Digital Particle Image Velocimetry Technique Research Articles

Three-Dimensional Flow Measurement by Digital Holographic Particle Image Velocimetry with Spatio-Temporal Derivative Method (Evaluation of the Measurement Accuracy by Numerical Simulation)

In this paper, we present a digital holographic particle image velocimetry (DHPIV) technique with a spatio-temporal derivative method for velocity measurement in 3D space and the results of evaluating on its measurement accuracy. In this technique, hologram patterns are observed as digital images using an electronic camera, such as CCD or CMOS, and image reconstruction is carried out on a personal computer. Since an in-line observation system is utilized in conventional digital holography, a numerically reconstructed image is considerably enlarged in the depth direction and its depth resolution is extremely low; hence, the measurement accuracy in the depth direction is inaccurate in a digital holographic measurement. To overcome this difficulty, we apply a spatio-temporal derivative method to this technique for the detection of particle displacement along the z-axis. In a numerical simulation, measurement accuracy is evaluated for a multi particle model and a cubic cavity flow model. Furthermore, we examine the effect of noise on displacement measurement accuracy for numerically constructed noisy hologram patterns.

A temporal PIV study of flame/obstacle generated vortex interactions within a semi-confined combustion chamber

Experimental data obtained using a new multiple-camera digital particle image velocimetry (PIV) technique are presented for the interaction between a propagating flame and the turbulent recirculating velocity field generated during flame–solid obstacle interaction. The interaction between the gas movement and the obstacle creates turbulence by vortex shedding and local wake recirculations. The presence of turbulence in a flammable gas mixture can wrinkle a flame front, increasing the flame surface area and enhancing the burning rate. To investigate propagating flame/turbulence interaction, a novel multiple-camera digital PIV technique was used to provide high spatial and temporal characterization of the phenomenon for the turbulent flow field in the wake of three sequential obstacles. The technique allowed the quantification of the local flame speed and local flow velocity. Due to the accelerating nature of the explosion flow field, the wake flows develop ‘transient’ turbulent fields. Multiple-camera PIV provides data to define the spatial and temporal variation of both the velocity field ahead of the propagating flame and the flame front to aid the understanding of flame–vortex interaction. Experimentally obtained values for flame displacement speed and flame stretch are presented for increasing vortex complexity.

An investigation of the effect of viscosity on mixing in an oscillatory baffled column using digital particle image velocimetry and computational fluid dynamics simulation

Abstract In this paper, we examine the effect of viscosity on the performance of the oscillatory baffled column (OBC) using the digital particle image velocimetry (DPIV) technique and computational fluid dynamics (CFD) codes. A selection of Newtonian (1–210 cP) and shear thinning fluids is investigated. The effects of viscosity on mixing are carefully observed by analysing flow patterns generated by both the DPIV and CFD methods. A ratio of the plane-averaged axial over the radial velocity is defined to quantify such the viscosity effects. For the given geometry the velocity ratio approaches to 2 very quickly at increased oscillatory Reynolds numbers, regardless of the Newtonian and non-Newtonian fluids used. An empirical critical ratio of 3.5 is identified, below which the system mixes sufficiently. This would act as the guide for industrial applications where a viscous fluid is mixed in an aqueous solution in the OBC.

Viscoelastic effects on interfacial dynamics in air–liquid displacement under gravity stabilization

We have investigated the role of liquid elasticity in the dynamics of air–liquid interfaces during immiscible fluid displacement flows. Our experimental studies of coating flows with gravity stabilization in an eccentric cylinder geometry for both a viscous Newtonian fluid and a series of elastic Boger fluids have uncovered two new elastically driven phenomena. First, elasticity is shown to create steady, two-dimensional ‘sharp interfaces’ under creeping flow conditions in forward-roll coating. Digital particle image velocimetry (DPIV) and DEVSS finite element techniques are applied to investigate the effect of gravity on both the flow field and the state of polymeric stresses near the free surface. Secondly, a new class of elastically driven stable substructures is shown to form and persist along the tip, i.e. the centre stagnation line, of the two-dimensional `sharp interfaces' even after the onset of ribbing.

