Digital Particle Image Velocimetry Research Articles

Advanced blood flow assessment in Zebrafish via experimental digital particle image velocimetry and computational fluid dynamics modeling

Over the past few decades, Zebrafish has become a widely used vertebrate model for cardiovascular research. Easy genetic manipulation, low cost, high fecundity, embryonic transparency, and ability to survive in the early stages of development without active circulation are among the advantages of Zebrafish. Cardiac malformations can be induced through genetic manipulations for elucidating the influence of mechanobiological stimuli on the development and progress of the cardiovascular diseases. For this purpose, a reliable in vivo assessment of cardiac function and disturbed hemodynamics is required. Therefore, it is necessary to accurately determine the complex blood flow patterns and associated hemodynamic shear stresses within the developing heart and cardiovascular system. In the traditional approach, brightfield microscopy is used to track the motion of cells in two-dimensions (2D). However, with the development of advanced modalities such as light-sheet fluorescent microscopy, it is now possible to perform 4D (three-dimensional space + time) imaging of Zebrafish embryo and larvae. The integration of digital particle image velocimetry (DPIV) and computational fluid dynamics (CFD) provide an opportunity for detailed investigations using in vivo images. In this review, DPIV and CFD methods are explained for blood flow assessment, and recent relevant research findings from Zebrafish studies are summarized.

Cycle-to-cycle flow variations in a square duct with a symmetrically oscillating constriction

Spatially and temporally resolved digital particle image velocimetry measurements are presented of flow complexities in a nominally two-dimensional, symmetric, duct with an oscillating constriction. The motivation for this research lies in advancing the state-of-the-art in applying integral control volume analysis to modeling unsteady internal flows. The specific target is acoustic modeling of human phonation. The integral mass and momentum equations are directly coupled to the acoustic equations and provide quantitative insight into acoustic source strengths in addition to the dynamics of the fluid-structure interactions in the glottis. In this study, a square cross-section duct was constructed with symmetric, computer controlled, oscillating constrictions that incorporate both rocking as well as oscillatory open/close motions. Experiments were run in a free-surface water tunnel over a Strouhal number range, based on maximum jet speed and model length, of 0.012–0.048, for a fixed Reynolds number, based on maximum gap opening and maximum jet speed, of 8000. In this study, the constriction motions were continuous with one open-close cycle immediately following another. While the model and its motions were nominally two-dimensional and symmetric, flow asymmetries and oscillation frequency dependent cycle-to-cycle variations were observed. These are examined in the context of terms in the integral conservation equations.

A contralateral wing stabilizes a hovering hawkmoth under a lateral gust

Previous analysis on the lateral stability of hovering insects, which reported a destabilizing roll moment due to a lateral gust, has relied on the results of a single wing without considering a presence of the contralateral wing (wing-wing interaction). Here, we investigated the presence of the contralateral wing on the aerodynamic and flight dynamic characteristics of a hovering hawkmoth under a lateral gust. By employing a dynamically scaled-up mechanical model and a servo-driven towing system installed in a water tank, we found that the presence of the contralateral wing plays a significant role in the lateral static stability. The contralateral wing mitigated an excessive aerodynamic force on the wing at the leeward side, thereby providing a negative roll moment to the body. Digital particle image velocimetry revealed an attenuated vortical system of the leading-edge vortex. An excessive effective angle of attack in the single wing case, which was caused by the root vortex of previous half stroke, was reduced by a downwash of the contralateral wing. The contralateral wing also relocated a neutral point in close proximity to the wing hinge points above the actual center of gravity, providing a practical static margin to a hovering hawkmoth.

