Two-color Particle Image Velocimetry Research Articles

Injection of single and multiple vortices in an opposed-jet burner

A thorough understanding of turbulent reacting flows is essential to the continued development of practical combustion systems. Combustor codes can be validated using data such as those generated in this study of a vortex interacting with a nonpremixed, opposed-jet hydrogen-air flame. When experimental results are compared with model predictions, the underlying flowfield must be matched carefully. Since the vortex-injection process used in the present experiments can result in many types of vortices, including multiple vortices, restrictions on the experimental operation of the burner are required as well as careful vortex characterization. Vortex-characterization data are acquired using digital, two-color particle-image velocimetry (PIV), and the hydroxyl (OH) layer produced by the flame is imaged using planar laser-induced fluorescence (PLIF). The PIV and OH PLIF measurements are performed simultaneously. Good agreement with previous numerical-modeling predictions is obtained when experiments and computations are performed using similar vortex conditions.

Optical Alignment Tolerances and Techniques for Particle Image Velocimetry

Toensuregoodparticleimagevelocimetrycorrelations,itisnecessarytocharacterizetheimportantvariablesthat control image acquisition and image analysis. Among these, image spot size as it changes across the entire imaging surfacebearsinvestigation.Therefore, wedescribeanerroranalysisforsubpixel resolutionofcorrelation peaksus- ingWhittaker' smethod.Theseresultsseta toleranceonoptimumimagespotsizes.Typicallensesarethenanalyzed usingray-tracesoftware,toascertainwhatopticalparametersandcameramisalignmentsareallowablewithinthese optimum tolerances. We then describe an alignment procedure that allows us to set a camera to those tolerances. O acquire high-quality particle image velocimetry (PIV) data, it is necessary to use a cross-correlation technique, for exam- ple, two-color or image-shifted PIV, to align carefully the optical system and use an optimized data analysis package. In recent work performed at the Colorado School of Mines (CSM), two PIV inves- tigations resultedin1 )an assessmentofthe effects ofparticleimage spot size on correlation accuracy, 2 ) determination of the minimum spot size for least error, and 3 ) the development of an experimental system that can reliably generate this minimum spot size. One of these projects involved the development of PIV analy- sis software that generates vector data from PIV images (Ref. 1; T. Drouillard and M. Linne, Experiments in Fluids, manuscript in preparation ). In that work, several techniques for resolving cor- relation peaks were evaluated. These included the basic centroid method, Whittaker' s reconstruction, an analytical three-pointGaus- sian e t, and a numerical e ve-point Gaussian e t using the Marquardt method. An assessment of reconstruction errors was included. The least error was produced by the e ve-point Gaussian e t using Mar- quardt nonlinear e tting, whereas Whittaker' s reconstruction was a close second. Furthermore, it was found that the size of the cor- relation peak (directly related to particle image spot size ) has the greatest effect on the accuracy of the reconstruction algorithm. This sets a tolerance on the image spot size. That work is discussed in detail by Drouillard and Linne (manuscript in preparation ). Asecond projectinvolvedthe development of a method by which a PIV experimental apparatus may beconsistently and reproducibly aligned to meet this tolerance on spot size. 2 This topic is of primary importance to chemical e lm-based techniques, where turnaround time is very long. It also has implications for digital systems, espe- cially when it is necessary to align a camera across a large system, from within a wind tunnel, for example. A portion of this inves- tigation included modeling photographic lens performance under ideal and nonideal conditions using computer-based geometric ray tracing.These resultsprovide insightinto howvariousexperimental cone gurations can lead to spot size variation, which in turn affects the accuracy of PIV analysis. Based on such results, a system for alignment of the experiment was developed. That work is the pri- mary focus of this paper.

Measurement of Velocity and Turbulence Scale Near the Spark Plug by Using a Two-Color PIV Method.

It has been known that the in-cylinder flow field structure at the time of ignition timing has an effect on the shape and propagating speed of initial flame. Particle image velocimetry (PIV), a planar measuring technique, is a very efficient tool for studying complicated flow fields such as in-cylinder flow and intake port flow, and relies on fast and efficient method to track numerous particles suspended in the flow. In this study, two-color scanning PIV is designed to measure the two-dimensional velocity data by using a rotating mirror and a beam splitter. This PIV method gives the information about the complex turbulent flow and vorticity including tumbling flow by using a spatial filtering technique. From this experimentation, we obtained the few-mm turbulent integral length scales with a high spatial resolution and showed that the mean velocity and the turbulence strength increased linearly with the increase of engine speed.

