The holographic particle-image velocimetry (HPIV) method of achieving highly resolved measurements of three-dimensional vector fields is being refined to make possible extensive investigations of the structure of turbulent wall flow. A PC-based interrogation system utilizing two-frame cross-correlation has been developed to analyze the volumetric images obtained by HPIV. The general optical set-up uses off-axis reference beams and illumination beams that propagate along the axial direction of the pipe very close to the wall. Side-scattering from the particles is used to record their images from two distinct views at two times on a multiplexed hologram. Restricting the volume that is illuminated enhances the signal-to-noise ratio and the spatial resolution. The signal-to-noise ratio and percentage of valid vectors versus illuminating beam diameter are computed to determine the system parameters necessary for optimal HPIV.
[Cj.02] Distortion Compensation for Generalized Stereoscopic Particle Image Velocimetry
Steven M. Soloff, Ronald J. Adrian, Zi-Chao Liu (University of Illinois at Urbana-Champaign)
Optical distortion caused by inaccurate optical alignment, lens nonlinearity, and/or refraction by optical windows, fluid interfaces, and other optical elements of an experiment causes inaccuracy by introducing variable magnification. Since fractional changes in the magnification have a one-to-one effect on the accuracy of measuring the velocity, it is important to compensate for such distortions. A general experimental calibration procedure is described which determines the magnification matrix of a distorted imaging system, and an algorithm is presented to compute accurate velocity field displacements from measurements of distorted PIV images. These procedures form a basis for generalized stereoscopic PIV procedures which permit easy electronic registration of multiple cameras and accurate recombination of stereoscopic displacement fields to obtain the three-dimensional velocity vector field.
[Cj.03] Advanced Off-axis Holographic Particle Image Velocimetry (HPIV) System
Y. Pu, Z. Huang, H. Meng (Laser Flow Diagnostics Lab, Mechanical Engineering Department, Kansas State University)
Off-axis Holographic PIV (HPIV) is a promising solution to the increasing demand for high spatial resolution 3D velocity field measurement. A fully automated off-axis HPIV system and data processing software have been implemented in Laser Flow Diagnostics Lab (LFD). High SNR achieved by the off-axis configuration relieves the demands for de-noising in the data processing stage and thus greatly improves the overall processing speed. The high SNR also brings a highly efficient, yet simple implementation of centroid finding algorithm. The core scheme of HPIV data processing, a fast "Concise Cross Correlation" (CCC) algorithm has been implemented which makes use of only particle centroid location and is thus much faster than conventional cross-correlation. Experimental data shows great promise of this system.
[Cj.04] Development of A Low Cost Automatic Holographic PIV System using In-line Recording Off-axis Viewing (IROV) with Artificial Intelligence (AI) Data Processing
J. Sheng, R. Elavarasan, H. Meng (Laser Flow Diagnostics, Mechanical Engineering Department, Kansas State University)
Holographic PIV (HPIV) is a promising 3D velocity field measurement technique providing high spatial-temporal resolution needed for understanding complex and turbulent flows. Due to the requirement of conventional PIV data processing scheme (high seeding density and signal-to-noise ratio), an HPIV system often evolves into a complicated off-axis configuration, which requires expensive equipment (injection-seeded pulsed laser), elaborate optical components, and accurate optical alignment. Although this configuration shows great potential of providing highly spatially resolved 3D instantaneous velocity fields, the complications involved prevent itself from being widely used. For this reason a low cost automatic HPIV technique is investigated. It combines In-line Recording Off-axis Viewing (IROV) holography with an Artificial Intelligence (AI) data processing system using Genetic Algorithm Particle Pairng (GAPP). The AI data processing lowers the restrictions on the holographic recording and reconstruction. IROV, furthermore, provides a holographic scheme to record 3D volumetric particle fields using low-cost equipment. In this work, different parameters of IROV configurations are studied for optimization. Moreover, an integrated hybrid automatic data processing system based on GAPP and correlation methods are developed. The complete system is used to study a traveling vortex.
This abstract was not submitted electronically.
