggests, involve active 726 I. K. Bartol and others movement of the effectors independent of the body to correct in maintaining this stability. We were particularly interested in disturbances (e.g. asymmetrical pectoral fin beats to correct a trimming self-correcting forces for pitching and yawing. rolling moment) (Webb, 2000, 2002). We approached this objective using three separate but Powered correction systems are used by many fishes under a interrelated techniques. For each approach, an anatomically multitude of conditions to regulate trajectories. For powered exact, stereolithographic model of the smooth trunkfish was systems, neural processing is required before the effectors make used. The three techniques applied were: (1) digital particle corrections and may ultimately be the limiting factor for actively image velocimetry (DPIV), which provides a global picture regulating trajectories (Webb, 2000). Many disturbances, of flow structures at various planes along the carapace; especially those that are external, have low predictability in (2) pressure distribution measurements, which provide terms of direction, period or amplitude. Under these conditions, information on what is happening at the surface of the accurate phasing of correction forces with perturbations is carapace, a region that is difficult to resolve using DPIV; and difficult. Without proper phasing, correction may even amplify (3) force balance measurements, which provide an integrated the disturbance through ‘pilot-induced error’ (Webb, 1998, view of forces acting on the carapace. 2000). Trimming self-correcting systems, on the other hand, can accommodate these unpredictable events more effectively, since response latency periods are not required. Consequently, fish Materials and methods relying more heavily on trimming control seemingly are better Model construction adapted to more turbulent regimes than fish emphasizing One Lactophrys triqueter L. (17.0 cm total length, 128.8 g) powered-control systems. Support for this hypothesis is was captured by hook and line in Puerto Rico, frozen provided by Webb (1998), who discovered that the smallmouth immediately, and shipped to the University of California, Los bass Micropterus dolomieu, a fish that relies heavily on powered Angeles (UCLA), where it was stored in a –70°C freezer until control, has greater difficulty holding station in the wake of it was prepared for computerised tomography (CT) scanning. cylinders than the river chub Nocomis micropogon, a fish that This specimen was in excellent condition and had carapace relies heavily on self-correcting trimming control. features that closely resembled those of smooth trunkfish Whether using trimming or powered hydrodynamic systems, considered in other studies conducted in our laboratory. controlling trajectories and avoiding growth of disturbances Preparation protocol involved removing the dorsal and anal into energy-wasting displacements is essential to organisms for fins, positioning the pectoral fins flush against the body, and effective and economical swimming. However, surprisingly aligning the caudal fin with the longitudinal axis of the fish. little is known about stability of locomotory motions in fishes Once the exterior of the carapace was thawed completely, the and other aquatic organisms, as noted by Blake (1981), Weihs specimen was dried off with a towel and placed on a low- (1993, 2002) and Webb (2000, 2002). An intriguing fish that density block of Styrofoam (floral frog). The Styrofoam block resides in a highly turbulent environment and may take elevated the fish above the scanner bed during CT scanning, advantage of both trimming and powered control is the smooth which made it easier to distinguish ventral portions of the trunkfish Lactophrys triqueter. The smooth trunkfish is a neo- carapace from the bed in cross-sectional exposures. tropical reef-dwelling fish that has a rigid bony carapace The specimen was scanned using a GE CT/i high-speed consisting of hexagonal plates (or scutes), which encases about scanner (General Electric Medical Systems, Milwaukee, WI, 70–75% of its body (Tyler, 1980). The carapace, a feature that USA). A continuous scan was performed through the entire distinguishes it from most flexible-bodied marine fishes, is fish using a 1.0 mm collimation at 120 kVp/200 mA–1 and a 1 s predominantly triangular in cross-section, with one dorsal rotation time. A 50% overlap was used so that images were and two prominent ventro-lateral keels (Fig. 1). In smooth reconstructed every 0.5 mm. A total of 340 consecutive, two- trunkfishes and other marine boxfishes (Teleostei: dimensional (2-D), cross-sectional exposures were generated Ostraciidae), the carapace limits body movements to locations longitudinally from the snout to the posterior edge of the posterior to the caudal peduncle. As a result, these fishes rely caudal fin. An appropriate threshold (contrast) was selected for heavily on complex combinations of movement of their five maximum resolution of the carapace, and the images were fins for swimming. converted to digital stereolithography (STL) files using a Field observations and recent studies on the swimming Marching Cubes algorithm, a high-resolution 3-D surface physiology of boxfishes indicate that they are capable of construction algorithm (Lorensen and Cline, 1987). Exact 3-D remarkably low recoil motions, both when moving in flumes physical models of polymerized epoxy resin, one in halves and in highly variable velocity fields in nature, resulting in and the other whole, were created from the files using smooth, energy-efficient, rectilinear swimming trajectories stereolithographic rapid-prototyping (Solid Concepts, Inc., (Gordon et al., 2000; Hove et al., 2001). Although there are Valencia, CA, USA). some studies describing and analyzing swimming in rigid- bodied ostraciiform fishes (Blake, 1977, 1981, 1983a,b; Hove Measurements of CT images et al., 2001), little is known about how this remarkable stability A variety of morphological measurements were collected is achieved. The objective of our study was to understand what from the 2-D, cross-sectional CT images to characterize the role the uniquely shaped carapace of the smooth trunkfish plays unique shape of the carapace. Before measurements were The carapace and swimming stability in trunkfish 727 taken, the CT images were converted from a UCLA PACS in- 0.02 ms and separated by a period of 1 ms during paired image house format to a TIFF format. Measurements were collected collection. At least 60 sequential images (30 paired images) from every fourth image using NIH Image v. 1.61 (National were considered for processing for each trial. In all images Institute of Health, http://rsb.info.nih.gov/nih-image/). For except those collected in the wake, the model was subtracted each CT image considered, carapace width (distance from out of the video image prior to processing. The interrogation ventro-lateral keel tip to ventro-lateral keel tip), carapace window (i.e. the area over which the particle shifts were height (distance from dorsal keel tip to midline of ventral averaged) was 322 pixels with a 16 pixel offset (50% overlap). region), eye ridge width (distance from eye ridge tip to edge The displacement of particles within the interrogation windows ridge tip), dorsal keel angle (determined using the apex of the comprising the paired images was calculated using cross- keel (vertex) and the two widest points on either side of correlation (Willert and Gharib, 1991). Outliers, defined as keel apex), and both ventro-lateral keel angles (determined particle shifts that were 3 or more pixels greater than particle similarly to dorsal keel angles) were measured along the length shifts of neighboring particles, were removed and corrected, of the carapace (Fig. 1). To characterize levels of lateral and and data were smoothed to remove high frequency fluctuations ventral concavity/convexity, a segment connecting the apex of within the video images. each ventro-lateral keel and the apex of the dorsal keel (SL) The resulting flow displacement fields were divided by the and a segment connecting the apices of the ventro-lateral keels time difference between the paired images to determine the (SV), respectively, were drawn on all CT images considered velocity field of the flow. Mean velocity and mean vorticity using NIH image. The maximum distance(s) the body indented fields were determined for each trial using FlowVisionTM and/or extended relative to the segment and location(s) of software. Circulation of regions of concentrated vorticity maximum indentation/extension along the segment were attached to the model was determined by carefully tracing recorded. along the outline of the model where the region of concentrated vorticity was present, tracing along the remaining vorticity Digital particle image velocimetry contour found external to the model, and then computing the Willert and Gharib (1991) and Raffel et al. (1998) provide area integral within the tracing using an in-house code at the detailed descriptions of the DPIV technique for flow field California Institute of Technology. Circulation of vortices in measurements. A general description of the experimental the wake was determined by tracing along the outlines of each techniques relevant to our study is given here. To reduce glare region of concentrated vorticity and computing the area from laser light, one of the two fabricated models was painted integral of the tracing. black, which did not obscure the hexagonal scute topography DPIV measurements were collected at transverse planes at the surface of the carapace. For DPIV measurements made along the length and within the wake of the model as it was along the body, the tail of the model was removed and replaced positioned at various pitching angles of attack in the water with a 10 cm rod (also painted black and 1.0 cm in diameter, tunnel. The water tunnel was set at a speed of 44 cm s–1 which was similar in dimension to the caudal peduncle). The (2.6 BL s–1), where BL=body length, which corresponds to rod was connected to a sting that entered the water tunnel from roughly half the sustained speed range of boxfish (J. Hove, above. For DPIV measurements in the wake, the tail was unpublished swim tunnel data). The water tunnel was also set reattached, and the sting was affixed to a machined connector at low speeds (10 cm s–1, 0.6 BL s–1) and high speeds located at the caudal peduncle. We used a water tunnel with a (90 cm s–1, 5.3 BL s–1) periodically to confirm that DPIV results 30 cm×30 cm×100 cm test section (Model 503, Engineering at the intermediate speed were qualitatively applicable over the Laboratory Design, Inc., Lake City, MN, USA) seeded with range of swimming speeds of the fish. Data were collected at silver-coated hollow glass spheres (14 µm in diameter). Two 2° intervals between –10 and +10°, and at 5° intervals at more pulsed ND:YAG lasers (wavelength=532 nm, power rating negative and positive angles of attack, up to –30 and +30°, 50 mJ; New Wave Research, Fremont, CA, USA), a series of respectively. At all pitching angles of attack, flow patterns front-surface mirrors, and a cylindrical lens were used to were examined in the wake of the model (5 cm downstream generate and align an illuminated sheet approximately 1.0 mm from the caudal fin) and at five locations along the body of the thick. The laser sheet was projected beneath the water tunnel model: the eye ridge, the point of maximum girth, the midpoint in a transverse (YZ) plane. A Pulnix CCD video camera (TM- between the point of maximum girth and the posterior edge of 9701) with a frame size of 480×768 pixels and frame rate of the carapace, the posterior edge of the carapace and the caudal 30 Hz was positioned downstream of the working section peduncle. Several (2–3) trials were performed at each location (unobstructed views of oncoming flow were made possible at each pitching angle of attack. The model was oriented through a Plexiglas exit tank). sideways, upside-down, and right-side-up in the water tunnel The video camera, lasers and a Coreco OC-TC10-DIGSE during the various angles of attack to ensure that no zones frame grabber (National Instruments, Inc., Austin, TX, USA) around the carapace were ignored because of shadowing. The were synchronized using a video timing box and FlowVisionTM upside-down orientation worked best for positive pitching software (PixelFlow, Pasadena, CA, USA). The video camera angles, while the sideways orientation worked best for negative generated timing signals for phase-locking the frame grabber, pitching angles. Lift on the carapace was computed at each lasers and camera. Generally, the two lasers were pulsed for angle of attack considered using a 2-D airfoil lift equation 728 I. K. Bartol and others

