Time-resolved Digital Particle Image Velocimetry Research Articles

Phase-averaged and cycle-to-cycle analysis of jet dynamics in a scaled up vocal-fold model.

Phase-averaged and cycle-to-cycle analysis of key contributors to sound production in phonation is examined in a scaled-up vocal-fold model. Simultaneous temporally and spatially resolved pressure and velocity measurements permitted examination of each term in the streamwise integral momentum equation. The relative sizes of these terms were used to address the issue of whether transglottal pressure is a surrogate for vocal-fold drag, a quantity directly related to sound production. Further, time traces of transglottal pressure and volume flow rate provided insight into the role of cycle-to-cycle variations in voiced sound production which affect voice quality. Experiments were conducted using a 10× scaled-up model in a free-surface water tunnel. Two-dimensional vocal-fold models with semi-circular ends inside a square duct were driven with constant opening and closing speeds. The time from opening to closed, To , was half the oscillation period. Time-resolved digital particle image velocimetry (DPIV) and pressure measurements along the duct centreline were made for 3650 ≤ Re ≤ 8100 and equivalent life frequencies from 52.5 to 97.5 Hz. Results showed that transglottal pressure does serve as a surrogate for the vocal-fold drag. However, smaller but non-negligible momentum flux and inertia terms, caused by the jet and vocal-fold motions, may also contribute to vocal-fold drag. Further, cycle-to-cycle variations including jet switching and modulation are inherent in flows of this type despite their high degrees of symmetry and repeatability. The origins of these variations and their potential role in sound production and voice quality are discussed.

Experimental Study on Wake Development in a Three-cylinder Model within a Tunnel in Transitional Flow

ABSTRACTThe characteristics of the flow and heat transfer in transitional flow are not clear and especially the lack of some reliable experimental data hinders the numerical study of the heat exchanger. Based on this, a two-dimensional time-resolved digital particle image velocimetry from the American-TSI-corporation was adopted. Experiments were carried out in an open-loop, recirculating water tunnel that operated at steady-state conditions. The experimental object is a three-cylinder with an equilateral triangle configuration having center-to-center distance ratio of 3. Reynolds number based on the tube diameter and the velocity of inlet is set at 70, 90, 120, 150, 200, 250 and 300. The flow parameters were quantified to research the effect of Reynolds number on flow field for supporting some facts to analyze the heat transfer. Time-mean flow field, turbulence statistics and velocity processed by proper orthogonal decomposition were investigated at different Reynolds numbers. The wake closure length, oscillation and Karman vortex in the wake of the three-cylinder model were discussed in detail.

Use of a rotating cylinder to induce laminar and turbulent separation over a flat plate

An innovative and easy technique using a rotating cylinder system has been implemented in a water tunnel experiment to generate an adverse pressure gradient (APG). The strength of the APG was varied through adjustment in the rotation speed and location of the cylinder. Then the technique was used for inducing a laminar separation bubble (LSB) and turbulent boundary layer (TBL) separation over a flat plate. A theoretical model to predict the pressure variation induced on the plate consists of an inviscid flow over a reverse doublet-like configuration of two counter rotating cylinders. This model quantified the pressure distribution with changes of cylinder speed and location. The dimensionless velocity ratio (VR) of the cylinder rotation rate to the mainstream velocity and gap to diameter ratio were chosen as the two main ways of varying the strength of the APG, which affects the nature and extent of the LSB as well as TBL separation. The experimental parametric study, using time-resolved digital particle image velocimetry, was then conducted in a water tunnel. The variation in height (h), length (l), and the separation point (S) of the LSB was documented due to the variation in the APG. The similar type of experimental parametric study was used to explore the unsteady, turbulent separation bubble in a 2D plane aligned with the flow and perpendicular to the plate. The mean detachment locations of TBL separation are determined by two different definitions: (i) back-flow coefficient (χ) = 50%, and (ii) location of start of negative mean skin friction coefficient (Cf). They are in good agreement and separation bubble characteristics agreed well with results obtained using different methods thus proving the validity of the technique.

