Reconstruction of transient three-dimensional density distributions using digital speckle tomography

A form of digital speckle tomography has been proposed for calculating transient three-dimensional density distributions. Multiple CCD images have captured movements of speckles due to the variation in density among three angles of view simultaneously and instantaneously so that asymmetrical and transient flows can be measured. Initially, the variations in density of one-nozzle and two-nozzle test sections have been investigated by the use of a cross-correlation method involving the speckle displacements. The results have been compared with measurements made by Mach-Zehnder interferometry to confirm the accuracy. Then, the speckle movements for downward flow of carbon dioxide from a circular small opening with a vertical partition (CO2-air exchange flow) have been obtained by the cross-correlation tracking method so that those distances can be transformed into deflection angles of laser rays for density gradients. The transient three-dimensional density fields have been reconstructed from the deflection angles by tomography.