Particle tracking velocimetry in three-dimensional flows

The photogrammetric determination of three-dimensional particle coordinates from a 3-camera system is described in Part I. In Part II we describe a fully automated tracking scheme for the determination of a sequence of velocity vectors within a three-dimensional observation volume of a fluid flow. From this sequence long-time particle trajectories are reconstructed.The tracking scheme is tested on trajectories obtained using the Kinematic Simulation Inertial Model (KSIM). Estimates of the yield of links between adjacent data sets of particle positions and of the yield of long-time particle trajectories are obtained. The limits of efficient tracking as a function of the spacing-displacement ratio p = _o/ut are also obtained. The effect of noise, in the form of the apparent appearance and disappearance of particles between one image and the next, and of jitter, which is the error in the determination of particle coordinates, is examined. It is shown that noise reduces the number of links per frame, but does not increase the number of erroneous links which is always small. However, the yield of long trajectories drops sharply with increasing noise. A small level of jitter, on the other hand, does not significantly influence any of the results.The tracking scheme is used on two sets of particle coordinate data obtained from real flows: a non-turbulent flow in a small water tank and a turbulent open channel flow.     

Particle tracking velocimetry in three-dimensional flows

Particle Tracking Velocimetry (PTV) is a well-known technique for the determination of velocity vectors within an observation volume. However, for a long time it has rarely been applied because of the intensive effort necessary to measure coordinates of a large number of flow marker particles in many images. With today's imaging hardware in combination with the methods of digital image processing and digital photogrammetry, however, new possibilities have arisen for the design of completely automatic PTV systems. A powerful 3D PTV has been developed in a cooperation of the Institute of Geodesy and Photogrammetry with the Institute of Hydromechanics and Water Resources Management at the Swiss Federal Institute of Technology. In this paper hardware components for 3D PTV systems wil be discussed, and a strict mathematical model of photogrammetric 3D coordinate determination, taking into account the different refractive indices in the optical path, will be presented. The system described is capable of determining coordinate sets of some 1000 particles in a flow field at a time resolution of 25 datasets per second and almost arbitrary sequence length completely automatically after an initialization by an operator. The strict mathematical modelling of the measurement geometry, together with a thorough calibration of the system provide for a coordinate accuracy of typically 0.06 mm in X, Y and 0.18 mm in Z (depth coordinate) in a volume of 200 × 160 × 50 mm³.     

Stereoscopic particle image velocimetry applied to liquid flows

A twin-camera stereoscopic system has been developed to extend conventional high image-density Particle Image Velocimetry (PIV) to three-dimensional vectors on planar domains. The stereoscopic velocimeter performs with extremely high accuracy. Translation tests have yielded errors (rms) of 0.2% of full-scale for the in-plane displacement, and 0.8% of full-scale for the out-of-plane component, both of which agree with the errors predicted by an uncertainty analysis. In addition, modified techniques in hardware and software have enabled the stereoscopic system to perform successfully when acquiring images through a thick liquid layer, wherein previously the aberrations arising due to the liquid-air interface have restricted the use of such systems. With these techniques, the stereoscopic system, in combination with a simple method for image-shifting, is able to accurately measure threedimensional velocity fields in liquids. This is demonstrated by measurements of the helical, three-dimensional flow induced by a rotating disk in glycerine.     

Simultaneous velocimetry/accelerometry measurements in a turbulent two-phase pipe flow

A two-color digital particle image velocimetry and accelerometry (DPIV and DPIA) measurement technique is described that records the velocity and acceleration fields of both the solid and liquid phases simultaneously. Measurements were taken at turbulent conditions of a vertical pipe flow using glass spheres as the solid phase and fluorescent particles to indicate fluid phase motion. Nd-YAG pulse lasers acted as illumination sources and images were recorded by two monochrome CCD cameras. The two-color aspect of the technique was realized by placing optical filters in front of the cameras to discriminate between the phases. Cross-correlations and auto-correlations were applied to determine velocity and acceleration fields of the two phases. Results showing some of the capabilities of the technique as applied to a two-phase pipe flow experiment are provided. For the condition studied, it was found that there was turbulence suppression due to the solid phase and that the statistics associated with the acceleration probability distribution were different for the solid and fluid phases.     

Particle velocimetry in high-gradient/high-curvature flows

Particle-velocimetry techniques typically rely on the assumption that particle velocities match fluid velocities. However, this assumption may be invalid if external forces or inertia cause the particle motion to differ from that of the flow. In this paper, particle motion through premixed stagnation flames is modeled, including Stokes-drag and thermophoretic forces. The finite time interval employed in particle-tracking techniques can act as a low-pass filter in flow regions with large curvature in the velocity field. To account for this effect, the modeled-particle-tracking profile for a specified time interval is estimated from the predicted particle position in time and compared to measurements. The implementation presented here is applicable to other simulated flow fields and allows direct comparisons with particle-velocimetry measurements. Expressions are also derived that allow particle-tracking data to be corrected for these effects.     

