Digital Particle Image Velocimetry Research Articles

The development of an automated PIV image processing software—SmartPIV

Since the popularity of digital particle image velocimetry technique (DPIV), many PIV image processing algorithms have been proposed. Amongst them, fast Fourier transform (FFT) Cross Correlation, Discrete Window Offset Cross Correlation, Iterative Multigrid Cross Correlation, Iterative Image Deformation Cross Correlation and cross correlation based particle tracking methods are widely used algorithms and have been extensively studied by researchers. All of these algorithms have their advantages and disadvantages in terms of computational load and measurement accuracy. To choose a suitable algorithm, researchers not only need to understand the complex principles of these algorithms, but also need to find out their applicable flow conditions. This could greatly increase work load for PIV users who focus more on flow structure itself instead of PIV algorithms. It is therefore necessary to develop a method which can choose PIV algorithms wisely according to the input PIV images. This paper firstly reviews the development of PIV algorithm with mainly focus on analysing advantages and disadvantages of six widely used algorithms. By using both synthetic and real PIV images, comparative studies are then carried out among these algorithms. The tests give a rate for the performance of the algorithms and provide a parameter to automatically separate pattern match and particle tracking algorithms. Based on qualitative and quantitative analysis, an automated PIV image processing method—SmartPIV is proposed and tested by both synthetic and real PIV images. For all the three test cases, the SmartPIV successfully picks the most suitable algorithm and gives very promising results.

Comparison of macroturbulence measured using decomposition of PIV, ADV and LSPIV data

Visualization and length-scale estimates are provided for turbulence in open channel flow over a gravel-bed. Results obtained using the digital particle image velocimetry (PIV) followed by post-processing with proper orthogonal decomposition showed higher and lower velocity regions aligned along a stagnation plane extending from the bed at approximately a 20° angle. Periodicity of second and fourth quadrant events equalled 0.20 s and agreed well with the outer variable defined bursting period. Normalized turbulence length scales were measured after performing the decomposition of the turbulent fields collected with various instrumentation techniques. Acoustic Doppler velocimetry and PIV had turbulence length scales equal to the flow depth, and the good agreement highlights the ability of the former to estimate length scales in streams. Large scale PIV provided length scales equal to 1.5 times the flow depth, but care should be used if directly inferring scales of macroturbulence from the free-surface.

Correlation of fluid refractive index with calibration coefficient for micro-defocusing digital particle image velocimetry

Using the 3D micro-defocusing digital particle image velocimetry (μ-DDPIV) method, Yoon and Kim (2006 Meas. Sci. Technol. 17 2897–905) found that an experimental calibration was necessary to correlate the particle images with corresponding depthwise particle locations for a specified test fluid. Furthermore, the calibration needed to be repeated for different fluids with different refractive index (RI) values. In an effort to alleviate the cumbersome requirement of repeated calibrations, this note presents an experimentally extrapolated correlation between the fluid RI and the calibration coefficient (∂z/∂D) that can now be used to apply the μ-DDPIV technique for a wide range of fluids without requiring individual calibrations.

Studies of C 4 and C 10 methyl ester flames

The oxidation of three model biodiesel fuels, namely methyl butanoate (C 5H 10O 2, CAS No. 623-42-7), methyl crotonate (C 5H 8O 2, CAS No. 623-43-8), and methyl decanoate (C 11H 22O 2, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally.

Aerodynamics of gliding flight in common swifts

Gliding flight performance and wake topology of a common swift (Apus apus L.) were examined in a wind tunnel at speeds between 7 and 11 m s(-1). The tunnel was tilted to simulate descending flight at different sink speeds. The swift varied its wingspan, wing area and tail span over the speed range. Wingspan decreased linearly with speed, whereas tail span decreased in a nonlinear manner. For each airspeed, the minimum glide angle was found. The corresponding sink speeds showed a curvilinear relationship with airspeed, with a minimum sink speed at 8.1 m s(-1) and a speed of best glide at 9.4 m s(-1). Lift-to-drag ratio was calculated for each airspeed and tilt angle combinations and the maximum for each speed showed a curvilinear relationship with airspeed, with a maximum of 12.5 at an airspeed of 9.5 m s(-1). Wake was sampled in the transverse plane using stereo digital particle image velocimetry (DPIV). The main structures of the wake were a pair of trailing wingtip vortices and a pair of trailing tail vortices. Circulation of these was measured and a model was constructed that showed good weight support. Parasite drag was estimated from the wake defect measured in the wake behind the body. Parasite drag coefficient ranged from 0.30 to 0.22 over the range of airspeeds. Induced drag was calculated and used to estimate profile drag coefficient, which was found to be in the same range as that previously measured on a Harris' hawk.