Optical technique DPIV in measurements of granular material flows, Part 1 of 3—plane hoppers

The aim of this paper is to present the evolution of plug flow developing in three densely packed granular materials in the model of a hopper made of Plexiglas. For this purpose, the digital particle image velocimetry (DPIV) technique is applied to analyse the flow field of the granular material. When discharge starts, a plug flow zone in the flowing material expands upward. This zone changes its width reaching the upper surface of the material. The plug flow evolution as a function of time is described using DPIV. This technique yields the velocity profiles of flowing granular materials, velocity magnitude contours, vector fields, velocity distributions on certain levels in the model and traces of the selected particles. The results obtained for the evolution of the vertical velocity, height and width of the plug flow zone as a function of time, measured at the symmetry axis of the model for the amarantus seed are compared to the results obtained by Waters and Drescher. Measurements of the stagnant boundary as a function of time are compared to the results available in the literature.

Numerical and Experimental Investigations into The Effect of Gap Between Baffle and Wall on Mixing in an Oscillatory Baffled Column

We report our numerical and experimental investigation of the effect on mixing of implementing 0, 2, 4 and 6 mm gaps between the outer edge of baffles and the inner diameter of an Oscillatory Baffled Column (OBC). Large eddy simulation and digital particle image velocimetry techniques have been used to obtain the flow patterns and quantify the flow field and turbulence, such as the velocity ratio, integral length scale and mixing time scale versus the oscillatory Reynolds numbers. It is shown that the global mixing intensity and the turbulence length scale decrease, while the Kolmogorov time scale increases with increasing gap size in the range of experimental conditions tested in this work, which suggests a need for new configuration of OBC with baffle gap for certain applications involving particulates.

A method for three-dimensional particle sizing in two-phase flows

A method is devised for true three-dimensional (3D) particle sizing in two-phase systems. Based on a ray-optics approximation of the Mie scattering theory for spherical particles, and under given assumptions, the principle is applicable to intensity data from scatterers within arbitrary interrogation volumes. It requires knowledge of the particle 3D location and intensity, and of the spatial distribution of the incident light intensity throughout the measurement volume. The new methodology is particularly suited for Lagrangian measurements: we demonstrate its use with the defocusing digital particle image velocimetry technique, a 3D measurement technique that provides the location, intensity and velocity of particles in large volume domains. We provide a method to characterize the volumetric distribution of the incident illumination and we assess experimentally the size measurement uncertainty.

Velocity fields and streamline patterns of miscible displacements in cylindrical tubes

Displacements of a viscous fluid by a miscible fluid of a lesser viscosity and density in cylindrical tubes were investigated experimentally. Details of velocity and Stokes streamline fields in vertical tubes were measured using a DPIV (digital particle image velocimetry) technique. In a reference frame moving with the fingertip, the streamline patterns around the fingertip obtained from the present measurements confirm the hypothesis of Taylor (1961) for the external patterns, and that of Petitjeans and Maxworthy (1996) for the internal patterns. As discussed in these papers, the dependent variable, m, a measure of the volume of viscous fluid left on the tube wall after the passage of the displacing finger, is a parameter that determines the flow pattern. When m>0.5 there is one stagnation point at the tip of the finger; when m<0.5 there are two stagnation points on the centerline, one at the tip and the other inside the fingertip, and a stagnation ring on the finger surface with a toroidal recirculation in the fingertip between the two stagnation points. The finger profile is obtained from the zero streamline of the streamline pattern.

