A color-coded particle tracking velocimeter with application to natural convection

Abstract

This paper introduces a novel concept of the image-processing technique to visualize a two-dimensional flow field and to generate streaks with embedded time marks in real time. Grey levels of a pixel are used in this work to encode the temporal information rather than the intensity of particle trajectories used in conventional image-processing systems. To do this, the incoming video image is first binarized by a step function predefined in an input look-up table, with the height of the step varying with time to bear temporal marks. A real-time arithmetic-logic unit and an image-frame buffer are used to accumulate the images. These processes result in continuous streaks with embedded time marks which have many advantages over those recorded by conventional techniques. Pseudo-color output look-up tables and color monitor provide more direct and intuitive means for human visual perception. This color-coded particle tracking velocimetry was applied to investigate natural convection in an enclosure heated from below with an isothermal extrusion. By quantifying the color-coded streaks, velocity vectors were obtained at a discrete number of points with less labor. The present technique provides an elaborate tool for simultaneous qualitative and quantitative realizations of steady and slowly varying flow felds.

Bichromatic particle streak velocimetry bPSV

“…The study by Wung and Tseng (1992) can be seen as an early precursor of the technique presented in this paper since this is the first time that temporal information was coded along the streak structure by changing the intensity of the illumination during exposure. In the recent past many approaches were introduced to extend this method that was originally developed to measure two velocity components in a plane (2D2C), to measure a third velocity component (Wung and Tseng 1992;Müller et al 2001) or volumetric data (Sinha and Kuhlman 1992;Rosenstiel and Rolf-Rainer Grigat 2010;Biwole et al 2009;Dixon et al 2011). A newly proposed method, published in Dixon et al (2011), even uses a holographic PSV technique to measure volumetric flow features.…”

“…On the other hand, strobing the illumination during one exposure creates a problem because the streak fractions generated by the strobing have to be matched correctly for the extraction of a velocity vector. A slightly different approach that does not need a matching of streak fractions was used by Wung and Tseng (1992) who change the illumination intensity asymmetrically during the exposure. In our approach, intensity information codes velocity information.…”