Tetsuo Saga

Standard images for particle-image velocimetry

Particle-image velocimetry (PIV) offers lots of advantages for studying fluid mechanics. Many PIV techniques and systems have been developed. However, no standard evaluation tool for evaluating the effectiveness and accuracy of the PIV systems has been established. To popularize PIV practically, for each PIV system there should be some means of evaluating the performance. PIV involves two processes, i.e. capturing the image for visualization and the image analysis. In order to evaluate the image analysis, the use of standard images has been proposed. Using these images, anybody can evaluate the effectiveness and accuracy of the PIV image analysis. The standard PIV images can be grouped into three categories, i.e. standard PIV images for two-dimensional, custom-made images with tunable parameters and images for a transient flow. The standard PIV images that we have developed are distributed via the web site http://www.vsj.or.jp/piv as part of a collaboration with the Visualization Society of Japan. They can be applied to investigate the performance of any PIV technique. The standard PIV images that we have developed have already been accessed by more than 3 000 researchers around the world.

A study on a lobed jet mixing flow by using stereoscopic particle image velocimetry technique

In a continuing effort to study the mixing enhancement by large-scale streamwise vortices generated by a lobed nozzle, a high-resolution stereoscopic particle image velocimetry (PIV) system was used in the present study to conduct three-dimensional measurements of air jet flows exhausted from a lobed nozzle and a conventional circular nozzle. The three-dimensional instantaneous and ensemble-averaged velocity fields, instantaneous and ensemble-averaged streamwise vorticity distributions and turbulent kinetic energy distributions were used to analyze the characteristics of the mixing process in the lobed jet flow compared with conventional circular jet flow. The existence of large-scale streamwise vortices in the lobed jet mixing flow is revealed clearly from the stereoscopic PIV measurement results. The instantaneous streamwise vorticity distributions revealed that the large-scale streamwise vortices generated by the corrugated trailing edge of the lobed nozzle break into smaller, but not weaker streamwise...

Development of Genetic Algorithm Based 3D-PTV Technique

A new 3D-PTV technique has been developed using GA (Genetic Algorithm). The measurement system consists of three CCD cameras, Ar-ion laser, an image grabber and a host computer. One-to-one correspondences are made by taking advantage of the combinatorial optimization of the genetic algorithm for the whole particles of the two images during the time interval of image frames. Two fitness functions are introduced in order to enhance the correspondences of the particles. One is based on the concept of continuum theory and the other one is based on the minimum error of threedimensional distance. Performance of the developed algorithm is tested using a set of virtual images constructed by the use of LES data set on an impinging jet. The developed 3D-PTV system is successfully applied to the measurement of flow characteristics of the wake of a circular cylinder.

Research on the vortical and turbulent structures in the lobed jet flow using laser induced fluorescence and particle image velocimetry techniques

The changes in vortical and turbulent structure in the near field (X / D < 3.0) of jet mixing flow caused by a lobed nozzle were investigated experimentally in the present study. The techniques of planar laser induced fluorescence and particle image velocimetry were used to accomplish flow visualization and velocity field measurements. The experimental results showed that, compared with a circular jet flow, lobed jet mixing flow was found to have a shorter laminar region, a smaller scale of the spanwise Kelvin-Helmholtz vortices, earlier appearance of small scale turbulent structures and a larger region of intensive mixing in the near field of the jet mixing flow. The central line velocity decay of the lobed jet mixing flow was also found to be much faster than that in a conventional circular jet. All these indicated that the lobed nozzle provides better mixing performance than does a conventional circular nozzle.

3D-PTV measurements of the wake of a sphere

3D-PTV measurements were conducted on the wake of a sphere. The Reynolds number is 1130 with diameter d = 30 mm. The measurement system consisted of three CCD cameras, an image grabber, an Ar-ion laser and a host computer. Structural analysis was adopted for the analysis on the wake using the obtained velocity profiles. Statistical turbulent properties were quantitatively clarified using the time-consecutive instantaneous three-dimensional vectors obtained by the 3D-PTV system. The spatial velocity profiles of the sphere wake seemed to have a strong relationship with the spiral motion of the vortices that were shed from the surface of the sphere.