Particle image velocimetry measurements in complex geometries

 One of the advantages of Particle Image Velocimetry (PIV) is its ability to determine the instantaneous flow field over two- or three-dimensional domains. Yet PIV has had limited application to complex flow passages because of the difficulty in replicating these geometries with optically transparent materials. In this work, we describe a method for overcoming this difficulty using rapid prototyping techniques. As an illustrative example, the technique has been used to characterize flow in a model of the human nasal cavity. Received: 12 January 1999/Accepted: 30 September 1999     

Particle tracking velocimetry with an ant colony optimization algorithm

A new concept algorithm based on the ant colony optimization is developed for the use in 2-D and 3-D particle tracking velocimetry (PTV). In the particle matching process of PTV, the ant colony optimization is usually aimed at minimization of the sum of the distances between the first-frame and second-frame particles. But this type of minimization often goes unsuccessfully in the regions where the particles are located very close to each other. In order to avoid this flaw, a new type of minimization is attempted using a physical property corresponding to the flow consistency or the quasi-rigidity of particle distribution patterns. Specifically, the ant colony optimization is now aimed at minimization of the sum of the relaxation of neighbor particles. In the present study, the new algorithm is applied to sets of 2-D and 3-D synthetic particle images as well as the experimental images with successful results.     

Particle image velocimetry （PIV） measurements of tip vortex wake structure of wind turbine

Large-view flow field measurements using the particle image velocimetry (PIV) technique with high resolution CCD cameras on a rotating 1/8 scale blade model of the NREL UAE phase VI wind turbine are conducted in the engineering-oriented Φ3.2 m wind tunnel.The motivation is to establish the database of the initiation and development of the tip vortex to study the flow structure and mechanism of the wind turbine.The results show that the tip vortex first moves inward for a very short period and then moves out...     

Particle tracking velocimetry beneath water waves. Part I: visualization and tracking algorithms

 A novel particle tracking system working with a high particle concentration for the measurement of flow fields beneath water waves is described. It features a 1–4 cm thick light sheet parallel to the main wave propagation direction so that the seeding particles stay long enough in the illuminated area to enable tracking over several wave periods. An area of up to 14.0×10.0 cm² is observed by a CCD camera with up to 200 fields/s. The polychromatic scattering theory of small particles in a light sheet illumination is investigated, enabling the segmentation of individual particles at high particle concentration. Received: 12 July 1995/Accepted: 18 April 1997     

Multiline hydroxyl tagging velocimetry measurements in reacting and nonreacting experimental flows

A compact micro-lens optical system is developed that produces a 7×7 multi-line optical grid for Hydroxyl Tagging Velocimetry (HTV) and generates at least 49 resolvable velocity vectors. Single-photon photodissociation of ground-state H₂O by a ~193-nm ArF excimer laser writes a 7×7 beam molecular grid with very long gridlines of superequilibrium OH and H photoproducts in either room air flowfields or in H₂-air flames due to the presence of H₂O vapor. The displaced OH tag line positions are revealed through fluorescence by A²⁺ (v=0)X²_i (v=0) OH excitation using a ~308-nm pulsed frequency-doubled dye laser. Time-of-flight analysis software determines the instantaneous velocity field in either an air nozzle or in a hydrogen/air flame. The OH tag lifetime is measured and compared to theoretical predictions using detailed chemistry. The lifetime of the OH tag is significantly enhanced by the presence of O atoms from 193-nm photodissociation of O₂.     

Study of vortex breakdown by particle tracking velocimetry (PTV)

Using a quasi three-dimensional instantaneous measurement technique, which combines particle tracking velocimetry (PTV) with volume scanning, first quantitative experimental results of the unsteady and asymmetric interior region of vortex breakdown were obtained. The study was carried out in a low speed flow through a cylindrical tube. A vortex was generated by a set of guidevanes and subjected to an adverse pressure gradient causing its breakdown. By scanning a pulsed illuminated planar laser light sheet, a set of meridional and azimuthal cuts of the flow was obtained. With PTV the recorded particle paths in the cuts were processed in order to obtain the instantaneous two-dimensional velocity field, mean streamlines and vorticity distribution. Moreover, the three-dimensional shape of the appearing breakdown, visualized with fluorescent dye, was reconstructed from the cuts. The results revealed that the shape of the bubble nearly equals the streamsurface of the stagnation point. According to the conditions in the water tunnel a single tilted vortex ring at the open rear part of the bubble dominates the interior flow structure of the bubble as first noted by Sarpkaya (1971). The vortical flow is bulged over the bubble, restored and intensified at the lower end. The gathered data lead to the conclusion that the vortex axis remains parallel to the centerline.     