–2 (Tritton, 1998): Ls=–ρWuΓ, where Ls is lift per unit span USA). Static pressure (Nm ) at each of the 36 ports was –2 –3 –2 (kg s ), ρW is freshwater density (kg m ), u is flow speed expressed relative to static pressure (Nm ) at a tunnel wall (m s–1), and Γ is mean circulation of each vortex comprising port. the pair in the wake of the model (m2 s–1). These vortices Data were collected while each model was positioned at 2° represent both the linear and non-linear contributions to slender intervals for pitching angles of attack of –30 to +30° and wing-body lift (Küchemann, 1978). To obtain total lift L, Ls yawing angles of attack of 0 to 30° (for yawing angles of attack was multiplied by the span of the ventral region of the boxfish the pressure ports were both exposed to flow (i.e. positioned in model. the near field carapace side) and shielded from flow (i.e. Additional DPIV studies were performed at transverse positioned in the far field carapace side) for each of the angles planes along the length of the model as it was positioned at of attack). For the angles considered, data were acquired at various yawing angles of attack (0 to +30°) in the water tunnel 100 Hz for 10 s using LabVIEW software (National set at the same speed described above. Data were collected at Instruments, Inc.). Wind tunnel speed was set according to the yawing angles of attack of 0, 10, 20 and 30° at the following Reynolds number considered in water tunnel trials. Pressure locations: the eye ridge, the point of maximum girth, the coefficients (CP) were calculated by dividing the pressure –2 2 midpoint between the point of maximum girth and the posterior difference above (Nm ) by dynamic pressure [ρAU /2; where –3 –1 edge of the carapace, the posterior edge of the carapace, and ρA is air density (kg m ) and U is wind speed (m s )], caudal peduncle. Several (2–3) trials were performed at each determined using a Pitot-static tube positioned in the wind location at each yawing angle of attack. As was the case for tunnel. pitching studies, the models were positioned in different orientations to ensure that no shadowed region was Force measurements overlooked. The upside-down orientation worked best for The smooth trunkfish model used in DPIV experiments was yawing studies. also used in force measurement experiments. The model was mounted caudally to a sting in a water tunnel with a test section Pressure measurements 61 cm×46 cm×244 cm in dimension. Force measurements were The second smooth trunkfish model, constructed in halves collected using three Interface 2.27 kg strain gauge load cells but identical in dimensions to that employed in DPIV (Interface, Inc. Scottsdale, AZ, USA) [two load cells measured experiments, was used for pressure experiments. One of the forces normal to flow (lift), one load cell measured forces model halves was hollowed out, and 36 holes were drilled in parallel to flow (drag)] connected to an in-house force balance lateral and ventral regions of the carapace. The holes were (Lisoski, 1993). Data output from the load cells were amplified drilled along body transects similar to those examined in the using three Interface SGA amplifiers/conditioners and DPIV studies. Urethane tubing (i.d. 0.068 cm, o.d. 0.129 cm) recorded with a Dash 8 Series data recorder (Astro-Med, Inc.). was inserted into the holes and glued in place so that the tubing Data were collected at 200 Hz for 10 s for each angle was flush with the model surface. The two model halves were considered. As was the case for pressure experiments, force glued together. Tubing exited the model through a 1.0 cm data were collected every 2° from pitching angles of attack of diameter, 10 cm long rod attached to the posterior section of –30 to +30° (some additional angles <–30° and >+30° were the model. The rod was used to mount the model to a sting in also considered). Flow speed during trials was identical to that a 61 cm wind tunnel (model 407, Engineering Laboratory considered in DPIV experiments [44 cm s–1 (2.6 BL s–1)]. Design, Inc., Lake City, MN, USA). Tubing exiting the model Coefficients of drag (CD) and lift (CL) were calculated 2 was connected to a Scanivalve 48-channel rotating pressure using steady-state equations [CD=2×D/(ρW×Af×u ) or 2 scanner (Scanivalve Inc., Liberty Lake, WA, USA) and a CL=2×L/(ρW×Ap×u ), where D is total drag (N), L is total lift –3 barocel electric manometer and capacitative differential (N), ρW is water density (kg m ), Af is maximum frontal area 2 pressure sensor (Barocel Datametrics, Wilmington, MA, (m ), Ap is planform surface area of the ventral region of