Validation of CFD and Thermal Hydraulics Codes by Digital Particle Image Velocimetry

Abstract Experimental validation of thermal hydraulics and CFD codes of scaled down models and full models of Nuclear Reactor components is very important for reactor system designers and reactor safety experts for optimal design and providing best safety features of the reactor systems. Over the period of time several techniques have been developed for this purpose. Among them Digital Part icle Image Velocimetry (DPIV), a laser based optical technique, is the recent, innovative, promising and flexible enough one so that it can be adapted to a large variety flows. By this technique test time is reduced significantly. DPIV is a non-intrusive optical measurement technique, which allows capturing several thousand velocity vectors within large flow fields instantaneously. DPIV technique is being used in very wide and different applications, from micro flows over combustion to supersonic flows for both industrial needs and research. 2-D DPIV facility having non-intrusive, instantaneous flow visualization with high spatial and temporal resolution features has been developed. It is used for measurement of flow velocity vectors at many (e.g. thousands) points in a flow field simultaneously and it provides instantaneous vector fields and displays velocity vectors in rea l-t ime. DPIV combines flow visualization with accurate quantitative velocity data. 2-D Velocity vectors are obtained by analysis of displacement of tracer particles on images in a known time. The development of visualizat ion tools are powerful enough to at tack the complexity of a resulting flow field but also flexible enough so that they can be adapted to a large variety of different flow. DPIV system has Laser light source, Laser beam delivery system, High speed camera interface, DPIV image acquisition & analysis software, CCD camera, computer controlled synchronizer, seeding particles & PC. This system will enhance our capability of thermal hydraulics, neutronics and mult iphysics modelling by experimental support & actual measurement of parameters, in addition to codes we had with us. Application of DPIV is must for the flow in the separation and reattachment regions where it is directly related to drag and heat transfer coefficients and for Vortex shedding. This facility is being used for basic research of code verification, code development, design optimizations and safety analysis.

Vortex and wake-induced vibrations of a tandem arrangement of two flexible circular cylinders with near wake interference

Results showing the dynamic response of a tandem arrangement of two vertical high aspect ratio (length over diameter) and low mass ratio (mass over mass of displaced fluid) flexible cylinders vibrating at low mode number are presented in this paper. Two circular cylinder models were aligned with the flow, so the downstream or trailing cylinder was immersed in the wake of the leading one. Centre-to-centre distances from 2 to 4 diameters were studied. The models were very similar in design, with external diameters of 16 mm and a total length of 1.5 m. Reynolds numbers up to 12 000 were achieved with reduced velocities, based on the fundamental natural frequency of the downstream cylinder in still water, up to 16. The trailing model had a mass ratio of 1.8 with a combined mass-damping parameter of 0.049, whilst the corresponding figures for the leading cylinder were 1.45 and 0.043, respectively. The dynamic response of the trailing model has been analysed by studying cross-flow and in-line amplitudes, dominant frequencies and modal amplitudes. The dynamic response of the leading one is analysed by means of its cross-flow amplitudes and dominant frequencies and it is also related to the motion of the trailing cylinder by studying the synchronisation between their instantaneous cross-flow motions. Planar digital particle image velocimetry (DPIV) was used to visualise the wake. Different response regimes have been identified based on the type of oscillations exhibited by the cylinders: vortex-induced (VIV), wake-induced (WIV) or combinations of both.

Vortex-enhanced propulsion

It has been previously suggested that the generation of coherent vortical structures in the near-wake of a self-propelled vehicle can improve its propulsive efficiency by manipulating the local pressure field and entrainment kinematics. This paper investigates these unsteady mechanisms analytically and in experiments. A self-propelled underwater vehicle is designed with the capability to operate using either steady-jet propulsion or a pulsed-jet mode that features the roll-up of large-scale vortex rings in the near-wake. The flow field is characterized by using a combination of planar laser-induced fluorescence, laser Doppler velocimetry and digital particle-image velocimetry. These tools enable measurement of vortex dynamics and entrainment during propulsion. The concept of vortex added-mass is used to deduce the local pressure field at the jet exit as a function of the shape and motion of the forming vortex rings. The propulsive efficiency of the vehicle is computed with the aid of towing experiments to quantify hydrodynamic drag. Finally, the overall vehicle efficiency is determined by monitoring the electrical power consumed by the vehicle in steady and unsteady propulsion modes. This measurement identifies conditions under which the power required to create flow unsteadiness is offset by the improved vehicle efficiency. The experiments demonstrate that substantial increases in propulsive efficiency, over 50 % greater than the performance of the steady-jet mode, can be achieved by using vortex formation to manipulate the near-wake properties. At higher vehicle speeds, the enhanced performance is sufficient to offset the energy cost of generating flow unsteadiness. An analytical model explains this enhanced performance in terms of the vortex added-mass and entrainment. The results suggest a potential mechanism to further enhance the performance of existing engineered propulsion systems. In addition, the analytical methods described here can be extended to examine more complex propulsion systems such as those of swimming and flying animals, for whom vortex formation is inevitable.