Investigation of Flow Field at the Inlet of a Turbocharger Compressor Using Digital Particle Image Velocimetry

Abstract The flow field at the inlet of a turbocharger compressor has been studied through stereoscopic particle image velocimetry (SPIV) experiments under different operating conditions. It is found that the flow field is quite uniform at high mass flow rates; but as the mass flow rate is reduced, flow reversal from the impeller is observed as an annular ring at the periphery of the inlet duct. The inception of flow reversal is observed to occur in the mid-flow operating region, near peak efficiency, and corresponds to an incidence angle of about 15.5 deg at the inducer blade tips at all tested speeds. This reversed flow region is marked with high tangential velocity and rapid fluctuations. It grows in strength with reducing mass flow rate and imparts some of its angular momentum to the forward flow due to mixing. The penetration depth of the reversed flow upstream from the inducer plane is found to increase quadratically with decreasing flow rate.

Local Scale Displacement Fields in Grains\u2013Structure Interactions Under Cyclic Loading: Experiments and Simulations

Soils encounter cyclic loading conditions in situ, for example during the earthquakes and in the construction sequences of pavements. Investigations on the local scale displacements of the soil grain and their failure patterns under the cyclic loading conditions are relatively scarce in the literature. In this study, the local displacement fields of a dense sand layer interacting with a rigid footing under the plane-strain condition are examined using both experiments and simulations. Three commonly used types of cyclic loading conditions were applied on the footing. Digital particle image velocimetry (DPIV) is used to measure the local scale displacement fields in the soil, and to understand the evolution of the failure envelopes in the sand media under the cyclic loading conditions. The experimental results are compared with corresponding finite element analysis (FEA), in which experimentally-characterised constitutive relations are fed as an input into the FEM simulations. For comparison purposes, the case of footing subjected to the quasi-static loading condition was also studied. In general, the results show a good level of agreement between the results of the experiments and simulations conducted here. Overall, relatively shallower but wider displacement fields are observed under the cyclic loading, when compared with that of the quasi-static load test. The vorticity regions are highly localized at the shear bands in the sand media under the ultimate load. The research contributes to new understanding on the local scale displacement fields and their link to the bearing capacity of the footing under the cyclic loading environments.

Influence of aspect ratio on wing–wake interaction for flapping wing in hover

The interaction between an insect wing and its wake during hovering flight is inevitable. The effect of this wing–wake interaction (WWI) during stroke reversal is an intricate phenomenon that is difficult to forecast. Previous studies have mostly concentrated on the effect of aspect ratio (AR) on the leading-edge vortex (LEV) at the middle of stroke motion. However, the effect of AR on LEV during stroke reversal, which is where the intricate WWI phenomenon occurs, has not been fully exploited. This study aimed at revealing how AR affects WWI during stroke reversal at Re of ~ 104, and the role LEV plays. The experimental time-course measurement of force showed that the WWI at stroke reversal was strengthened by decreasing AR irrespective of pitching duration. A time-varying digital particle image velocimetry (DPIV) measurement revealed a jet-like flow induced by a pair of counter-rotating trailing edge vortex (TEV) as the primitive source of the interaction. This study found a WWI that appeared at the wing root contrary to the conventional type in literature, which appeared at the wing tip. The size of the LEV at the wing root played a key role by facilitating the effectiveness of the wing rotation. AR = 2 wing benefited much from this form of WWI by generating strong coherent shear layer at the end of stroke, and relatively weak LEV during stroke reversal. Thus, this type of WWI might be mostly beneficial for low AR wings (AR < 3).

Physical modelling of lateral sand–pipe interaction

This paper presents a series of physical modelling tests performed to measure the resistance developing during lateral dragging of a rigid pipe buried in loose to very dense dry sand. The experiments were performed in a small-scale prototype developed to model sand–pipe interaction during relative ground movement episodes while accurately controlling the density and uniformity of sand around the pipe. Digital imaging and particle image velocimetry equipment are integrated with the rig, so as to track the evolution of the failure surface developing in sand with increasing pipe displacements. Auxiliary components of the rig allow investigation of the effects of pipe kinematic constraints and embedment method on the results obtained. Accordingly, the measurements obtained with the developed prototype are compared against results from similar studies, with the intention of shedding some light on the scatter observed in published data, and on the provisions from different pipe stress analysis guidelines. It is shown that current simplified methods may underestimate the lateral reaction developing on pipes in very dense sand beds, and analysis models built around these methods may under-predict pipe strains. To alleviate this, a modified expression is proposed for estimating the peak reaction of lateral elastoplastic soil springs, and an upper bound of this reaction is provided for design purposes.