Experimental study of laminar and turbulent boundary layer separation control of shark skin

The Shortfin Mako shark (Isurus oxyrinchus) is a fast swimmer and has incredible turning agility, and has flexible scales known to bristle up to 50° in the flank regions. It is purported that this bristling capability of the scales may result in a unique pass flow control method to control flow separation and reduce drag. It appears that the scales have evolved to be only actuated when the flow over the body is reversed; thereby inducing a method of inhibiting flow reversal close to the surface. In addition, bristled scales form cavities which could induce boundary layer mixing and further assist in delaying flow separation. To substantiate the hypothesis, samples of skin from the flank region of the mako have been tested in a water tunnel facility under various strengths of adverse pressure gradient (APG). Laminar and turbulent separation over the skin was studied experimentally using time-resolved digital particle image velocimetry, where the APG was generated and varied using a rotating cylinder. Shark skin results were compared with that of a smooth plate data for a given amount of APG. Both the instantaneous and time-averaged results reveal that shark skin is capable of controlling laminar as well as turbulent separation. Under laminar conditions, the shark skin also induces an early transition to turbulence and reduces the degree of laminar separation. For turbulent separation, the presence of the shark skin reduces the amount of backflow and size of the separation region. Under both flow conditions, the shark skin also delayed the point of separation as compared to a smooth wall.

Tracking and Quantifying Developmental Processes in C. elegans Using Open-source Tools.

Quantitatively capturing developmental processes is crucial to derive mechanistic models and key to identify and describe mutant phenotypes. Here protocols are presented for preparing embryos and adult C. elegans animals for short- and long-term time-lapse microscopy and methods for tracking and quantification of developmental processes. The methods presented are all based on C. elegans strains available from the Caenorhabditis Genetics Center and on open-source software that can be easily implemented in any laboratory independently of the microscopy system used. A reconstruction of a 3D cell-shape model using the modelling software IMOD, manual tracking of fluorescently-labeled subcellular structures using the multi-purpose image analysis program Endrov, and an analysis of cortical contractile flow using PIVlab (Time-Resolved Digital Particle Image Velocimetry Tool for MATLAB) are shown. It is discussed how these methods can also be deployed to quantitatively capture other developmental processes in different models, e.g., cell tracking and lineage tracing, tracking of vesicle flow.

Reynolds number dependency of an insect-based flapping wing

SummaryAerodynamic characteristics depending on Reynolds number (Re) ranges were studied to investigate the suitable design parameters of an insect-based micro air vehicle (MAV). The tests centered on the wing rotation timing and Re ranges, and were conducted to understand the lift augmentations and unsteady effects. A dynamically scaled-up flapping wing controlled by a pair of servos was installed underwater with a micro force/torque sensor. A high-speed camera and a laser sheet were also put in front of the water tank for the time-resolved digital particle image velocimetry (DPIV). The lift augmentations clearly appeared at low Re and were well reflected on the insect’s flight range. In the case of the high Re, however, the peak standing for the wing–wake interaction was delayed, and the pitching-up rotation was not able to lead to another lift enhancement, i.e., rotational lift. In such Re, the mean CL and the L/D of the advanced rotation were substantially decreased from those of the other rotations. The DPIV results at high Re well described turbulent characteristics such as the irregular, unstable, and high-intensity vortex structures with a short temporal delay. In the advanced rotation, the LEV in the rotational phase could not maintain the attachment. Thus, the rotational lift was not able to work. On the contrary, the temporal response delay benefitted the wing in the delayed rotation. Therefore, the wing in the delayed rotation had both a similar level of the mean CL and a higher marked L/D than those of the advanced rotation. Such results indicate that the high Re could interrupt lift augmentation mechanisms, and these augmentations would not be suitable for a heavier MAV. In conclusion, using adequate wing kinematics to acquire estimations of the weight and range of the Re is highly recommended at the aerodynamic design step.

Flexible margin kinematics and vortex formation of Aurelia aurita and Robojelly.

The development of a rowing jellyfish biomimetic robot termed as “Robojelly”, has led to the discovery of a passive flexible flap located between the flexion point and bell margin on the Aurelia aurita. A comparative analysis of biomimetic robots showed that the presence of a passive flexible flap results in a significant increase in the swimming performance. In this work we further investigate this concept by developing varying flap geometries and comparing their kinematics with A. aurita. It was shown that the animal flap kinematics can be replicated with high fidelity using a passive structure and a flap with curved and tapered geometry gave the most biomimetic performance. A method for identifying the flap location was established by utilizing the bell curvature and the variation of curvature as a function of time. Flaps of constant cross-section and varying lengths were incorporated on the Robojelly to conduct a systematic study of the starting vortex circulation. Circulation was quantified using velocity field measurements obtained from planar Time Resolved Digital Particle Image Velocimetry (TRDPIV). The starting vortex circulation was scaled using a varying orifice model and a pitching panel model. The varying orifice model which has been traditionally considered as the better representation of jellyfish propulsion did not appear to capture the scaling of the starting vortex. In contrast, the pitching panel representation appeared to better scale the governing flow physics and revealed a strong dependence on the flap kinematics and geometry. The results suggest that an alternative description should be considered for rowing jellyfish propulsion, using a pitching panel method instead of the traditional varying orifice model. Finally, the results show the importance of incorporating the entire bell geometry as a function of time in modeling rowing jellyfish propulsion.