Study of vortex breakdown by particle tracking velocimetry (PTV)

The spiral-type breakdown of a slender vortex was quantitatively investigated with PTV. Multiexposed pictures of the illuminated meridional midplane were processed to obtain the instantaneous 2-D velocity field and vorticity distribution. The periodic change of flow patterns with respect to time is clearly shown in a time-series of pictures. The 2-D velocity fields contained a stagnation point in the midplane located outside of the centerline. Additionally it was observed, that this point rotates around the centerline in the same way as the outer flow. A comparison with measurements of a bubble-type breakdown indicates a strong similarity to the spiral-type breakdown. The results reveal that the slope, winding and diameter of the spiral vortex-core determine the different observable forms. The first part of the deflected vortex-core near the breakdown point causes an asymmetric backflow due to induction, the strength of which depends on the slope of the deflected vortexcore. This is responsible for the radial distance between stagnation point and centerline. In case of the observed bubble-type breakdown the spiral is compressed which results in a stable stagnation point at the centerline.     

A multi-scale architecture for multi-scale simulation and its application to gas-solid flows

A multi-scale hardware and software architecture implementing the EMMS （energy-minimization multi-scale） paradigm is proven to be effective in the simulation of a two-dimensional gas-solid suspension. General purpose CPUs are employed for macro-scale control and optimization, and many integrated cores （MlCs） operating in multiple-instruction multiple-data mode are used for a molecular dynamics simulation of the solid particles at the meso-scale. Many cores operating in single-instruction multiple- data mode, such as general purpose graphics processing units （GPGPUs）, are employed for direct numerical simulation of the fluid flow at the micro-scale using the lattice Boltzmann method. This architecture is also expected to be efficient for the multi-scale simulation of other comolex systems.     

2-Dimensional particle tracking velocimetry (PTV): Technique and image processing algorithms

An automated particle track velocimeter (PTV) was constructed to determine the fluid velocity field in a transparent test section (an engine throttle body assembly is used as an example) by analyzing images of scattered light from hollow nylon particles as they move with the flow. The light from individual particles was imaged on an SIT vidicon at video rates, digitized on an FG100CD frame buffer board and analyzed automatically in an IBMPC/AT. The essence of this technique is a novel processing algorithm which converts particle track segments to flow velocity vectors at the rate of 2–3 images/minute with this hardware. An application of the technique to a gas phase flow ( 6.5 m/s) through this throttle body assembly is presented.     

Molecular Tagging Velocimetry (MTV) measurements in gas phase flows

 Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a “steady flow rig” model of an internal combustion engine. Received: 29 August 1997/Accepted: 16 September 1998     

Micro particle-image velocimetry of bead suspensions and blood flows

We present and discuss velocity profiles of microflows obtained by micro particle-image velocimetry in a transmission setup. We have measured suspensions of beads in water and on human blood, using the red blood cells as a natural particle seeding. The limitations imposed by our optical system on the spatial resolution normal to the focal plane, the so-called focal depth, have also been analyzed. The first direct observations of the influence of the focal depth on the observed velocity profiles are presented. Good agreement is obtained between observations and calculated profiles modified by the finite focal depth through a weight function.     

Extended glare point velocimetry and sizing for bubbly flows

A novel measuring technique for bubbly flows, named glare point velocimetry and sizing (GPVS), was developed in order to measure both bubble size and velocity with high accuracy in a 2D plane. This is accomplished by observing glare points in-focus under an observation angle of 96°. When a second laser-sheet is added, even higher accuracies are obtained and the relative refractive index of the bubble can be measured. It also allows non-spherical bubbles to be rejected and arbitrary angles to be used (e.g. 90°). The accuracy of the size and refractive index measurements was found to be within 1.3%.     

Flow tagging velocimetry in incompressible flow using photo-activated nonintrusive tracking of molecular motion (PHANTOMM)

We report the development of a new optical flow tagging velocimetry technique for hydrodynamic flows. The method utilizes highly water-soluble caged dye Photo-Activated Fluorophores (PAF's) which serve as fluorescent tracers, with essentially indefinite lifetime. Demonstration experiments are presented in a bench-top poiseulle flow and a 5,000 gallon water channel facility. Results of experiments designed to quantify critical optical characteristics of the caged dye PAF's are also presented, as is a comparison with other, similar, optical velocimetry approaches.The authors wish to acknowledge R. B. Miles and D. Nosenchuck for several stimulating discussions, and T. Frobose and P. Howard for providing technical support. The work was sponsored by ARPA-G. Jones, Technical Monitor, and the National Science Foundation.     

Endoscopic 2D particle image velocimetry (PIV) flow field measurements in IC engines

The paper describes camera and laser endoscopes designed for particle image velocimetry (PIV) applications like measurements in IC engines or turbomachinery. Endoscopic PIV measurements through 8-mm optical access on an IC engine are presented and compared with the measurements using standard optical access through a window.