Flow Velocity Field Measurement of Vertical Upward Oil–Water Two-Phase Immiscible Flow Using the Improved DPIV Algorithm Based on ICP and MLS

Flow velocity field measurement is important for analyzing flow characteristics of oil–water two-phase immiscible flow in vertical well. Digital particle image velocimetry (DPIV) is an effective velocity field measurement method that has overcome single point measurement limitation of traditional instruments. However, multiphase flow velocity fields generated by DPIV are often accompanied by local false vectors caused by image mismatching, which leads to measurement results with low accuracy. In this paper, the reasons for oil–water two-phase immiscible flow image mismatching in inner diameter 125 mm vertical pipe is identified by studying the DPIV calculation process. This is mainly caused by image noise and poor window following performance that results from poor deformation performance of the interrogation window. To improve deformation performance of the interrogation window, and thus improve the accuracy of the algorithm, iterative closest point (ICP) and moving least squares (MLS) are introduced into the window deformation iterative multigrid algorithm in DPIV postprocessing algorithm. The simulation showed that the improved DPIV algorithm had good matching performance, and thus the false vector was reduced. The experimental results showed that, in light of the present investigation, on average, the improved DPIV algorithm is found to yield an accuracy improvement of ~6%; the measurement uncertainty and reproducibility of the improved DPIV algorithm were 0.149 × 10−3 m/s and 1.98%, respectively.

Thrust and Hydrodynamic Efficiency of the Bundled Flagella.

The motility mechanism of prokaryotic organisms has inspired many untethered microswimmers that could potentially perform minimally invasive medical procedures in stagnant fluid regions inside the human body. Some of these microswimmers are inspired by bacteria with single or multiple helical flagella to propel efficiently and fast. For multiple flagella configurations, the direct measurement of thrust and hydrodynamic propulsion efficiency has been challenging due to the ambiguous mechanical coupling between the flow field and mechanical power input. To address this challenge and to compare alternative micropropulsion designs, a methodology based on volumetric velocity field acquisition is developed to acquire the key propulsive performance parameters from scaled-up swimmer prototypes. A digital particle image velocimetry (PIV) analysis protocol was implemented and experiments were conducted with the aid of computational fluid dynamics (CFD). First, this methodology was validated using a rotating single-flagellum similitude model. In addition to the standard PIV error assessment, validation studies included 2D vs. 3D PIV, axial vs. lateral PIV and simultaneously acquired direct thrust force measurement comparisons. Compatible with typical micropropulsion flow regimes, experiments were conducted both for very low and higher Reynolds (Re) number regimes (up to a Re number = 0.01) than that are reported in the literature. Finally, multiple flagella bundling configurations at 0°, 90° and 180° helical phase-shift angles were studied using scaled-up multiple concentric flagella thrust elements. Thrust generation was found to be maximal for the in-phase (0°) bundling configuration but with ~50% lower hydrodynamic efficiency than the single flagellum. The proposed measurement protocol and static thrust test-bench can be used for bio-inspired microscale propulsion methods, where direct thrust and efficiency measurement are required.