Mathematical simulation and physical modeling of unsteady fluid flows in a water model of a slab mold

Fluid flow of water in a model of a slab caster has been simulated using the large eddy simulation (LES) computational approach, and the simulated results are compared with experimental measurements performed using digital particle image velocimetry (DPIV) techniques. Simulation results agree acceptably well with the experimental measurements of instantaneous velocity fields. Flow patterns change with time as a consequence of the vertical oscillation of the jet core. These oscillations are originated by the residual Reynolds stresses that characterize turbulent flows. The asymmetry of fluid flows caused by these stresses provides biased flows. Thus, turbulence originates natural biasing effects without the influence of other operating factors such as the slide gate opening, gas bubbling, or inclusions clogging of the submerged entry nozzle (SEN). Instantaneous velocities follow periodical behaviors with time whose frequencies increase with increases of flow rate of liquid. Periodical flow changes originate velocity spikes, at some given casting speed, which are physically and mathematically identified. These sudden changes of fluid velocities are responsible of unsteady phenomena associated with fluid dynamics during steady operations of the mold.

A flow field study of an elliptic jet in cross flow using DPIV technique

The digital particle image velocimetry (DPIV) technique has been used to investigate the flow fields of an elliptic jet in cross flow (EJICF). Two different jet orientations are considered; one with the major axis of the ellipse aligned with the cross flow (henceforth referred to as a low aspect ratio (AR) jet), and the other with the major axis normal to the cross flow (henceforth referred to as a high aspect ratio jet). Results show that the vortex-pairing phenomenon is prevalent in the low aspect ratio jet when the velocity ratio (VR)≥3, and is absent in the high aspect ratio jet regardless of the velocity ratio. The presence of vortex pairing leads to a substantial increase in the leading-edge peak vorticity compared to the lee-side vorticity, which suggests that vortex pairing may play an important role in the entrainment of ambient fluid into the jet body, at least in the near-field region. In the absence of vortex pairing, both the leading-edge and the lee-side peak vorticity increase monotonically with velocity ratio regardless of the aspect ratio. Moreover, time-averaged velocity fields for both AR=0.5 and AR=2 jets reveal the existence of an “unstable focus” (UF) downstream of the jet, at least for VR≥2. The strength and the location of this focus is a function of both the velocity ratio and aspect ratio. In addition, time-averaged vorticity fields show a consistently higher peak-averaged vorticity in the low aspect ratio jet than in the high aspect ratio jet. This behavior could be due to a higher curvature of the vortex filament facing the cross flow in the low aspect ratio jet, which through mutual interaction may lead to higher vortex stretching, and therefore higher peak-averaged vorticity.

A DPIV Study of Liquid Steel Flow in a Wide Thin Slab Caster Using Four Ports Submerged Entry Nozzles

Influence of SEN design and its depth below the meniscus on liquid steel flow inside a large width thin slab mold was studied using a 1:1 scale water model. Fluid flow characterization was performed through Digital Particle Image Velocimetry techniques, measurements of bath oscillations and tracer dispersion experiments. Two designs of SEN with four ports (two lateral ports with two bottom ports) were investigated. SEN-1 has a wider bottom base and SEN-2 has a narrow one. Both SEN's provide fluid flows in the mold with upper and lower recirculating flows. Jets emerging from the lateral and bottom ports do not overlap although the surface area of the lateral jet of SEN-1 works inefficiently as compared with that of SEN-2. SEN-1 promotes higher bath oscillations than SEN-2 because the lateral jets travel almost horizontally toward the narrow face. This enhances fluid velocities along the narrow wall until the bath surface. A recirculating flow is formed close to the SEN-1 in the upper bath surface that provokes also high bath oscillations. Design of SEN-2 avoids these flow defects yielding more stable fluid flows. Rising the SEN depths alters negatively the flow characteristics, although SEN-2 is better than SEN-1. Generally speaking both designs yield complex turbulent and unsteady flows and a better SEN design is suggested.