Aerodynamics of the flying snake Chrysopelea paradisi: how a bluff body cross-sectional shape contributes to gliding performance

A prominent feature of gliding flight in snakes of the genus Chrysopelea is the unique cross-sectional shape of the body, which acts as the lifting surface in the absence of wings. When gliding, the flying snake Chrysopelea paradisi morphs its circular cross-section into a triangular shape by splaying its ribs and flattening its body in the dorsoventral axis, forming a geometry with fore-aft symmetry and a thick profile. Here, we aimed to understand the aerodynamic properties of the snake's cross-sectional shape to determine its contribution to gliding at low Reynolds numbers. We used a straight physical model in a water tunnel to isolate the effects of 2D shape, analogously to studying the profile of an airfoil of a more typical flyer. Force measurements and time-resolved (TR) digital particle image velocimetry (DPIV) were used to determine lift and drag coefficients, wake dynamics and vortex-shedding characteristics of the shape across a behaviorally relevant range of Reynolds numbers and angles of attack. The snake's cross-sectional shape produced a maximum lift coefficient of 1.9 and maximum lift-to-drag ratio of 2.7, maintained increases in lift up to 35 deg, and exhibited two distinctly different vortex-shedding modes. Within the measured Reynolds number regime (Re=3000-15,000), this geometry generated significantly larger maximum lift coefficients than many other shapes including bluff bodies, thick airfoils, symmetric airfoils and circular arc airfoils. In addition, the snake's shape exhibited a gentle stall region that maintained relatively high lift production even up to the highest angle of attack tested (60 deg). Overall, the cross-sectional geometry of the flying snake demonstrated robust aerodynamic behavior by maintaining significant lift production and near-maximum lift-to-drag ratios over a wide range of parameters. These aerodynamic characteristics help to explain how the snake can glide at steep angles and over a wide range of angles of attack, but more complex models that account for 3D effects and the dynamic movements of aerial undulation are required to fully understand the gliding performance of flying snakes.

Impact of the Combustor-Turbine Interface Slot Orientation on the Durability of a Nozzle Guide Vane Endwall

The combustor-turbine interface is an essential component in a gas turbine engine as it allows for thermal expansion between the first stage turbine vanes and combustor section. Although not considered as part of the external cooling scheme, leakage flow from the combustor-turbine interface can be utilized as coolant. This paper reports on the effects of orientation of a two-dimensional leakage slot, simulating the combustor-turbine interface, on the net heat flux reduction to a nozzle guide vane endwall. In addition to adiabatic effectiveness and heat transfer measurements, time-resolved, digital particle image velocimetry (TRDPIV) measurements were performed in the vane stagnation plane. Four interface slot orientations of 90 deg, 65 deg, 45 deg, and 30 deg located at 17% axial chord upstream of a first vane in a linear cascade were studied. Results indicate that reducing the slot angle to 45 deg can provide as much as a 137% reduction to the average heat load experienced by the endwall. Velocity measurements indicate the formation of a large leading edge vortex for coolant injected at 90 deg and 65 deg while coolant injected at 45 deg and 30 deg flows along the endwall and washes up the vane surface at the endwall junction.

Effects of Orientation and Position of the Combustor-Turbine Interface on the Cooling of a Vane Endwall

First stage, nozzle guide vanes and accompanying endwalls are extensively cooled by the use of film cooling through discrete holes and leakage flow from the combustor-turbine interface gap. While there are cooling benefits from the interface gap, it is generally not considered as part of the cooling scheme. This paper reports on the effects of the position and orientation of a two-dimensional slot on the cooling performance of a nozzle guide vane endwall. In addition to surface thermal measurements, time-resolved, digital particle image velocimetry (TRDPIV) measurements were performed at the vane stagnation plane. Two slot orientations, 90 deg and 45 deg, and three streamwise positions were studied. Effectiveness results indicate a significant increase in area averaged effectiveness for the 45 deg slot relative to the 90 deg orientation. Flowfield measurements show dramatic differences in the horseshoe vortex formation.