Flow around an array of four equispaced square cylinders

Experiments were conducted to study the flow around an array of four square cylinders arranged in square configuration under a subcritical Reynolds number Re = 8000. The goal is to systematically examine the effects of the cylinders’ center-to-center pitch (with 6 pitch ratios P/D = 2–5) and the array orientation with reference to oncoming flow (with 7 incidence angles α = 0°–45°), i.e., 42 cases in total. The flow field was measured using digital Particle Image Velocimetry (PIV), in conjunction with direct measurement of the hydrodynamic forces (lift and drag) on each cylinder using a piezoelectric load cell. The flow interference among the cylinders in the array is complex, which could be non-synchronous, quasi-periodic or synchronized. The flow patterns, Reynolds shear stress distributions, temporal variation and power spectra of drag and lift coefficients on each cylinder, force statistics and vortex shedding frequencies are discussed in detail. Similar to that of four-circular-cylinder array, the flow could be broadly classified into three regimes (shielding, reattachment and impinging) governed by the combined value of P/D and α. However, it differs noticeably from that of the four-circular-cylinder array, including: (i) a lack of reattachment regime for the inline square configuration (α = 0°); (ii) the unique feature of attractive lift for the pair of cylinders arranged side-by-side; and (iii) higher sensitivity to the variation in incidence angle. A new criterion is proposed to quantitatively classify the different flow regimes based on the averaged RMS drag/lift coefficients of the four cylinders in the array.

A combined digital image correlation/particle image velocimetry study of water-backed impact

Several aeronautical and naval structures are routinely exposed to impulsive loading conditions that elicit complex fluid-structure interactions. An archetypal problem that has attracted considerable attention is the impact on a compliant plate, fixed on a water surface. Despite significant progress in mathematical modeling of the impact, several technical questions remain open due to the lack of a simultaneous experimental characterization of the structural response and fluid flow. Here, we seek to fill this gap of knowledge through the integration of digital image correlation (DIC) and particle image velocimetry (PIV). We employ DIC to measure the apparent in-plane displacement of the plate, from which we reconstruct its out-of-plane deflection. On the other hand, PIV is utilized to measure the velocity field of the water, from which we infer the pressure field in the fluid and the hydrodynamic loading on the plate. We examine a number of aggregated measures, including the mid-span deflection, strain energy stored in bending and stretching of the plate, modal contribution factors, hydrodynamic loading, and added mass coefficient. Just as our approach constitutes a significant methodological step forward in the study of unsteady fluid-structure interactions, our experimental results contribute new evidence for an improved understanding of water-backed impact.

Effect of diameter ratio on the flow-induced vibrations of two rigidly coupled circular cylinders in tandem

We present an experimental study on the effects on the vortex-induced vibrations (VIV), of the relative size of two rigidly connected circular cylinders in a tandem arrangement, with a fixed centre-to-centre separation of 1.3D, where D represents the diameter of the upstream cylinder. This separation distance was selected after the results obtained by Jiménez-González and Huera-Huarte (2017), where a high VIV sensitivity region was identified at such location in the wake of a cylinder oscillating in cross-flow. The flow-induced response of the system, which has one degree of freedom, has been analysed for several values of the diameter ratio d∕D≤1, where d is the diameter of the downstream cylinder. As the value of d∕D increases, the VIV response is attenuated, it being associated with important reductions in the transverse and in-line force coefficients. In addition, Digital Particle Image Velocimetry (DPIV) has allowed us to study the changes in the near wake and forcing introduced by the presence of different downstream cylinders. Thus, the flow dynamics is clearly driven by the relative size of the cylinders, and therefore, the size of the gap region. In particular, for small values of d∕D, a moderate disruption in the vortex formation and shear layer interaction of the wake of the upstream cylinder takes place. For intermediate values of d∕D, the downstream cylinder generates a strong local wake that results in the shedding of two co-rotating vortices at each side of the system, while markedly undermining the shedding from the upstream cylinder. In the limit of d∕D=1, a stagnant region develops in the gap between the cylinders, and the tandem behaves as a single bluff body, featuring a very attenuated flow-induced response.

Experimental investigation of tip-leakage flow in an axial flow fan at various flow rates

We investigate the evolution of tip-leakage vortex (TLV) in a low-pressure axial flow fan using digital particle image velocimetry. The blade rotational speed is fixed at 1000 rpm, and the Reynolds number is 547000 based on the blade tip radius and tip velocity. The evolution of TLV in downstream is highly influenced by the incoming flow rate. The TLV breakup occurs for the peak efficiency and stall conditions and sometimes for a higher flow rate, but does not occur for the highest flow rate considered. Thus, the migration speed of the TLV for the peak efficiency condition is faster due to the TLV breakup than those for higher flow rates. The scatter plot of the TLV center location indicates that the TLV wanders around its mean location and the region swept by the wandering motion increases as the TLV migrates downstream. High turbulent kinetic energy exists at the phase-averaged TLV center and its upstream location, respectively, owing to the wandering motion and the interaction between the TLV and main axial flow.