Two-color digital PIV employing a single CCD camera

An extension of two color particle image velocimetry (PIV) is described where the color images are recorded onto a single high-resolution (3060×2036 pixel) color CCD sensor. Unlike mono-color CCD sensors, this system not only eliminates the processing time and the subsequent digitization time of film-based PIV but also resolves the directional ambiguity of the velocity vector without using conventional image-shifting techniques. For comparing the spatial resolutions of film and CCD data, a calibration experiment is conducted by recording the speckle pattern onto 35 mm color film and using a CCD sensor under identical conditions. This technique has been successfully implemented for simulated turbine film-cooling flows in order to obtain a more detailed characterization of the coolant-injection phenomenon and its interaction with freestream disturbances.

Manipulation of a jet in a cross flow

Abstract The effect of forcing on the mixing and velocity field of a jet in a cross flow is investigated for a jet-to-cross flow velocity ratio of R=1.0 using reactive-Mie-scattering flow-visualization and two-color Particle Image Velocimetry (PIV) techniques. Reactive-Mie-scattering images are used to assess the effect of forcing on molecular mixing, whereas double-exposed two-color PIV images are employed to obtain instantaneous velocity and corresponding vorticity distributions. The jet emanates from a square conduit and is manipulated by four piezoelectric actuators – one mounted along each side of the jet conduit near the jet-exit plane. Time-and phase-averaged distributions of velocity and rms velocity fluctuations in the x–y plane z=0 are also obtained. A novel approach is employed for processing jet and cross-flow fluids based on the size of the seeding particle. The present results show that when the jet is unforced, the “wake” of the jet contains a domain in which the magnitude of the velocity is very low compared to that of the cross-flow velocity, suggesting that the fluid within this domain is almost stagnant. When the jet is excited, the low-velocity domain in its wake is substantially reduced through vigorous mixing. Manipulation of the upstream and downstream segments of the jet shear layer also leads to an increase in the jet penetration into the cross flow (up to 30% as compared to the unforced case) and substantial mixing enhancement. Unlike in streamwise forcing where the jet fluid remains mainly in the central portion of the spanwise surface, spanwise forcing induces strong spanwise jet undulations.

Vortex-flame interactions and extinction in turbulent jet diffusion flames

Unsteady vortex-flame interactions and subsequent local extinction have been investigated in double-concentric turbulent methane jet diffusion flames stabilized on a thick-walled fuel tube using joint Mie scattering/thin-filament pyrometry (MS/TFP) and two-color particle image velocimetry (PIV). Particle images visualized the evolution and development of large-scale vortices in the internal fuel jet shear layer and external annulus air jet boundary. The real-time MS/TFP technique captured the events when the internal vortices poked through the flame zone, and the PIV measurements revealed the instantaneous velocity fields in flames interfered by the internal or external vortices. The visible (blue) flame zone turns luminous due to soot formation downstream (>5 jet diameter), where the flame bulges out in response to a radial movement of the internal or external vortices. By contrast, in the near-jet region where the flame zone was strained and relatively immobile, two modes of local extinction were observed: (1) interference by the internal jet-fluid vortices extending outward to the flame zone and (2) interference by the external annulus-fluid yortices perturbing air entrainment. In both cases, the entrainment velocity increased locally, and cold air passed through the quenched holes in the flame zone into the internal vortical structure in the intermittent mixing layer. Local flame extinction is attributed to two distinct unsteady mechanisms: (1) a sudden increase in the fuel diffusive influx as a result of a large Peclet number for mass transfer associated with the internal vortex and (2) an increase in the convective influx as a result of enhanced air entrainment due to the external vortex's squeezing motion.

Two-color particle image velocimetry technique for an internal combustion engine

A two-color particle image velocimetry (PIV) technique has been applied to a single-cylinder motored research engine. Two-color PIV is a quantitative planar velocity measurement technique that can unambiguously determine the velocity magnitude and direction. The work includes the development of an interrogation system, a series of computer simulations to determine the performance of the technique under various conditions, the comparison of these results to similar ones obtained for an autocorrelation PIV system, and a test of the technique by reconstructing the velocity field of a uniform jet flow. The technique was then applied to the in-cylinder flow field of a motored single-cylinder, cup-in-head, research engine. A total of 27 instantaneous velocity fields were obtained at a single measurement plane for a single operating condition of the engine. The data were analyzed to yield ensemble-averaged velocity and velocity fluctuation.