[Cj.06] Particle Image Velocimetry for Microfluidics
Carl Meinhart, Richard Chiu (University of California - Santa Barbara), Juan Santiago (University of Illinois)
A micro PIV system is being developed to measure velocity vectors with spatial resolutions of the order of several microns. Advancements in microfabrication technologies have facilitated the development of many new microfluidic devices. These devices present new and challenging fluid problems at the micro-scale, which are not present at the macro-scale. In general, analysis of fluid problems at the micron-scale have often been limited to measuring only bulk properties of the flow field. Lanzillotto et al. (1996) have used x-ray tomography to obtain whole-field velocity measurements in 500 - 1000 micron diameter tubes. They report velocity-vector spacings of roughly 40 microns. The current research is an attempt to extend the super-resolution PIV technique of Keane et al. (1995), to address fluid mechanics problems in microfabricated devices. Here, 50 - 200 nm diameter particles are imaged using an epi-fluorescent microscope and a scientific-grade CCD camera. Particles are illuminated by a continuously emitting Mercury arc lamp or by a pulsed Nd:YAG laser. The spatial resolution of optical techniques are fundamentally limited by the diffraction of light. We will discuss the ultimate spatial and temporal resolution limits of PIV for micro-flows, and report recent experimental.
[Cj.07] Turbulent Flow Investigations Using High Resolution Planar Holographic Particle Image Velocimetry
J. Kostas, J. Soria (Monash University, AUSTRALIA), A. Lozano (LITEC-CSIC, SPAIN)
Particle Image Velocimetry (PIV) is becoming a well established experimental technique to measure two components of the velocity in a planar region of a flow field. An experimental method will be described which shows how its proven capabilities can be further extended by using holographic recording to register the particle displacements. Among other unique characteristics, holography enables the acquisition of multiple images on a single plate, and the recording of three dimensional images. These features coupled with cross-correlation digital PIV (CCDPIV) analysis of the digitised reconstructed hologram are used to circumvent some of the limitations of conventional PIV. Some of these possibilities are demonstrated in this study by applying the technique to a high Reynolds number swirling flow using a lens-less off-axis orthogonal recording geometry.
[Cj.08] Measurement of Velocity Field Spectra by means of PIV
B. Alkislar, L. Lourenco (Department of Mechanical Engineering, Florida Aamp;M University and Florida State University)
A Particle Image Velocimeter has been developed for the measurement of field velocity spectra. The velocity spectra is computed from the velocity autocorrelation function, which is obtained by correlating corresponding velocity fields at variable time delays. The PIV system features a single microcomputer that controls the dual Kodak ES1.0 camera arrangement, and is capable of acquiring up to 128 image pairs, per camera, at a maximum frame rate of 30 Hz. The system also synchronizes a pair of dual Nd-Yag lasers for the flow illumination. Using this system field and pointwise spectra have been obtained and compare very well with those obtained by means of Hot Wire Anemometry. The special requirements in the PIV velocity measurement accuracy as well as the system's limitations due to quantization noise are discussed.
[Cj.09] Developments Of A Large Scale Three-Dimensional Particle Image Velocimetry System
S.M. Walker (Stanford University), L. Lourenco, A. Krothapalli (FSU)
A method for performing three-dimensional fluid velocity measurements has been developed using the Particle Image Velocimetry, PIV, measurement technique. Using PIV in conjunction with the photographic method commonly referred to as the Scheimpflug condition, it is possible to focus on a plane in space from an oblique angle. A two camera PIV system then allows the resolution of a fully three-dimensional flow field. This method was developed primarily to permit the use of Off- Axis PIV systems in large wind tunnels. In this application of the method it is intended to be used for two-dimensional large scale measurements. The technique has been demonstrated using small scale three dimensional flows.
[Cj.10] Studying turbulence and subgrid-scale motion using three-dimensional holographic PIV measurements
Bo Tao, Joseph Katz, Charles Meneveau (Department of Mechanical Engineering and Center for Environmental and Applied Fluid Mechanics, The Johns Hopkins University)
An off-axis holographic PIV system has been developed for the study of turbulence and complex flow phenomena. An automated image processing system is used for measuring the velocity distribution of a fully developed turbulent flow in a square duct at Re_h = 1.23 \times 10^5. A single 3-D vector map consists of 97x97x87 vectors. Improvements in analysis procedures using image compression enable reconstruction of the hologram at a magnification of 500 pixels/mm and about 100 pixel displacements between exposures, improving accuracy without payoffs in analysis time. The data is used in a study of subgrid-scale motion and modeling. The 3-D structure of fields such as subgrid dissipation and subgrid stress divergence is evaluated, giving a significantly more complete picture than our previous 2-D studies. As a preliminary step, it is shown that quantities such as the mean energy dissipation, its modeled counterpart using existing models such as the Smagorinsky and similarity models, and correlation coefficients, are in the expected range of values. Qualitative observations on the structure of the SGS force field are made.