Fig. 1. Anterior, posterior and lateral views of a smooth trunkfish Lactophrys triqueter. Scale bars, 1 cm. The carapace and swimming stability in trunkfish 729 the carapace (m2), and u is water tunnel speed (m s–1)]. broad (140°) and rounded, but then sharpened posteriorly down Pitching moments (M) about the center of mass of the to an angle of 24° at a position 82% CL. At locations >82% CL, models were computed using the equation: ventro-lateral keel angles increased, reaching a maximum M=d1(L2–L1)+(L1+L2)×(d2cosα)+D(d2sinα), where d1 is angle of 54° at the posterior edge of the carapace. The dorsal distance (m) between load cell beams in the force balance, d2 keel was first noticeable at a location 27% CL, forming in the is distance (m) between center of force balance and center of middle of the eye ridge. At this location the dorsal keel was mass of model, L1 is lift (N) measured at load cell closest to 98°. The dorsal keel generally became sharper posteriorly until model, L2 is lift (N) measured at load cell farthest from model, approximately 69% CL, where the keel angle was 34°. In more D is drag (N) on the model, and α is angle of attack (degrees) posterior regions, the dorsal keel began to broaden again, of the model. Pitching moment coefficients (CM) were especially at the posterior edge of the carapace where the keel 2 computed using the equation: CM=2M/(ρWApcU ), where c is angle was 108° (Fig. 2). the chord length of carapace (m). Significant regions of convexity were present laterally along the carapace, especially in areas just posterior to the eye (25–40% CL) and near the edge of the carapace (>84% CL) Results (Fig. 3). Along a given dorso-ventral transect, maximum Measurements of CT images lateral convexity occurred 0.84–2.85 cm below the dorsal keel The carapace of the smooth trunkfish is predominantly (Figs 1, 3). Ventral convexity, which was always greatest triangular in cross-section, has no significant ornamentation, equidistant from the ventro-lateral keels along transverse and has 1 dorsal and 2 ventro-lateral keels (Fig. 1). Maximum transects, actually decreased in magnitude posteriorly until body width (6.4 cm) and maximum body height (7.1 cm) of the 71% CL, at and after which no significant convexity was scanned smooth trunkfish specimen occurred at locations 51% present (Figs 1, 3). Lateral concavity was present in locations and 48% of carapace length CL, respectively (37% and 35% from 38–92% CL. Along dorso-ventral transects, maximum of the total length TL of the fish, respectively) (Fig. 2). The eye lateral concavity occurred 0.38–1.23 cm above the ventro- ridge, which was fairly consistent in width (1.8–2.0 cm) at most lateral keels (Figs 1, 3). Ventral concavity was also present in of the transverse sections where it was present, began at a locations from 58 to 94% CL. Along dorso-ventral transects, location 14% CL and ended at 37% CL. Ventro-lateral keels maximum ventral concavity occurred 0.62–1.37 cm from the were first noticeable at a location 14% CL as well. At this ventro-lateral keels (Figs 1, 3). anterior region of the carapace, the ventro-lateral keels were Digital particle image velocimetry At positive pitching angles of attack, regions of 160 concentrated, discrete vorticity began to develop near the 7 anterior edges of the ventro-lateral keels at a longitudinal 140 6 location corresponding to the eye ridge (Fig. 4). These regions of concentrated vorticity intensified posteriorly along the 120 5 ventro-lateral keels, both in terms of peak vorticity and ) s circulation, until two well-developed, counter-rotating vortices ree

(cm) 100 g s 4 ultimately formed at the posterior edge of the carapace. The on

i vortices left the body completely at the caudal peduncle. Some s e (de n 80 l

3 g regions of concentrated, attached dorsal vorticity formed along n ime

A the body as well, especially around the eye ridge, where vortex D 60 2 circulation was stronger dorsally than ventrally at positive angles of attack <20°. However, peak dorsal vorticity and 1 40 dorsal circulation did not grow steadily along the body, and did not reach the circulation magnitudes observed in ventral areas 0 20 0 10 20 30 40 50 60 70 80 90 100 at the posterior edge of the carapace and at the caudal peduncle. Percentage of carapace length In the wake, dorsal and ventral vortices merged with the vorticity representing the linear lift, producing two well- Carapace width (cm) Mean ventro–lateral keel angle developed, counter-rotating vortices with a prominent Carapace height (cm) Dorsal keel angle downwash of flow between them (Fig. 4). Eye ridge width (cm) At negative pitching angles of attack, regions of Fig. 2. Characteristics of the carapace of a CT-scanned smooth concentrated, attached vorticity also began to develop around trunkfish (17.0 cm total length, 12.3 cm carapace length) plotted as a the ventro-lateral keels at a longitudinal location corresponding function of percentage of carapace length. Dimensions (carapace to the eye ridge (Fig. 5). As was the case for positive angles width, carapace height, and eye ridge width) are depicted on the left of attack, circulation and peak vorticity of ventral vortices y-axis, while mean ventro-lateral keel angle and dorsal keel angle are increased posteriorly, developing into two counter-rotating depicted on the right y-axis. vortices at the posterior edge of the carapace, and ultimately 730 I. K. Bartol and others leaving the body completely at the caudal peduncle. However, At pitching angles of attack close to 0°, the same general while regions of concentrated vorticity developed above the patterns of vortex development occurred: regions of ventro-lateral keels at positive pitching angles of attack, concentrated, attached vorticity formed at the anterior edges of regions of concentrated vorticity formed below the ventro- the ventro-lateral keels and grew in terms of peak vorticity and lateral keels at negative pitching angles of attack (Figs 4, 5). circulation posteriorly, while weaker regions of dorsal vorticity Furthermore, vortical flow rotation changed, depending on formed sporadically with no consistent antero-posterior angle of attack. When viewed from the rear, flow around the circulation growth (Fig. 7). However, circulation and peak right ventro-lateral keel was counterclockwise at positive vorticity of ventral regions of concentrated, attached vorticity angles of attack and clockwise at negative angles of attack (Fig. were lower at angles of attack near 0° than at more positive or 6). Some regions of weaker, concentrated, attached vorticity more negative angles of attack (Figs 6, 7). Irrespective of angle formed around the eye ridge in dorsal locations at negative of attack, circulation of attached ventral vortices was always angles of attack, but as was the case at positive angles of attack, greatest at the posterior edge of the carapace. Interestingly, at the dorsal vortices did not grow steadily along the body like an angle of attack of 0°, vortical flow rotation and position the ventral vortices. In the wake, two well-developed vortices resembled conditions at positive angles of attack. were present with a prominent upwash of flow between them When the carapace was positioned at yawing angles of (Fig. 5). attack, vortex development occurred adjacent to far-field