A ctenophore (comb jelly) employs vortex rebound dynamics and outperforms other gelatinous swimmers.

Gelatinous zooplankton exhibit a wide range of propulsive swimming modes. One of the most energetically efficient is the rowing behaviour exhibited by many species of schyphomedusae, which employ vortex interactions to achieve this result. Ctenophores (comb jellies) typically use a slow swimming, cilia-based mode of propulsion. However, species within the genus Ocyropsis have developed an additional propulsive strategy of rowing the lobes, which are normally used for feeding, in order to rapidly escape from predators. In this study, we used high-speed digital particle image velocimetry to examine the kinematics and fluid dynamics of this rarely studied propulsive mechanism. This mechanism allows Ocyropsis to achieve size-adjusted speeds that are nearly double those of other large gelatinous swimmers. The investigation of the fluid dynamic basis of this escape mode reveals novel vortex interactions that have not previously been described for other biological propulsion systems. The arrangement of vortices during escape swimming produces a similar configuration and impact as that of the well-studied ‘vortex rebound’ phenomenon which occurs when a vortex ring approaches a solid wall. These results extend our understanding of how animals use vortex–vortex interactions and provide important insights that can inform the bioinspired engineering of propulsion systems.

Flow Characteristics for Two-Strand Tundish in Continuous Slab Casting Using PIV

With the development of continuous casting technology, there has been an increase in the stringent requirements for the cleanliness and quality of steel being produced. The flow state of molten steel in tundish is the key to: Optimizing the residence time of molten steel in the tundish; homogenizing the temperature of molten steel; and removing inclusions by floatation. Hence, from theoretical and practical aspects, it is imperative to examine and analyze the flow field of molten steel in the tundish in order to ensure the desired molten steel flow. In this study, a two-strand tundish with 650 mm × 180 mm slab casting is considered as the subject for this research. According to the similarity theory, combined with the geometrical shape and dimension of the prototype tundish, a tundish model with a geometric similarity ratio of 2:3 is established in the laboratory. Digital particle image velocimetry (PIV) is employed to measure and examine the flow fields at different casting speeds for a tundish containing different flow control devices. The flow in the tundish is typically turbulent and also consists of a vortex motion; it exhibits both random and ordered characteristics. Results reveal that the presence of baffles with 15° holes can cause an upward-directed flow in the outlet section and give rise to a large circulation. When the casting speed is doubled, the overall velocity of the flow field and turbulent intensity increase, leading to an increase in the molten steel surface velocity.

Aerodynamic effects of deviating motion of flapping wings in hovering flight

Nature’s flyers adopt deviating motion when they fly, however, the effect of deviating motion on the aerodynamics of flapping wings is not yet clearly understood. In this study, the aerodynamic characteristics of figure-of-eight deviating motion were investigated by comparing the aerodynamic force obtained from an experiment and the quasi-steady (QS) model in wide ranges of both the pitch and deviation amplitude. A flapping-wing robotic manipulator with a one-ton water tank was used in the experiment. Digital particle image velocimetry (DPIV) was also conducted in several chordwise cross-sections and different time instants. The results showed that the increment of deviating angle dramatically reduced the aerodynamic lift in the middle of the up- or downstroke from the quasi-steady estimation. The DPIV showed a vortex loop, which inhibits lift generation, at the middle of the up- or downstroke in deviating motion. The deviating motion delayed a development of the vortices, and more stroke angle was necessary for trailing edge vortex shedding. The QS model could not properly predict the aerodynamic force of the flapping wing with significant deviation angles. The error between the experiments and the QS estimation with regard to the aerodynamic lift was as high as 35% in the middle of the up- or downstroke with a high deviation amplitude (), reducing the cycle-averaged value of the lift coefficient up to 30%.