Radial velocities in axisymmetric jets and plumes

The results of two independent experimental investigations into the radial velocities of jets and plumes are reported. Micro Acoustic Doppler and Digital Particle Image Velocimetry techniques are employed to make the measurements. The radial velocity data shows good agreement with integral model predictions. Other mean and fluctuating statistics are generally consistent with previous results, although it is notable that the axial fluctuation statistics are generally higher when compared to previously reported data.

Experimental study on flow characteristics of a sleeved jet into a main crossflow

AbstractExperiments were carried out on the hydraulic mechanism of the thermal shock caused by cold jet injection at a T‐junction with thermal sleeve in the reactor cooling system using digital particle imaging velocimetry (DPIV) technique to measure the flow in the main duct and in the annular space of the sleeve tube. The flow and vorticity characteristics were investigated at jet‐to‐crossflow velocity ratios of 0.5 to 4. There was a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which resulted in decreasing the influence of the jet on the downstream wall. The intensive vorticity in the near wake mainly originated from the shear layer vorticity of the jet and the annular discharge stream. The intensive vorticity soon broke down and dissipated, and further developed into the counterrotating vortex pair in the far wake. The flow in the annulus was closely dependent on R, and thermal protection of the sleeve would become evident at higher R. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 24–31, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10131

Kinetic energy in grid turbulence: comparison between data and theory

The properties of turbulence induced in a viscous fluid by oscillating a grid within it are investigated. Vertical and horizontal components of fluctuating velocities are measured using the Digital Particle Image Velocimetry technique (DPIV). Vertical profiles of turbulent kinetic energy k, obtained from the fluctuating velocities, are presented and compared with theoretical predictions obtained using the k-epsilon turbulence model.

The role of free-surface turbulence and surfactants in air–water gas transfer

Laboratory measurements of gas-transfer velocity and free-surface hydrodynamics were made for oscillating grid-stirred turbulence. Air–water gas-transfer velocities were determined from the waterside dissolved oxygen mass balance, and an innovative digital particle image velocimetry technique was used to measure the two-dimensional free-surface flow field. The impact of adventitious and purposely introduced surfactant films on free-surface turbulence and gas-transfer rates was explored quantitatively. As expected, bulk turbulence was unable to provide a unique relationship for the gas-transfer rates observed, owing predominantly to surfactant effects. However, the surface divergence computed from the free-surface velocity field was capable of reconciling the gas-transfer data. This is possible evidence confirming that the free-surface divergence is an important process involved in interfacial gas transport. A relationship for the gas-transfer velocity in terms of the surface divergence and the surface pressure is presented that successfully relates the interactions between surface renewal, surfactants, and gas transfer.

Evaluation of Turbulent Integral Length Scale in an Oscillatory Baffled Column Using Large Eddy Simulation and Digital Particle Image Velocimetry

In phase-separated synthesis, two fundamental factors control the formation of droplets in a reactor system: the turbulent integral length scale and the rate of droplet breakage and coalescence. In this paper, we report the evaluation and validation of the former in an oscillatory baffled column (OBC) using large eddy simulation (LES) and digital particle image velocimetry (DPIV), respectively. In LES the Navier–Stokes equations are spatially filtered, which eliminates eddies whose scales are smaller than a filter width. Large eddies are directly resolved, while small eddies are modelled using sub-grid scale models that describe the interactions between the large eddies and the unresolved smaller scales. Eddies are the essential ingredient for mixing in OBC and the methodology of the LES is particularly suited for flows in OBC. In parallel to the LES work, a high-resolution DPIV is also employed. Since it works as a low-pass filtering system, which is analogous to the spacious filtering in LES, it is therefore possible to validate the numerical simulation using the DPIV technique. The results show that the turbulent integral length scale in the OBC is of an order around about 10 –3 m, and the Kolmogorov's time scale of a range from 0.2 to 24 ms depending on the oscillatory Reynolds numbers. Such information is vitally important in controlling chemical processes whose reaction time has an equal or similar value as the time scales shown here. We believe that the drop formation and subsequently size distribution in the OBC are also affected by these scales.