1.00 0.35 A C Ventral max. convexity 0.30 Lateral max. convexity ) ) 0.80 0.25 cm cm ( ( 0.20 0.60 0.15 m concavity m concavity m convexity m convexity 0.40

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axim 0.05 concavity M

M 0.20 0 Lateral max. concavity 0 –0.05 0 102030 40 50 60 70 80 90 100 0 102030 40 50 60 70 80 90 100

3.00 1.40 ) ) B D cm cm ( ( 1.20 2.50

1.00 2.00 m concavity m concavity m convexity m convexity u u 0.80 1.50 maxim maxim 0.60 f f

1.00 Lateral max. convexity 0.40 Lateral max. concavity Ventral max. concavity Location o Location o 0.50 0.20 0 102030 40 50 60 70 80 90 100 0 102030 40 50 60 70 80 90 100 Percentage of carapace length Percentage of carapace length Fig. 3. Concavity and convexity measurements of the carapace of a CT-scanned smooth trunkfish (17.0 cm total length, 12.3 cm carapace length) plotted as a function of percentage of carapace length. Maximum convexity (A) and concavity (C) are the maximum distances that the body extends and indents, respectively, relative to a segment connecting either the tips of the dorsal and ventro-lateral keels (SL) (lateral measurements) or the tips of the two ventro-lateral keels (SV) (ventral measurements). The location of maximum lateral convexity (B) is the distance from the dorsal keel to the point of maximum extension along the segment, SL. The location of maximum lateral/ventral concavity (D) is the distance from the ventro-lateral keel to the point of maximum indentation along segments SL (lateral) or SV (ventral). Data on lateral maximum convexity and concavity are mean measurements of the two sides of the carapace. Data on ventral concavity are mean measurements of the two regions of ventral concavity found along SV. In B the location of ventral maximum convexity occurs equidistant between ventro- lateral keels. The carapace and swimming stability in trunkfish 731 D Γ , respectively) are included V ω P and V Γ , respectively) and a ventral vortex ( D ω P and beneath the vorticity contour plots for measurements along body. In wake, dorsal and ventral distinctions are not necessary since dorsal vortices merge. The shadows beneath the models represent areas that were shielded from laser light. Mean circulation magnitude and mean peak vorticity values for a dorsal vortex ( Fig. 4. Velocity vector (B) and vorticity contour (C) fields around in the wake of smooth trunkfish model positioned at a pitching angle of attack +10°. The data are viewed in transverse planes at various locations along the body and wake. Each plot is the mean result of 30 velocity fields (1 representative trial). From left to right, locations (A) are: eye ridge, maximum girth, midpoint between girth and posterior edge of the carapace, caudal peduncle and wake. 732 I. K. Bartol and others , respectively) V ω P and V Γ , respectively) and a ventral vortex ( D ω P and D Γ are included beneath the vorticity contour plots for measurements along body. In wake, dorsal and ventral distinctions are not necessary since vortices merge. The shadows on the sides of or above models represent areas that were shielded from laser light. Mean circulation magnitude and mean peak vorticity values for a dorsal vortex ( Fig. 5. Velocity vector (B) and vorticity contour (C) fields around in the wake of smooth trunkfish model positioned at a pitching angle of attack –10°. The data are viewed in transverse planes at various locations along the body and wake. Each plot is the mean result of 30 velocity fields (1 representative trial). From left to right, locations (A) are: eye ridge, maximum girth, midpoint between girth and posterior edge of the carapace, caudal peduncle and wake. The carapace and swimming stability in trunkfish 733 V Γ , respectively) and a ventral vortex ( D ω P and D Γ , respectively) are included beneath the vorticity contour plots. V ω P and representative trial). The shadows underneath or to the side of models represent areas that were shielded from laser light. Mean circulation magnitude and mean peak vorticity magnitude values for a dorsal vortex ( Fig. 6. Velocity vector (B) and vorticity contour (C) fields around the posterior edge of the carapace of smooth trunkfish model positioned at pitching angles attack (left to right): +20°, +10°, 0°, –10° and –20°. The data are viewed in transverse planes, sampling locations are indicated (A). Each plot is the mean result of 30 velocity fields (1 734 I. K. Bartol and others , D ω P and D Γ , respectively) are included beneath the V ω P and V Γ vorticity contour plots for measurements along the body. In wake, dorsal and ventral distinctions are not necessary since ventral and dorsal vortices merge. respectively) and a ventral vortex ( underneath the model represent areas that were shielded from laser light. Mean circulation magnitude and mean peak vorticity values for a dorsal vortex ( Fig. 7. Velocity vector (B) and vorticity contour (C) fields around in the wake of smooth trunkfish model positioned at an angle of attack 0°. The data are viewed in transverse planes at various locations along the body and in wake. Each plot is mean result of 30 velocity fields (1 representative trial). From left to right, the locations (A) are: eye ridge, maximum girth, midpoint between girth and posterior edge of the carapace, posterior edge caudal peduncle and wake. The shadows The carapace and swimming stability in trunkfish 735 , respectively) and a D ω P and D Γ , respectively) are included beneath the vorticity contour plots. V ω P and V Γ ventral vortex ( the mean result of 30 velocity fields (1 representative trial). The shadows underneath models represent areas that were shielded from laser light. Circulation magnitude and peak vorticity magnitude values for a dorsal vortex ( Fig. 8. Velocity vector (B) and vorticity contour (C) fields around the posterior edge of the carapace of smooth trunkfish model positioned at yawing angles attack (left to right): 0°, 10°, 20° and 30°. The data are viewed in transverse planes, sampling locations are indicated using illustrations of models viewed from above (A). Each plot is 736 I. K. Bartol and others regions of the carapace (i.e. portions of the carapace that are circulation of far-field vortices intensified (Fig. 8). As was the shielded somewhat from oncoming flow), but little case with pitching angles of attack, peak vorticity and concentrated vorticity formed in near-field locations of the circulation of attached vortices were always greatest at the carapace (i.e. portions of the carapace that are exposed to posterior edge of the carapace, irrespective of angle of attack. oncoming flow) (Fig. 8). At the far-field side of the carapace (the far-field side in this case is the right side of the carapace Pressure measurements when viewed from the rear), a clockwise region of Locations of the concentrated, attached vorticity observed in concentrated, attached vorticity developed around the eye DPIV experiments were closely correlated with locations of ridge/anterior edge of the dorsal keel and grew posteriorly in low pressure detected in pressure experiments. Along the eye terms of circulation and peak vorticity along the keel. The ridge dorso-ventral transect, regions of attached vorticity resulting vortex left the body at either the posterior edge of the developed around the eye ridge and above the ventro-lateral carapace or caudal peduncle. One or two counterclockwise keel at positive pitching angles of attack. At these locations regions of concentrated, attached vorticity developed at the along the transect, which correspond to pressure ports B4 and anterior edge of the ventro-lateral keel and grew posteriorly B1, respectively, low pressure was also detected (Figs 4, 9). along the keel, before leaving the body at either the posterior Along the maximum girth dorso-ventral transect, regions of edge of the carapace or the caudal peduncle. As yawing angles concentrated vorticity were detected laterally and above the of attack deviated farther from 0°, peak vorticity and ventro-lateral keel, which correspond to pressure ports B7 and

