A Comparison Between Analog LDA, Photon Correlation LDA and Rate Correlation Techniques

In this paper, four different LDA system configurations are compared with respect to their application to time-periodic flows. The four configurations are: (i) a phase-locked loop analog tracker followed by a multichannel analyzer which give the distribution of velocity and acceleration, (ii) a digital correlator that produces the photon correlation function which yields the velocity distribution as well, (iii) an Erdmann/Gellert type one beam rate correlation setup that measures velocity correlations, and (iv) an extended rate correlation setup using cross beam geometry and Bragg shift frequency offset for the selection of single velocity components. The comparison includes measurement time, computational effort and the applicability to special flow situations. As a test flow we used the well known Taylor vortices, which show various flow patterns depending on the Reynolds number, especially a region where the velocity varies periodically.