A simultaneous PIV and heat transfer study of bubble interaction with free convection flow

Whole field velocity and point temperature and surface heat flux measurements were performed to characterise the interaction of a single rising ellipsoidal air bubble with the free convection flow from a heated flat surface immersed in water at different angles of inclination. Two thermocouples and a hot film sensor were used to characterise heat transfer from the surface, while a time-resolved digital particle image velocimetry technique was used to map the bubble induced flow in a plane parallel to the surface. Heat flux fluctuations, preceding and following the bubble passage, were shown to correlate with the variation in both local flow velocities and fluid temperatures. The largest increases in heat transfer were recorded when both flow and temperature effects combined to enhance the convective cooling simultaneously. Such conditions were shown to be most likely met when the block was inclined at 45°, thus forcing the bubble to slide closer to the heated surface and hence to the thermal boundary layer.

On the interaction of a vertical shear layer with a free surface

New experiments have been conducted using a combined free-surface gradient detector (FSGD) and digital particle image velocimetry (DPIV) technique to study the interaction between a vertical shear layer, created by a surface-piercing splitter plate, and a free surface. The emphasis of this study is on understanding aspects of the interaction between the free-surface deformation (FSD) and the near-surface turbulence through the correlations between the elevation and the vorticity fields, and the spectral behaviour of the near-surface pressure. The Reynolds number of the present study, based on visual thickness and the velocity average of the two streams, is 12 100. Mean results for the velocity and vorticity fields show that self-similarity is achieved. Instantaneous data sets show that at the free surface, vortex tubes within the main rollers connect normally with the free surface as is evidenced by strong vorticity as well as the strong deformations at the free surface. The instantaneous data sets also show that the streamwise vortices near the braid regions, while weaker than those seen in the main rollers, also reconnect with the free surface. Statistical analyses show that the FSD is strongly correlated with the near-surface vorticity field, as the correlation coefficients are quite high ( (George et al. 1984) found for a shear layer in unbounded flow.

Turbulence measurements on a bunsen burner inserted with perforated plugs of different hole sizes using DPIV

The digital particle imaging velocimetry (DPIV) technique is applied to measure simultaneously flow velocity and the turbulence integral length scale on a Bunsen burner of 36 mm in diameter. A perforated plug is inserted at 45 mm upstream of the burner exit for turbulence adjustment. The plug consists of a hexagonal array of circular holes at a diameter of 2, 4, or 6 mm. The opening of these three plugs is controlled to be approximately the same at 55 % of the plug surface area. Radial profiles of the mean and rms velocities, and the longitudinal and transverse integral length scales are reported at 40 mm downstream of the burner exit. The turbulence properties are found similar for the 4- and 6-mm plugs with the decay of the centreline rms velocity following a power–law relationship expected for conventional grid-generated turbulence. Unstable reattachment of the flow is likely to occur in the holes of the relatively thick 2-mm plug and may interact with the downstream flow development. The integral length scales do not show a clear dependency on the hole diameter of the plugs. This is likely due to low grid Reynolds numbers as well as effects of intense shear-layer turbulence, much higher than generated by the plugs. Further improvement on the current plug design as a turbulence regulator is suggested for future use in studying low-turbulence premixed flames.

Quantitative flow visualization: toward a comprehensive flow diagnostic tool.

Quantitative flow visualization has many roots and has taken several approaches. The advent of digital image processing has made it practical to extract useful information from every kind of flow image. In a direct approach, the image intensity or color (wavelength or frequency) can be used as an indication of concentration, density and temperature field, or gradients of these scalar fields in the flow.(1) For whole-field velocity measurement, the method of choice for experimental fluid mechanicians has been digital particle image velocimetry (DPIV). This paper presents a novel approach to extend the DPIV technique from a planar method to a full three-dimensional volume mapping technique.