Evaluation of the Cross Correlation Method by Using PIV Standard Images

Effects of various parameters for PIV image acquiring and processing on the final velocity field is studied by using PIV standard images (Okamoto et al., 1997) to evaluate the cross correlation method. The studied parameters include the size of interrogation window, the size of search window, the number of tracer particles, the diameter of tracer particles, out-of-plane velocity and average image velocity or the time interval between two images. In order to improve the PIV sub-pixel accuracy, the validity of the “sub-pixel interpolation” process also is discussed in the paper. Some useful conclusions are suggested for the optimal parameter selection for a final PIV result with high accuracy.

Mixing Process in a Lobed Jet Flow

A high-resolution stereoscopic particle image velocimetry (PIV) system was used to conduct three-dimensional measurement of an air jet exhausted from a lobed nozzle. The characteristics of the mixing process in the lobed jet e ow are revealed clearly and quantitatively from the stereoscopic PIV measurement results. The instantaneous velocity and streamwise vorticity distributions revealed that the large-scale streamwise vortices generated by the lobed nozzle break into smaller but not weaker streamwise vortices as they travel downstream. This is the proposed reason why a lobed nozzle would enhance both large-scale mixing and small-scale mixing reported by other researchers. The overall effect of the lobed nozzle on the mixing process was evaluated by analyzing the ensemble-averaged streamwise vorticity distributions. The strength of the ensemble-averaged streamwise vortices wasfoundto decayveryrapidlyoverthee rsttwo diameters (e rstsixlobeheights ),thento decayatamoremoderate rate farther downstream. The averaged turbulent kinetic energy proe le also indicated that most of the intensive mixing between the core jet and ambient e ow occurred within the e rst two diameters. These results indicate that the maximum effectiveness region for the lobed nozzle to enhance mixing is about the e rst two diameters of the lobed nozzle (e rst six lobe heights ).

Wavelet Multiresolution Analysis of Stereoscopic Particle-Image-Velocimetry Measurements in Lobed Jet

A wavelet-based vector multiresolution technique was developed and applied to a high-resolution stereoscopic particle image velocimetry system for studying the three-dimensional multiscale structurefeatures of the lobed jet mixing e ow. The instantaneous three-dimensional velocity vectors were successfully decomposed into large- and small-scale velocity e elds, and existence of very strong multiscale turbulent structures were cone rmed in the near e eld of the lobed jet. Within the central scale range of 16 mm, the instantaneous pairs of large-scale streamwise and horseshoe vortices can be clearly observed around the lobe peak regions and lobe troughs, respectively, and begin to breakdown and spread outward at downstream. The stronger small-scale streamwise vortices were found in the lobe regions, which e rst spread outward along the lobes and then develop to the whole measured e owe eld. These small-scale streamwise vortices also play an important role in the enhance mixing process. The stronger alternate positive and negative peaks of small-scale axial velocity component, which appear at the trailing edge of the lobed nozzle in the near e eld, indicate the existence of the Kelvin ‐Helmholtz or normal vortices.

Simultaneous Velocity and Concentration Measurements of a Turbulent Jet Mixing Flow

Abstract: A method for the simultaneous measurement of velocity and passive scalar concentration fields by means of particle image velocimetry (PIV) and planar laser induced florescence (PLIF) techniques is described here. An application of the combined PIV‐PLIF system is demonstrated by performing simultaneous velocity and concentration measurements in the near field of a turbulent jet mixing flow. The distributions of the ensemble‐averaged velocity and concentration, turbulent velocity fluctuation, concentration standard deviation, and the correlation terms between the fluctuating velocities and concentration in the near field of the turbulent jet flow are presented as the measurement results of the simultaneous PIV‐PLIF system.

Simultaneous measurements of all three components of velocity and vorticity vectors in a lobed jet flow by means of dual-plane stereoscopic particle image velocimetry

Results from an advanced particle image velocimetry (PIV) technique, named as dual-plane stereoscopic PIV technique, for making simultaneous measurements of all three components of velocity and vorticity vectors are presented for a lobed jet flow. The dual-plane stereoscopic PIV technique uses polarization conservation characteristic of Mie scattering to achieve simultaneous stereoscopic PIV measurements at two spatially separated planes. Unlike “classical” PIV systems or conventional stereoscopic PIV systems, which can only get one component of vorticity vectors, the present dual-plane stereoscopic PIV system can provide all three components of velocity and vorticity distributions in fluid flows instantaneously and simultaneously. The evolution and interaction characteristics of the large-scale streamwise vortices and azimuthal Kelvin–Helmholtz vortices in the lobed jet flow are revealed very clearly and quantitatively from the simultaneous measurement results of the dual-plane stereoscopic PIV system. A...