Vortex rings in radially confined domains

The dynamics of vortex rings generated within confined domains are relevant to important hydrodynamic processes such as flow past heart valves or severe arterial constrictions. However, despite their importance, these flows have not received much attention to date. This study examines the development and evolution of radially confined vortex rings. Time-resolved digital particle image velocimetry was used to investigate two levels of radial confinement and a range of vortex ring strengths. We found that for severely confined vortex rings, the formation time and peak circulation values were unaffected for L/D 0 < 4 cases and slightly affected for larger L/D 0 cases. After pinch-off, circulation decay was observed with an approximately constant normalized circulation decay rate. We found that with increasing circulation strength, the nondimensional time delay between the pinch-off and the onset of circulation decay reduced due to an increased vortex ring diameter within the confinement domain and a reduction in the necessary time for the surface induced and core vorticity regions to interact. This study uncovers the dynamics of radially confined vortex rings and show that the nondimensional rate of circulation decay is dependent on the vortex ring confinement ratio (ratio of the vortex ring orifice diameter to the diameter of the outer cylinder), and the time delay between the vortex pinch-off and the onset of circulation is dependent on the vortex ring circulation strength.

The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-resolved digital particle image velocimetry (TRDPIV) and a new variety of thin-film heat flux sensor called the heat flux array (HFA) are used simultaneously to measure the spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence (Tux¯=0.5–1.0%) are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are performed to examine the effects of turbulence with mean streamwise turbulence intensity of Tux¯=5.0% and an integral length scale of Λx¯=3.25 cm. At a Reynolds number of ReD¯=U∞¯D/υ=21,000, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity, integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients.

A Method for Identifying and Visualizing Foreign Particle Motion Using Time-Resolved Particle Tracking Velocimetry

Gas turbines for aircraft are designed for operation with a clean inlet air flow. This ideal operational condition is often violated during take-off and landing, where the probability of particle ingestion is high, with sand and dirt being the most commonly observed foreign particles. Current research on particle ingestion has identified several mechanisms that contribute to performance degradation in the turbine: erosion of internal and external surfaces, and flow blockages of film-cooling holes and internal cooling passages. The focus of the study given in this paper is to present a method that identifies the motion of foreign particles within an internal ribbed passage. The method uses a high-resolution, flow field interrogation method known as time-resolved digital particle image velocimetry (TRDPIV). Observations from the two-phase flows showed that particle collisions occurred more frequently on the upstream surface of the ribs, especially in the inlet region. Results from these collisions included substantial particle breakup, and a particle rebounding phenomenon between the upper and lower walls. Comparisons are made to large eddy simulation predicted particle trajectories indicating some agreement, as well as phenomena that are not predicted due to the inherent assumption of the modeling.

Developing and fully developed turbulent flow in ribbed channels

Wall-mounted roughness features, such as ribs, are often placed along the walls of a channel to increase the convective surface area and to augment heat transfer and mixing by increasing turbulence. Depending on the relative roughness size and orientation, the ribs also have varying degrees of increased pressure losses. Designs that use ribs to promote heat transfer encompass the full range of having only a few streamwise ribs, which do not allow fully developed flow conditions, to multiple streamwise ribs, which do allow the flow to become fully developed. The majority of previous studies have focused on perturbing the geometry of the rib with little attention to the spatially and temporally varying flow characteristics and their dependence on the Reynolds number. A staggered rib-roughened channel study was performed using time-resolved digital particle image velocimetry (TRDPIV). Both the developing (entry region) and a fully developed region were interrogated for three Reynolds numbers of 2,500, 10,000, and 20,000. The results indicate that the flow was more sensitive to Reynolds number at the inlet than within the fully developed region. Despite having a similar mean-averaged flowfield structure over the full Reynolds number range investigated, the population and distribution of coherent structures and turbulent dissipation within the fully developed region were also found to be Reynolds number dependent. Exploring the time-accurate flow characteristics revealed that in addition to vortices shed from the rib shear layer, the region of the rib wake was governed by a periodic process of bursting of the wake vortices resulting in the intermittent ejection of the inter-rib recirculation region into the core flow. This periodic process was the driving mechanism resulting in mixing and heat transfer augmentation. A quadrant-splitting burst analysis was also performed to determine the characteristic frequency and duration of inter-rib bursting as well as the wake shedding frequency, both of which were determined to be Reynolds number dependent.