1.00 0 Angle of attack (°) D5 A B 0 B5 0.50 4 B4 10 B3 14 B2 –0.50 20 B1 0 A2 24 30 –0.50 –1.00

D7 –1.00 B6 B7 –1.50 B8 B9 –1.50 B10 B11 A4 –2.00 –2.00

P D5 B5 B4 B3 B2 B1 A2 D7 B6 B7 B8 B9 B10 B11 A4 C 0 0.20 C D –0.20 0.10

–0.40 0 –0.60 –0.10 –0.80 D9 B17 B16 –0.20 –1.00 B15 B18 B14 B19 B13 B20 –1.20 B12 –0.30 A6

–1.40 –0.40 D9 B17 B16B15 B14 B13 B12 A6 B18 B19 B20 Pressure port

Fig. 9. Pressure coefficients (CP) plotted as a function of location (pressure port) along various dorso-ventral transects on the smooth trunkfish model positioned at positive pitching angles of attack. The locations of the pressure ports included in each graph are highlighted in images of the model. Note pressure ports A2, A4 and A6 are located in the middle of the ventral region of the carapace, which is not visible in the images. The dorso-ventral transects considered are: (A) eye ridge, (B) maximum girth, (C) midpoint between maximum girth and the posterior edge of the carapace and (D) posterior edge of the carapace. Values are means ± 1 S.D. The carapace and swimming stability in trunkfish 737

B11, respectively. Low pressure at B11 was clearly apparent Along the eye ridge dorso-ventral transect, regions of at all positive pitching angles of attack along the transect. Low attached vorticity formed around the ventro-lateral keel and pressure at B7 was less apparent, but there was a drop in the eye ridge at negative pitching angles of attack. Consistent pressure at B7 relative to its dorsal neighbor B6 at angles of with this observed vorticity pattern, a clear decrease in attack >4° (Fig. 9). Along a transect half way between the point pressure was observed at B1, a pressure port just above the of maximum girth and the posterior edge of the carapace, both ventro-lateral keel (Fig. 10). As angles of attack increased, attached vorticity and low pressure were observed just above pressure dropped at B1, just as vortex circulation increased at the ventro-lateral keel (pressure port B12) at positive pitching more negative angles of attack. A drop in pressure was also angles of attack. At the posterior edge of the carapace, a strong observed at pressure ports near the eye ridge; for angles of vortex was observed above the ventro-lateral keel at positive attack 0 to –10°, pressure was low at B4, and for angles of pitching angles of attack; at this location (B20 on pressure attack –20 to –30°, pressure was low at B5. For dorso-ventral model) low pressure relative to other points along the dorso- transects located at maximum girth and at more posterior ventral transect was observed. Furthermore, pressure dropped locations, regions of concentrated vorticity were located at ports near regions of concentrated, attached vorticity as largely just below the ventro-lateral keel at negative angles of angles of attack increased, which is consistent with the attack, an area where no pressure port was located. As a result, observed increase in peak vorticity and circulation with higher there were no consistent declines in pressure at ports just angles of attack (Fig. 9). above the ventro-lateral keel (i.e. B11, B12 or B20) along

0.40 –0.10 D5 B A B5 0.20 B4 –0.20 B3 B2 0 B1 A2 –0.30 –0.20 –0.40

–0.40 D7 B6 Angle of attack (°) –0.50 B7 –0.60 0 B8 –4 B9 –10 B10 –14 –0.60 B11 –0.80 –20 A4 –24 –30 –1.00 –0.70 P D5 B5 B4 B3 B2 B1 A2 D7B6 B7 B8 B9 B10 B11 A4 C 0.10 –0.15 C D 0.05 –0.20 0 –0.25 –0.05

–0.30 D9 –0.10 B17 B16 –0.35 B15 B14 –0.15 B13 B18 B12 B19 A6 B20 –0.40 –0.20 D9 B17B16 B15 B14 B13 B12 A6 B18 B19 B20 Pressure port

Fig. 10. Pressure coefficients (CP) plotted as a function of location (pressure port) along various dorso-ventral transects on the smooth trunkfish model positioned at negative pitching angles of attack. The locations of the pressure ports included in each graph are highlighted in images of the model. Note pressure ports A2, A4, and A6 are located in the middle of the ventral region of the carapace, which is not visible in the images. The dorso-ventral transects considered are: (A) eye ridge, (B) maximum girth, (C) midpoint between maximum girth and the posterior edge of the carapace and (D) posterior edge of the carapace. Values are means ± 1 S.D. 738 I. K. Bartol and others dorso-ventral transects as observed at positive angles of at B1, the only location along the transect where regions of attack. Interestingly, at the point of maximum girth, there was concentrated vorticity were prominent. Along an antero- a noticeable drop in pressure at A4, a pressure port located in posterior transect where regions of attached dorsal vorticity the middle of the ventral region of the carapace that were frequently observed, pressure was lowest above and/or presumably was influenced by regions of ventrally located, behind the eye ridge (D4, B5) at both positive and negative concentrated vorticity (Figs 5, 10). angles of attack (Fig. 11), areas where regions of concentrated, Although peak vorticity and vortex circulation increased attached dorsal vorticity were frequently greatest. Dorsal consistently along the ventro-lateral keels from the anterior pressures at positive angles of attack were lower than at edges of the keels to the posterior edge of the carapace, negative angles of attack, which too is consistent with the pressure values did not decrease antero-posteriorly at positive observed vorticity patterns, i.e. there was stronger concentrated pitching angles of attack. Instead, pressure just above the vorticity around the eye ridge at positive as opposed to ventro-lateral keel decreased from the snout to the point of negative angles of attack. maximum girth (B11), but then increased posteriorly thereafter At yawing angles of attack, vortex formation occurred at the (Fig. 11). Pressure along the antero-posterior path followed by far-field side of the carapace, most prominently in regions regions of concentrated ventral vorticity at negative angles of adjacent to the dorsal and ventro-lateral keels. Low pressure attack was not measured because of a lack of pressure ports. was consistently detected in these regions along dorso-ventral However, along the D1–B20 antero-posterior transect transects. For example, at the point of maximum girth, pressure considered for positive angles of attack, pressure was lowest at D7 (port just below dorsal keel) and B11 (port just above

1.00 1.00 A B B18 B16 B7 B5 D4 D3 0.50 0.50 B20 B12 B11 B1 D1 0

0 –0.50 Angle of attack (°) –1.00 0 –0.50 4 10 –1.50 14 20 24 30 –1.00 –2.00 P D3 D4 B5 B7 B16 B18 D1 B1 B11 B12 B20 C 1.00 1.00 C D Angle of attack (°) 0 –4 0.50 –10 –14 0.50 –20 –24 –30 0

0 –0.50

–0.50 –1.00 D3 D4 B5 B7 B16 B18 D1 B1 B11 B12 B20 Pressure port

Fig. 11. Pressure coefficients (CP) plotted as a function of location (pressure port) along dorsal antero-posterior transects (A,C) and ventral antero-posterior transects (B,D) on the smooth trunkfish model positioned at different pitching angles of attack. Positive pitching angles of attack are depicted in A and B, while negative pitching angles of attack are depicted in C and D. The locations of the pressure ports included in each graph are highlighted in images of the model in A and B. Values are means ± 1 S.D. The carapace and swimming stability in trunkfish 739