Roll Control of Low-Aspect-Ratio Wings Using Articulated Winglet Control Surfaces

The use of tip mounted winglets with independently variable cant angles was investigated as a means of roll control on wings with an aspect ratio of one. Wind-tunnel testing was performed in which a six-axis force balance was used to measure the total aerodynamic load on wings with winglet control surfaces. Stereoscopic digital particle image velocimetry of the near-wake plane was used to show how the topology of the tip vortices changed with winglet deflection. Shifts in the location of the right tip vortex core are considered to be responsible for roll moment generation because they indicate changes in the symmetry of suction-side flow structures. All winglet deflections were observed to shift the right tip vortex core inboard, and thereby shorten the effective span of the wing. The effect of a winglet deflection may be approximated as a change in the wing aspect ratio and a lateral shift in the wing aerodynamic center. Prandtl’s lifting line theory provides a closed-form estimate for the reduction in lift caused by a winglet deflection. A geometrical argument was made to account for the induced roll moment. The right tip vortex core also shifts vertically, following the deflected wing tip. Vertical shifts in the right tip vortex result in an angle between the wing span line and a line connecting the two tip vortices. A positive angle is defined as the right tip vortex higher over the wing than the left, and it is accompanied by a positive roll moment. While in sideslip, the wing with no winglet deflection experiences a considerable roll moment as a result of a vertical and lateral shift in the two tip vortices. The articulated winglets are observed to partially mitigate these effects when the upstream winglet is actuated, and thus show promise as a direct means of disturbance rejection.

Velocimetry based on dye visualization for a pulsatile tubing flow measurement.

A dye visualization experiment was designed and operated as an analogy of x-ray angiographic imaging. The velocimetry technique based on dye visualization for a pulsatile tubing flow measurement is introduced. The optical flow method was utilized to recover the velocity field from the visualization images. For the same flow condition, a digital particle image velocimetry (PIV) system was also employed to measure the same flow field in the middle plane. The purpose of this study was to determine the accuracy of the velocity field estimation from the transmittance-based two-dimensional projection image of the three-dimensional volumetric flow field by dye visualization in comparison with the PIV measurement results. Compared to the PIV results in the middle plane, the averaged velocity magnitude from the dye visualization measurement was underestimated by about 16%-24% in the central region and by about 29%-43% in the outer region across the tube at two time instants of the cyclic pulsatile flow.

Effect on louver dampers configuration on gas maldistribution in a heat exchanger for waste heat recovery

This paper reports the results of visualization research involving a heat exchanger applied for waste heat recovery using of Digital Particle Image Velocimetry (DPIV) method. Five louver dampers configurations responsible for typical operation modes were tested. As a consequence, airflow maldistribution in the tube bundle section and bypass section were identified and described. Based on the measurements and analysis carried out, the authors have developed a way to implement design changes to optimize gas distribution inside the unit.

Investigation of Half-Ring Vortices Generated by Half-Submerged Thin Circular Plate Using Digital Particle Image Velocimetry Method

A half-ring vortex near the free surface interface of water can be generated by dragging a half-submerged thin circular plate in the fluid and quickly pulling it out. This results in faster moving water in front of the plate rolling over the slower water next to the plate and creating a vortex. This vortex tends to preserve its momentum which results in the vortex continually spinning and moving in the dragging direction. To investigate the ring’s motions, the Digital Particle Image Velocimetry (DPIV) method is used to study the flow of the vortex. Glitter is used as tracking particles which follow the flow of the water. The relationships between the radii of the ring and the plate as well as the dragging and vortex speeds are derived. In addition, the vortex size and speed appear not to be constant over time. It can be deduced experimentally that this half-ring vortex behaves similarly to a full vortex ring generated by the motion of a fully submerged circular plate.