Large-scale time-resolved digital particle image velocimetry (TR-DPIV) for measurement of high subsonic hot coaxial jet exhaust of a gas turbine engine

The development of a highly configurable triple digital particle image velocimetry (DPIV) system is described, which is capable of acquiring both continuous, statistically independent measurements at up to 14 Hz and time-resolved PIV data at MHz rates. The system was used at QinetiQ's Noise Test Facility (NTF) as part of the EU-funded CoJeN programme to obtain measurements from high subsonic (Mach ⩽ 0.9), hot (∼500 °C), large (1/10th) scale coaxial jet flows at a standoff distance of ∼1 m. High-resolution time-averaged velocity and turbulence data were obtained for complete coaxial engine exhaust plumes down to 4 m (20 jet diameters) from the nozzle exit in less than 1 h. In addition, the system allowed volumetric data to be obtained, enabling fast assessment of spatial alignment of nozzle configurations. Furthermore, novel six-frame time-series data-capture is demonstrated up to 330 kHz, used to calculate time–space correlations within the exhaust, allowing for study of spatio-temporal developments in the jet, associated with jet-noise production. The highly automated system provides synchronization triggers for simultaneous acquisition from different measurement systems (e.g. LDA) and is shown to be versatile, rugged, reliable and portable, operating remotely in a hostile environment. Data are presented for three operating conditions and two nozzle geometries, providing a database to be used to validate CFD models of coaxial jet flow.

A mechanism for mitigation of blade–vortex interaction using leading edge blowing flow control

The interaction of a vortical unsteady flow with structures is often encountered in engineering applications. Such flow structure interactions (FSI) can be responsible for generating significant loads and can have many detrimental structural and acoustic side effects, such as structural fatigue, radiated noise and even catastrophic results. Amongst the different types of FSI, the parallel blade–vortex interaction (BVI) is the most common, often encountered in helicopters and propulsors. In this work, we report on the implementation of leading edge blowing (LEB) active flow control for successfully minimizing the parallel BVI. Our results show reduction of the airfoil vibrations up to 38% based on the root-mean-square of the vibration velocity amplitude. This technique is based on displacing an incident vortex using a jet issued from the leading edge of a sharp airfoil effectively increasing the stand-off distance of the vortex from the body. The effectiveness of the method was experimentally analyzed using time-resolved digital particle image velocimetry (TRDPIV) recorded at an 800 Hz rate, which is sufficient to resolve the spatio-temporal dynamics of the flow field and it was combined with simultaneous accelerometer measurements of the airfoil, which was free to oscillate in a direction perpendicular to the freestream. Analysis of the flow field spectra and a Proper Orthogonal Decomposition (POD) of the TRDPIV data of the temporally resolved planar flow fields indicate that the LEB effectively modified the flow field surrounding the airfoil and increased the convecting vortices stand-off distance for over half of the airfoil chord length. It is shown that LEB also causes a redistribution of the flow field spectral energy over a larger range of frequencies.

In Vitro, Time-Resolved PIV Comparison of the Effect of Stent Design on Wall Shear Stress

The effect of stent design on wall shear stress (WSS) and oscillatory shear index (OSI) was studied in vitro using time-resolved digital particle image velocimetry (DPIV). Four drug-eluting stents [XIENCE V (Abbott Vascular), TAXUS Liberté (Boston Scientific), Endeavor (Medtronic), and Cypher (J&J Cordis)] and a bare-metal stent [VISION (Abbott Vascular)] were implanted into compliant vessel models, and the flow was measured in physiologically accurate coronary conditions featuring reversal and realistic offsets between pressure and flowrate. DPIV measurements were made at three locations under two different flow rates (resting: Re = 160, f = 70 bpm and exercise: Re = 300, f = 120 bpm). It was observed that design substantially affected the WSS experienced at the vessel walls. Averaged values between struts ranged from 2.05 dynes/cm(2) (Cypher) to 8.52 dynes/cm(2) (XIENCE V) in resting conditions, and from 3.72 dynes/cm(2) (Cypher) to 14.66 dynes/cm(2) (VISION) for the exercise state. Within the stent, the WSS dropped and the OSI increased immediately distal to each strut. In addition, an inverse correlation between average WSS and OSI existed. Comparisons with recently published results from animal studies show strong correlation between the measured WSS and observed endothelial cell coverage. These results suggest the importance of stent design on the WSS experienced by endothelial cells in coronary arteries.

