On the Techniques to Analyse the White Light Speckle Photograph to Obtain Velocity Fields

Abstract

Laser or white light speckle photograph method is one of the very useful techniques to measure velocity field. The speckle photographs are usually lighted by fine laser beams and generate the Young's fringes. Velocity vectors are analysed by counting the fringe spacings and directions using Fourier Transform method or the others. In this paper, Fast Fourier Transform (FFT) method is applied to analyse the speckle photographs directly. Two kinds of simplifying methods, named one dimensional FFT method and one dimensional compressing method, are also introduced to shorten the analysing time. Three methods are applied to analyse flow field around a circular cylinder. Obtained results are as follows; (1) Good agreements are obtained between the three methods except separated flow region. (2) The one dimensional compressing method spend significantly shorter analysing time, round 1/10, than the others.