Aquatic manoeuvering with counter-propagating waves: a novel locomotive strategy

Many aquatic organisms swim by means of an undulating fin. These undulations often form a single wave travelling from one end of the fin to the other. However, when these aquatic animals are holding station or hovering, there is often a travelling wave from the head to the tail, and another moving from the tail to the head, meeting in the middle of the fin. Our study uses a biomimetic fish robot and computational fluid dynamics on a model of a real fish to uncover the mechanics of these inward counter-propagating waves. In addition, we compare the flow structure and upward force generated by inward counter-propagating waves to standing waves, unidirectional waves, and outward counter-propagating waves (i.e. one wave travelling from the middle of the fin to the head, and another wave travelling from the middle of the fin to the tail). Using digital particle image velocimetry to capture the flow structure around the fish robot, and computational fluid dynamics, we show that inward counter-propagating waves generate a clear mushroom-cloud-like flow structure with an inverted jet. The two streams of fluid set up by the two travelling waves 'collide' together (forming the mushroom cap) and collect into a narrow jet away from the cap (the mushroom stem). The reaction force from this jet acts to push the body in the opposite direction to the jet, perpendicular to the direction of movement provided by a single travelling wave. This downward jet provides a substantial increase in the perpendicular force when compared with the other types of fin actuation. Animals can thereby move upward if the fin is along the bottom midline of the body (or downward if on top); or left-right if the fins are along the lateral margins. In addition to illuminating how a large number of undulatory swimmers can use elongated fins to move in unexpected directions, the phenomenon of counter-propagating waves provides novel motion capabilities for systems using robotic undulators, an emerging technology for propelling underwater vehicles.

Time-resolved vortex wake of a common swift flying over a range of flight speeds

The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s(-1). The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of 'wing root vortices' are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings.

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n -decane and n -dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O 2 /N 2 and S-8/O 2 /N 2 mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.

The effect of fin ray flexural rigidity on the propulsive forces generated by a biorobotic fish pectoral fin

A biorobotic pectoral fin was developed and used to study how the flexural rigidities of fin rays within a highly deformable fish fin affect the fin's propulsive forces. The design of the biorobotic fin was based on a detailed analysis of the pectoral fin of the bluegill sunfish (Lepomis macrochirus). The biorobotic fin was made to execute the kinematics used by the biological fin during steady swimming, and to have structural properties that modeled those of the biological fin. This resulted in an engineered fin that had a similar interaction with the water as the biological fin and that created close approximations of the three-dimensional motions, flows, and forces produced by the sunfish during low speed, steady swimming. Experimental trials were conducted during which biorobotic fins of seven different stiffness configurations were flapped at frequencies from 0.5 to 2.0 Hz in flows with velocities that ranged from 0 to 270 mm s(-1). During these trials, thrust and lift forces were measured, kinematics were recorded in three dimensions, and digital particle image velocimetry was used to evaluate flow hydrodynamics. The results of the trials revealed that slight changes to the fin's mechanical properties or to the operating conditions can have significant impact on the direction, magnitude and time course of the propulsive forces. In general, the magnitude of the 2-D (thrust and lift) propulsive force scaled with fin ray stiffness, and increased as the fin's flapping speed increased or as the velocity of the flow decreased.

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.

Measurements in the near-wall region of a boundary layer over a wall with large transverse curvature

Measurements of the near-wall velocity field of the flow over cylinders aligned with a uniform flow are presented. The broader objective of this investigation was to quantify and understand the role of transverse curvature in the limit as cylinder diameter approaches zero. The specific goal was to begin with a turbulent boundary layer over a larger radius cylinder and see what happens as the radius is reduced. Spatially and temporally resolved digital particle image velocimetry (DPIV) measurements were made on three different radius cylinders, 0.14 cm ≤a≤ 3.05 cm, extending along the length of a large free-surface water tunnel. Mean and fluctuating profiles are presented at a fixed streamwise location and free-stream speed. For the first time, spatially resolved measurements were made very close to the wall, permitting direct determination of wall shear stress, i.e.uτ, from near-wall velocity profiles. The measurements revealed a region close to the wall for small radii where the mean streamwise velocity profile is inflectional. This has significant implications on assumptions regarding what happens in the limit of a vanishing cylinder radius.