D7 B6 B7 Yawing angles of attack (°) B8 B9 0 B10 10 B11 20 A4 30 0 0.30 Far field Near field 0.20

0.10 –0.50 0

P –0.10 C –0.20 –1.00 –0.30

–0.40

–1.50 –0.50 D7 B6 B7 B8 B9 B10 B11 A4 D7 B6 B7 B8 B9 B10 B11 A4 Pressure port

Fig. 12. Pressure coefficients (CP) plotted as a function of location (pressure port) along a dorso-ventral transect at the maximum girth point of a smooth trunkfish model positioned at different yawing angles of attack. The graph on the left depicts conditions when the transect is in the far field, i.e. shielded from flow, whereas the graph on the right depicts conditions when the transect is in the near field, i.e. exposed to flow. The locations of the pressure ports included in each graph are highlighted in the image of the model. A4 is located in the middle of the ventral region of the carapace. ventro-lateral keel) was low when models were positioned at to drag ratio occurred at –16°. Nose-down pitching moments yawing angles of attack with the ports in the far field (Fig. 12). about the center of mass were detected at angles of attack >–2°, Moreover, a clear drop in pressure at lateral locations between while nose-up pitching moments about the center of mass were the dorsal and ventro-lateral keel (i.e. B7–B9) was frequently detected at angles of attack <–2° (Fig. 13). observed, which is where additional regions of concentrated When compared to lift measurements from force balance vorticity were detected using DPIV (Figs 8, 12). As circulation experiments, lift values predicted from DPIV circulation and peak vorticity increased with increasing yawing angle of values (using 2-D airfoil equations) were consistently of higher attack, pressure in regions of concentrated vorticity dropped. magnitude (paired sample t-test, mean difference=0.045, When the model was positioned so that the holes were in the d.f.=6, t-value=4.322, P=0.005). At positive angles of attack, near field, i.e. directly exposed to flows, a very different DPIV lift estimates were 29–33% higher in the positive pressure distribution occurred. As the pressure ports were direction than force balance lift measurements. At negative exposed to a larger normal velocity component in the near-field angles of attack, DPIV lift estimates were 25–38% higher in side (i.e. higher angles), pressure actually rose at all the lateral the negative direction than force balance lift measurements ports (Fig. 12). (Fig. 14).

Force balance measurements Discussion No obvious stall occurred at angles of attack up to ±30°, and This study is the first to our knowledge that applies multiple overall lift coefficients of the carapace were similar to aerodynamic and hydrodynamic engineering methods to coefficients of delta wings of similar aspect ratio (Fig. 13). The address an important question in the functional morphology of lift coefficient of the carapace was closest to 0 at –2° fish locomotion, i.e. what are the physical bases for the (CL=–0.005). At 0°, CL was 0.027. The drag coefficient of the exceptional hydrodynamic stability of swimming found in the carapace was lowest at –4° (CD=0.157) and was 0.167 at 0°. ostraciid fishes? The results demonstrate that: (1) it is possible At positive angles of attack, the lift-to-drag ratio was highest to obtain internally consistent, independent lines of data that at +20°, which is consistent with delta wings that often have collectively address the question; and (2) morphological maximum L/D ratios >15° (Bertin and Smith, 1989). At features of the carapace contribute to hydrodynamically stable negative angles of attack, the highest downward-directed lift swimming. This paper establishes the foundations for a larger, 740 I. K. Bartol and others similar, comparative study of carapace morphologies and fluid ventro-lateral keels provide channels for vortices to develop, and flow properties for each of the major variations in body form serve to fix developing vortices in place at various pitching and that occur in the ostraciids. Those results will be presented in yawing angles of attack. The steep lateral sides and prominent another paper. dorsal ridge also play important roles in isolating developing ventro-lateral flows, allowing counter-rotating vortices to form Morphological features without interaction with one another. The eye ridge, which was The results of this study indicate that prominent vortices form consistent in width and present at 14–37% CL, and dorsal keel, along the body when smooth trunkfish L. triqueter pitch and which became sharper posteriorly up until approximately yaw. The morphological features of the carapace play an 70% CL, are responsible for forcing flow separation and the instrumental role in the generation, growth and persistence of generation of vorticity in dorsal regions when smooth trunkfish these vortices. The anterior origins of the ventro-lateral keels, pitch and yaw, respectively. During yawing, vortex circulation which were present at locations approximately 14% CL, force growth along the carapace is facilitated by the dorsal keel when flow separation and the generation of vorticity during pitching one side of the keel is exposed to larger normal velocity and yawing. The ventro-lateral keels extend along the majority components than the other. However, during pitching, the lack of the length of the carapace, becoming sharper posteriorly up of multiple, laterally extended dorsal keels, which would allow to a location 82% CL and providing a large area for vorticity the two regions of vorticity shed from the eye ridge to grow in buildup along the body. These characteristics facilitate vortex strength posteriorly with minimal interaction with one another circulation growth in areas adjacent to the ventro-lateral (as in ventral regions), inhibits vortex circulation growth along keels. Regions of lateral concavity found 38–92% CL and the carapace. Consequently, vortex circulation in posterior 0.38–1.23 cm above the ventro-lateral keels and ventral regions of the carapace is weaker dorsally than ventrally during concavity present 58–94% CL and 0.62–1.37 cm below the pitching.

1.00 1.40 0.80 A B 1.20 0.60 1.00 0.40 0.20 0.80 L C D

0 C 0.60 –0.20 0.40 –0.40 Smooth trunkfish 0.20 –0.60 Delta wing –0.80 0 –40–30–20–100 10 20 30 40 –40–30–20–100 10 20 30 40 1.00 2.00 0.75 C 1.50 D 0.50 1.00 0.25 0.50 D / M

L 0 0 C –0.25 –0.50 –0.50 –1.00 –0.75 –1.50 –1.00 –2.00 –40–30–20–100 10 20 30 40 –40–30–20–100 10 20 30 40 Angle of attack (degrees) Angle of attack (degrees)

Fig. 13. Lift coefficients (CL; A), drag coefficients (CD; B), lift to drag ratios (L/D; C), and pitching moment coefficients about the center of mass (CM; D) for the smooth trunkfish model positioned at various pitching angles of attack. In the lift coefficient plot (A), smooth trunkfish coefficients are depicted as open circles, while delta wing coefficients are depicted as filled circles. The delta wing has a similar aspect ratio (0.83) to that of the smooth trunkfish. Delta wing data are from Schlichting and Truckenbrodt (1969). Positive pitching moment coefficients (D) indicate a nose-down pitching moment about the center of mass, whereas negative pitching coefficients indicate a nose-up pitching moment about the center of mass. The carapace and swimming stability in trunkfish 741

0.40 Tritton, 1998; Vogel, 1994). Consequently, for those ports Lift (DPIV) located near the vortex core, a low-pressure region is detected 0.30 Lift (force balance) and becomes more conspicuous as angles of attack increase. 0.20 Along some dorso-ventral transects, a high-pressure region adjacent to the low-pressure vortex core region was present )