Hydrodynamics of the escape response in bluegill sunfish,Lepomis macrochirus

Escape responses of fishes are one of the best characterized vertebrate behaviors, with extensive previous research on both the neural control and biomechanics of startle response performance. However, very little is known about the hydrodynamics of escape responses, despite the fact that understanding fluid flow patterns during the escape is critical for evaluating how body movement transfers power to the fluid, for defining the time course of power generation, and for characterizing the wake signature left by escaping fishes, which may provide information to predators. In this paper, we present an experimental hydrodynamic analysis of the C-start escape response in bluegill sunfish (Lepomis macrochirus). We used time-resolved digital particle image velocimetry at 1000 frames s(-1) (fps) to image flow patterns during the escape response. We analyzed flow patterns generated by the body separately from those generated by the dorsal and anal fins to assess the contribution of these median fins to escape momentum. Each escape response produced three distinct jets of fluid. Summing the components of fluid momentum in the jets provided an estimate of fish momentum that did not differ significantly from momentum measured from the escaping fish body. In contrast to conclusions drawn from previous kinematic analyses and theoretical models, the caudal fin generated momentum that opposes the escape during stage one, whereas the body bending during stage one contributed substantial propulsive momentum. Additionally, the dorsal and anal fins each contributed substantial momentum. The results underscore the importance of the dorsal and anal fins as propulsors and suggest that the size and placement of these fins may be a key determinant of fast start performance.

Fluid dynamics of feeding behaviour in white-spotted bamboo sharks

Although the motor control of feeding is presumed to be generally conserved, some fishes are capable of modulating the feeding behaviour in response to prey type and or prey size. This led to the 'feeding modulation hypothesis', which states that rapid suction strikes are pre-programmed stereotyped events that proceed to completion once initiated regardless of sensory input. If this hypothesis holds true, successful strikes should be indistinguishable from unsuccessful strikes owing to a lack of feedback control in specialized suction feeding fishes. The hydrodynamics of suction feeding in white-spotted bamboo sharks (Chiloscyllium plagiosum) was studied in three behaviours: successful strikes, intraoral transports of prey and unsuccessful strikes. The area of the fluid velocity region around the head of feeding sharks was quantified using time-resolved digital particle image velocimetry (DPIV). The maximal size of the fluid velocity region is 56% larger in successful strikes than unsuccessful strikes (10.79 cm2 vs 6.90 cm2), but they do not differ in duration, indicating that strikes are modulated based on some aspect of the prey or simply as a result of decreased effort on the part of the predator. The hydrodynamic profiles of successful and unsuccessful strikes differ after 21 ms, a period probably too short to provide time to react through feedback control. The predator-to-prey distance is larger in missed strikes compared with successful strikes, indicating that insufficient suction is generated to compensate for the increased distance. An accuracy index distinguishes unsuccessful strikes (-0.26) from successful strikes (0.45 to 0.61). Successful strikes occur primarily between the horizontal axis of the mouth and the dorsal boundary of the ingested parcel of water, and missed prey are closer to the boundary or beyond. Suction transports are shorter in duration than suction strikes but have similar maximal fluid velocity areas to move the prey through the oropharyngeal cavity into the oesophagus (54 ms vs 67 ms).

Nonlinear temporal filtering of time-resolved digital particle image velocimetry data

Nonlinear filtering methods have been developed to identify and replace outlying data points in velocity time series obtained with time-resolved digital particle image velocimetry (PIV) of the flow around a surface-mounted cube at a Reynolds number of 20,000. Nuances associated with the spectral computation of the cross-correlation are highlighted, including the requirement of zero-padding an image interrogation area to eliminate the circular components of the cross-correlation. Three nonlinear filtering methods for the replacement of outliers are applied to the velocity time series sampled at 1,000 Hz: a median filter, a decision-based Hampel filter, and a PIV-specific Hampel filter. The particular benefit of the PIV-specific Hampel filter is that it allows the retention of actual measured data, sometimes derived from alternate peaks in the cross-correlation function, while still providing for the removal of outliers when a consistent, nonoutlying measurement is not available.