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.

Two-phase interactions through turbulent events as described by fluid–particle correlations

An experimental investigation of a vertical upward, two-phase pipe flow was undertaken to measure kinematic parameters of the fluid and solid phases. The kinematic parameters included Reynolds stress distributions based on quadrant analyses that provided insight in understanding the behavior of two-phase kinematic correlation profiles. The data collected was based on a two-color digital particle image velocimetry (DPIV) technique that simultaneously measured the velocity fields of the fluid and solid phases. From quadrant analysis results, differences in Reynolds stress quadrant profiles between the single- and two-phase conditions were observed near the wall in the range 0.8> r/R>0.55, corresponding to wall distances between 35 and 75 viscous lengths ( y+). Correlation coefficients between the two phases were then calculated, using the fluctuating velocity components of each phase. The extent of the interaction between the two phases was tracked by the changing correlation values versus distance from the wall. The correlation of the fluid and solid phase velocities was highest in the core region of the pipe ( y+∼120), where the effect of turbulent events is reduced; low correlation coefficient values were found at y+<75, where differences of magnitudes, inflection points, etc. of burst and sweep event quadrant analysis profiles were observed. The extent of the influence of wall dynamic turbulent events on the solid phase was observed both by the differences in the relative Reynolds stress quadrant profiles and, more readily, by the changing values of two-phase axial and radial correlation coefficients determined from the simultaneous fluid and solid fluctuating velocities measured by the two-color DPIV methodology. These changing values of the correlation coefficients across the tube reflect the different responses of low inertia (fluid tracers) and high inertia (solid phase glass spheres) particles to the turbulent events. Similar profiles of the axial and radial correlation coefficients were observed, indicating that for the geometry and flow conditions considered, one velocity component of each phase was sufficient to track the spatial extent of turbulent event effects and their interactions with the fluid and solid phases. It is found that the two-color DPIV methodology and two-phase correlation results can give critical insight into the performance of thermal-fluid processes, as burst and sweep events have a large impact on the kinematics and dynamics of particles in the two-phase flow.

Experimental Study of a Starting Vortex Ring Generated by a Thin Circular Disk

The formation process of vortex ring generated by a thin circular disk was studied experimentally in this paper. A thin circular disk installed a linear motion stage was used to generate the vortex rings. Digital Particle Image Velocimetry (DPIV) was used to measure the velocity and vorticity fields. The finite-time Lyapunov exponent field corresponding to the vortex flow was computed to identify Lagrangian coherent structures of the starting vortex. The results reveal the existence of a flux window between repelling Lagrangian Coherent Structures (rLCS) and attracting Lagrangian Coherent Structures (aLCS), through which the shear flow is entrained into the vortex. The flux window is closed gradually during the starting process. Once the flux window shut down, the formation process of the vortex ring finishes, as the shear flow with vorticity cannot be entrained in the vortex ring.

Effect of a constant radial temperature gradient on a Taylor–Couette flow with axial wall slits

The flow between two concentric cylinders with the inner cylinder rotating with an imposed radial temperature gradient was studied using the digital particle image velocimetry method. Four models of a stationary outer cylinder without and with different numbers of slits (6, 9 and 18) were used. The Grashof number in this study was 1000. The results showed clearly that the buoyant force due to the temperature gradient caused the helical flow; this flow occurred when the Richardson number was larger than 0.0045. The constant temperature gradient considered in this research has little effect on the transition to a turbulent Taylor vortex flow in all models.

Low Reynolds number Couette flow facility for drag measurements

For this study a new low Reynolds number Couette facility was constructed to investigate surface drag. In this facility, mineral oil was used as the working fluid to increase the shear stress across the surface of the experimental models. A mounted conveyor inside a tank creates a flow above which an experimental model of a flat plate was suspended. The experimental plate was attached to linear bearings on a slide system that connects to a force gauge used to measure the drag. Within the gap between the model and moving belt a Couette flow with a linear velocity profile was created. Digital particle image velocimetry was used to confirm the velocity profile. The drag measurements agreed within 5% of the theoretically predicted Couette flow value.