N 0.10

( (e.g. B2 and B10 in Fig. 9). This high-pressure region is 0 indicative of a slow flow area where separated flow comes to orce reattachment, a process that is visible in the velocity vector and –0.10 Lift f vorticity contour fields for the respective transects (Fig. 4). The –0.20 farther the vortex core migrates from the carapace surface, the less effect it has on pressure at the surface because the induced –0.30 velocity drops with growing distance. This point is illustrated –0.40 by the observed increase in pressure along an antero-posterior –40 –30–20–100 10 20 30 40 transect above the ventro-lateral keel, beginning at maximum Angle of attack (degrees) girth and progressing posteriorly to the edge of the carapace. Despite an observed increase in vortex circulation along the Fig. 14. Lift forces acting on a smooth trunkfish model measured antero-posterior transect, localized surface pressures did not using a force balance (filled circles) and DPIV (open circles) plotted as a function of angle of attack. Values are means ± 1 S.D. In the continue to decrease along the transect because the vortex core force balance measurements, S.D. bars are often smaller than the migrated away from the carapace, thus having less effect on symbols used to denote values. surface pressure (see Figs 4, 11). This result highlights the importance of using pressure and DPIV data in conjunction; linking global flow features with their concomitant localized Self-correction for pitching effects on the smooth trunkfish carapace is critical for fully DPIV measurements, which provide a global picture of flow understanding flow effects. around the body within planar domains, indicate that pitching Force measurements, which afford an integrative view of the angle of attack has a large impact on ventral vortex formation. forces acting on the entire carapace, provide further support for As angles of attack increased from approximately 0° in the the flow patterns detected in DPIV and pressure experiments. positive direction, vortices with stronger peak vorticity and Based on DPIV and pressure results, vortices are generated circulation developed along concavities above ventro-lateral near the anterior of the fish and grow in strength as they move keels, beginning at the anterior origins of the ventro-lateral posteriorly down the carapace, most prominently in regions keels and reaching maximum strength at postero-lateral adjacent to ventro-lateral keels. Delta wings, which have regions of the carapace. As angles of attack increased from comparable planforms to smooth trunkfish, produce similar approximatey 0° in the negative direction, vortices with flows. In delta wings, a coiled vortex sheet with a core of high stronger peak vorticity and circulation developed along ventral vorticity forms at the leading edge of the wing and grows concavities below the ventro-lateral keels, beginning again at posteriorly along the wing generating lift – a process that the anterior origins of the ventro-lateral keels and reaching differs from lift created through bound circulation in maximum strength at postero-ventral regions of the carapace. conventional wings and leads to higher angles of attack for stall At angles near 0°, weak vortices with the lowest circulation (Bertin and Smith, 1989). The observed similarity in lift developed along the ventro-lateral keels. coefficients and L/D ratios between delta wings and smooth Pressure measurements, which provide useful information trunkfish is thus further evidence of vortex generation and on flow conditions at the surface of the carapace, an area subsequent growth along the ventro-lateral keels. Based on difficult to resolve with DPIV because large velocity gradients force measurements, lift coefficients were closest to 0 at –2° within the boundary layer require high particle densities and and were slightly positive at 0°. This too is in agreement with small interrogations windows to resolve, were consistent DPIV data. DPIV results indicate that lowest vortex circulation with the global flow patterns. In regions where attached, occurs at angles of attack near 0°, and vortices are generated concentrated vorticity was observed, areas of low pressure above ventro-lateral keels at 0°, providing beneficial lift for were consistently detected on the carapace surface. Moreover, counteracting negative buoyancy present in rigid-bodied as angles of attack deviated farther from 0°, either in a positive ostraciids (Blake, 1977). or negative direction, and circulation of attached vortices The vortical flow patterns summarized here produce increased, pressure dropped accordingly. These correlations trimming forces that self-correct for pitching motions, i.e. are in agreement with Bernoulli’s Law, which states that higher rotation in the vertical, head up/down longitudinal plane. local speeds result in lower static pressure. Near the core Attached vortices with the strongest peak vorticity and of each vortex, flow speeds are higher than the surrounding circulation generally develop posterior to the center of mass; fluid and consequently static pressure is low. As the vortex the center of mass of smooth trunkfish is located at a intensifies in strength and local speeds increase, pressure longitudinal location corresponding approximately to the point becomes more negative (McCormack and Crane, 1973; of maximum girth (Bartol et al., in press). These vortices form 742 I. K. Bartol and others above (positive angles of attack) or below (negative angles of vortices, which pick up vorticity and energize flow close to the attack) ventro-lateral keels that extend laterally at an angle of carapace surface. Based on the observed positive pressure 25–40° relative to a horizontal axis when viewed in cross coefficients at the posterior edge of the carapace in dorsal section. Consequently, suction derived from the presence of a locations, flow separation appears to occur in dorsal, posterior vortex above or below the ventro-lateral keels, which was regions of the carapace for angles of attack >4°. According to evident as low pressure zones in pressure experiments, should pressure distribution measurements of other aquatic organisms, act largely upward and posterior to the center of mass at flow separation occurs near the eye and anterior to the arms of positive angles of attack (which also occurs in delta wings) and squid Loligo pealei swimming tail-first (Vogel, 1987), at the downward and posterior to the center of mass at negative posterior quarter of the body of penguins Pygoscelis papua, P. angles of attack. Based on pitching moments recorded in force aseliae and P. antartica and tuna Trachurus mediterraneus balance measurements, where nose-down pitching moments (Aleyev, 1977; Bannasch, 2000), and at the posterior half of occurred and became progressively stronger as angles of attack the body of a swordfish Xiphias gladius (Aleyev, 1977). Using became more positive, and nose-up pitching moments occurred surgically implanted catheters, Dubois et al. (1974) found that and became progressively stronger as angles of attack became flow does not separate anterior to the caudal peduncle in more negative, this is exactly what happens. Therefore, the bluefish Pomatomus saltatrix. Results from both Dubois et al. ventro-lateral keels are effectively generating self-correcting (1974) and the present study are significant because they forces for pitching motions; the self-correcting couple is suggest that fin motion may not be necessary to keep flow proportional to the angle to which the fish is perturbed from a attached to the body, as suggested in other studies (Aleyev, horizontal swimming trajectory. 1977; Blake, 1983b).

