Particle Image Velocimeter Research Articles

Directionally-unambiguous, digital particle image velocimetry studies using a image intensifier camera

The paper describes the implementation of a Digital Particle Image Velocimeter (in the Particle Tracking mode) using an advanced, ultra-high speed gated, image intensifier, monochrome video camera. The camera was controlled locally by an internal computer chip with which the image capture and analysis software suite could communicate. The software was mounted on a PC and the captured images were stored on a (PC) frame board, which was synchronised with the camera. The camera could be triggered by external events or by keyboard or software prompt. The frame grabber had on-board storage for four frames of 512 by 512 pixels, each having a resolution of 256 grey levels. This allowed the implementation of a flexible and cost effective DPIV system with a wide dynamic range. Examples are given of the use of the system at very low illumination levels and in the presence of flow directional ambiguity associated with gravity wave motion. Phase sampling in a periodic flow is also demonstrated. The image intensifier hardware and control software allowed multiple exposures, of different duration, within the same digital frame. The camera sensitivity could be adjusted to suit different levels of illumination. This feature is shown to allow the capture of DPIV images with embedded time signatures.

Phase-conjugate holographic system for high-resolution particle-image velocimetry

A novel holographic particle-image velocimeter system has been developed for the study of threedimensional (3-D) fluid velocity fields. The recording system produces 3-D particle images with a resolution, a signal-to-noise ratio, an accuracy, and derived velocity fields that are comparable to high-quality two-dimensional photographic particle-image velocimetry (PIV). The high image resolution is accomplished through the use of low f-number optics, a fringe-stabilized processing chemistry, and a phase conjugate play-back geometry that compensates for aberrations in the imaging system. In addition, the system employs a reference multiplexed, off-axis geometry for the determination of velocity directions with the cross-correlation technique, and a stereo camera geometry for the determination of the three velocity components. The combination of the imaging and reconstruction subsystems makes the analysis of volumetric PIV domains feasible.

On-line particle-image velocimeter: an integrated approach

We describe an on-line particle-image velocimeter (PIV) and its implementation for the measurement of the velocity field in steady and unsteady flows. The instrument uses up-to-date solid-state sensors, dedicated electronic boards, and digital microcomputers to combine the image-acquisition and processing steps into a single, fully integrated unit. A calibration experiment evaluates the accuracy of the velocity measurement instrument. The experiment consists of the measurement of the velocity distribution of a laminar wall jet and shows that the velocity, as well as the vorticity profiles, obtained by using the on-line PIV compare well with those obtained by means of a conventional photographic PIV and a numerical prediction.

Polarization properties of a photorefractive Bi/sub 12/SiO/sub 20/ crystal and their application in an optical correlator

The polarization properties of Bi/sub 12/SiO/sub 20/ (BSO) crystals are investigated in detail theoretically and experimentally, and the results are used to describe the operation of an optical correlator for a particle image velocimeter (PIV) using a BSO crystal as the nonlinear optical element. The work is based on an extension of the optical beam-propagation (OBP) method to include all the significant optical properties of the BSO crystal when used in a two-wave mixing configuration, i.e., optical activity, field-induced birefringence, and anisotropic diffraction. The model is able to handle multiple gratings where the input beams do not have to be symmetric about the axis of propagation. Using the numerical model the polarization properties of the BSO crystal are analyzed and the operation of the correlator is explained. The model is able to take into account self-diffraction effects, and it is shown that these effects can have a significant influence in the setup employed for the optical correlator even when the diffraction efficiency is low. The predictions of the numerical model are verified by extensive experiments on the polarization state of the output of the correlator as a function of operating conditions and of the polarization state of the input beams.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Photorefractive particle image velocimetry: performance enhancement with bismuth silicon oxide crystals

The polarization properties of diffraction from bismuth silicon oxide crystals are used to enhance the signal-to-noise ratio of a photorefractive particle image velocimeter. We explain these diffraction characteristics by using a simple picture of the processing steps and the beam-propagation method, accounting for the multiplicity of the recorded gratings. Experimental measurements are also presented.

Optimization of particle image velocimeters: II. Multiple pulsed systems

For pt.I see ibid., vol.1, p.1202 (1990). The spatial resolution, detection rate and reliability of a particle image velocimeter depend critically upon careful selection of a number of parameters of the PIV system and the fluid motion. The set of six non-dimensional parameters that were most significant in optimizing double pulsed PIV performance is generalized for a multiple pulsed system and shown to be similarly significant. They are the data validation criterion, the particle image density, the relative in-plane image displacement, the relative out-of-plane displacement, the velocity gradient and the ratio of the mean image displacement to the interrogation spot diameter. It is shown that a single transformation can be applied to the method of interrogation by autocorrelation analysis to extend these results to interrogation by two dimensional Fourier transform analysis of the Young's fringes. Thus, the non-dimensional parameters are studied for the case of interrogation by autocorrelation analysis. It is shown that optimal parameters for a multiple pulsed system can be generalised from earlier results for a double pulsed system.

Optimization of particle image velocimeters. I. Double pulsed systems

The spatial resolution, detection rate, accuracy and reliability of a particle image velocimeter (PIV) depend critically upon the careful selection of a number of parameters of the PIV system and the fluid motion. An analytical model and a Monte Carlo computer simulation have been developed to analyse the effects of experimental parameters and to optimize the system parameters. A set of six nondimensional parameters that are the most significant in optimizing PIV performance are identified. They are the data validation criterion, the particle image density, the relative in-plane image displacement, the relative out-of-plane displacement, a velocity gradient parameter, and the ratio of the mean image diameter to the interrogation spot diameter. These parameters are studied for the case of interrogation by autocorrelation analysis. By a single transformation, these results can be applied to interrogation by two-dimensional Fourier transform analysis of the Young's fringes.

A directionally sensitive particle image velocimeter

Measurements of flow velocity by the particle image velocimetry (PIV) technique have a directional ambiguity. In common with the laser Doppler anemometer, there is an upper limit of turbulence measurable without spectral distortion. An image shifting instrument is described which enables both the directional ambiguity of the flow to be resolved and the upper limit of measurable turbulence to be increased.

Double exposure, multiple-field particle image velocimetry for turbulent probability density

A particle image velocimeter method is described in which double exposed fields of particles moving in a two-dimensional slice of a steady turbulent flow are photographed repeatedly to build up a statistical ensemble of flow field realizations on a single photographic plate. Each interrogation spot on the plate contains a sample of the probability density function of the two components of velocity that lie in the photographic object plane, assuming paraxial photography. Theory is developed showing how this sample can be measured by two-dimensional spatial correlation analysis, followed by deconvolution to remove the effects of finite particle image size. The probability density measurements are biased inherently against large velocities, but these effects can be minimized and/or corrected.