Dorsal and antero-posterior flows Self-correction for yawing Regions of attached, concentrated vorticity detected around The flow patterns and pressure distributions detected around the dorsal eye ridge in this study did not grow in a uniform and along the carapace at various yawing angles of attack manner antero-posteriorly during pitching, as was the case for indicate that the carapace also generates self-correcting forces ventral keel induced vorticity. Instead regions of concentrated for yawing, i.e. rotations in the left/right horizontal frontal dorsal vorticity largely broke down along the body posterior to plane. Dorsal and ventral vortices clearly formed on the far- the eye ridge, irrespective of pitching angle of attack. These field side of carapace, especially in areas adjacent to the dorsal results again are consistent with the observed pressure and ventro-lateral keels, when the smooth trunkfish model was distributions. If dorsal carapace morphology does not facilitate positioned at various yawing angles of attack. Circulation of the growth of vortices posteriorly, is there an advantage to attached vortices increased posteriorly along the carapace, such producing vorticity and creating the observed pressure that maximum vortex circulation occurred posterior to the distributions at the eye ridge, i.e. is there a hydrodynamic center of mass. Vortex circulation and peak vorticity also purpose of the eye ridge? Given that ambient pressure occurs increased as yawing angles increased. In areas where around the eye ridge regardless of angle of attack, the eye ridge concentrated attached vorticity was observed, low pressure was may generate the observed flows to ensure an optimal pressure detected, and as circulation of attached vortices increased with distribution around the eye. Maintaining ambient pressure increasing angle of attack, pressure dropped accordingly. around the eye is advantageous because the eye (and most Suction derived from the presence of vortices at far-field importantly the lens) will not be pushed in or pulled out as flow locations of the carapace acts largely opposite the direction of moves along the body, which improves eye function. Detection the yaw and posterior to the center of mass, thus providing of ambient pressures around the eyes has been reported in other trimming forces that self-correct for yawing motions. As with nekton, such as squid, bluefish, and tuna (Aleyev, 1977; pitching, the self-correcting couple is proportional to the angle Dubois et al., 1974; Vogel, 1987). In addition to creating an to which the fish is perturbed. optimal pressure distribution around the eye, the eye ridge may generate some lift to counteract the nose-down pitching Hydrodynamic stability in boxfishes moment produced by the ventro-lateral keels at a 0° angle of Hove et al. (2001) found that boxfishes exhibit some of the attack, thus allowing for more uniform lift generation about the smallest amplitude recoil moments known among fishes. As a center of mass to counteract negative buoyancy. This proposed result, they swim in smoother trajectories than either body- and function is analogous to the role of canards on delta wing caudal-fin (BCF) or single-complex median- and paired-fin aircraft, which provide nose-up trimming moment to (MPF) swimmers. Results from the three independent but counteract nose-down pitching (Bertin and Smith, 1989). complementary experimental approaches applied in this study Based on pressure distributions along antero-posterior indicate that the keeled bony carapace plays an important role transects, there is no appreciable flow separation from the in producing both longitudinal and latitudinal stability in at posterior edge of the carapace in ventro-lateral regions (i.e. least one species of boxfish, the smooth trunkfish L. triqueter. pressure coefficients are not greater than ambient pressure In addition to producing self-correcting forces, the bony (CP=0) and pressure gradients are gradual). Flow remains carapace of the smooth trunkfish also appears to incur high bound to the carapace, presumably because of the ventral drag; the drag coefficients reported in this study are higher than The carapace and swimming stability in trunkfish 743 those typically recorded in other fish, by as much as an order Drucker and Lauder, 2001b), flows around the caudal peduncle of magnitude (Blake, 1981, 1983a). Given that Anderson et al. and finlets (Nauen and Lauder, 2001), vorticity control in fish- (2001) found that live, flexible-bodied swimming fish have like propulsion (Beal et al., 2001), and boundary layer flows much greater friction drag than has been reported in the (Anderson et al., 2001). However, DPIV has one significant literature because of boundary layer thinning and streamwise limitation: it is a 2-D measurement platform that does not acceleration of the near-field, drag differences between boxfish consider all three flow-field components, which may ultimately and flexible-bodied fish may be less pronounced than expected. lead to some error when calculating propulsive and resistive Nonetheless, control for pitching and yawing is presumably forces. An example of such error was encountered in the present more important than drag reduction for fishes like smooth study. Lift forces determined using DPIV were consistently trunkfish. These fishes generally move relatively slowly, but higher than those measured directly using a force balance. A live in highly energetic waters with frequent external comparable situation was also detected by Noca et al. (1997) disturbances, like turbulence, that can lead to large when measuring instantaneous forces on a cylinder in an displacements and significant energy-wasting trajectories. The incompressible cross-flow. These errors occurred because a 2-D trimming self-corrective forces produced by the carapace limit rather than a 3-D velocity field was used to determine force. these displacements and are especially advantageous for the Future studies on live smooth trunkfish will be performed unpredictable velocity fields experienced by smooth trunkfish using defocusing digital particle image velocimetry (DDPIV), because rapid neural processing – a factor that may limit a new method that allows us to visualize and quantify flows in correction responses to sudden, erratic perturbations – is not three dimensions as they move along the body and around the required, as in powered control systems (Webb, 2000). In fins (Pereira et al., 2000; Pereira and Gharib, 2002). Since addition to the energy savings associated with self-correction this technique allows for the quantification of flows over a for disturbances, maintenance of smooth swimming significant volume of fluid, it is an improvement on, not only trajectories also presumably improves sensory acuity of both current 2-D DPIV systems, but also stereo-DPIV technology, hostile and target objects because it reduces complexity of where 3-D flows are imaged in a thin slice of fluid. Employing movement, a factor that improves sensory perception in other DDPIV technology to study stability in live rigid-bodied (Land, 1999; Kramer and McLaughlin, 2001). boxfishes promises to provide unprecedented information on Trimming and powered forces provided by the fins also the control and regulation of trajectories in rigid-bodied multi- presumably play an important role in the control and regulation propulsor swimmers. of trajectories. Like spotted boxfish, smooth trunkfish have four low aspect-ratio median and paired fins of nearly identical area that assuredly move in complex ways together with the List of symbols caudal fin to minimize recoil motions (Hove et al., 2001; I. Af maximum frontal area Bartol, unpublished morphological measurements of smooth Ap planform surface area of the ventral region of the trunkfish). The pectoral fins, in particular, may play an carapace important role in the structure of vortices forming along the BL body length of fish ventro-lateral keels since the pectoral fins are located near the c chord length of carapace path of body-induced vortex formation. Studies focusing on the CD drag coefficient interactions between flows over the fins and over the bodies of CL carapace length of fish live boxfishes are underway in our laboratories. Although fin CL lift coefficient flow and body flow interactions are present in live fishes, the CM pitching moment coefficient results of this study are applicable to freely swimming fishes CP pressure coefficient because the underlying physical functioning of the carapace D total drag self-correcting system is not affected by fin/body flow d1 distance between load cell beams in the force balance interactions. When angles of attack deviate from 0° in either d2 distance between center of force balance and center the positive or negative direction, vortex strength increases of mass of model along the carapace. The self-correcting effect of these vortices DPIV Digital particle image velocimetry will still occur, irrespective of whether vortex strength L total lift increases relative to magnitudes measured for the body alone, L1 lift measured at load cell closest to model or for some fin-modified value. L2 lift measured at load cell farthest from model Quantifying and visualizing flow patterns around oscillating Ls lift per unit span fins poses a significant challenge for understanding stability in M pitching moment smooth trunkfish. DPIV is a powerful tool that has been applied SL segment connecting the apices of ventro-lateral and to a number of interesting areas of fish locomotion, ranging from dorsal keels wakes around flexing bodies (Stamhuis and Videler, 1995; SV segment connecting the apices of the two ventro- Wolfgang et al., 1999; Müller et al., 2001), fin function (Drucker lateral keels and Lauder, 1999, 2000, 2001a; Wilga and Lauder, 1999, 2000), TL total length of the fish fluid forces during turning maneuvers (Wolfgang et al., 1999; u flow speed 744 I. K. Bartol and others

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