Tomo-PIV study of turbulence structure through a rapid contraction

Following the study of Mugundhan et al.1 in a 2.5:1 contraction, we perform Tomo-PIV and Shake-The-Box PTV measurements to investigate the 3-D vortical structures in the turbulent flow through a more rapid contraction. We use active grids in synchronous or random modes to reach turbulent Reynolds numbers Re_λ ≈220. We study three separate 1-D contractions, all with the same contraction ratio of 4, but widely different streamwise lengths. We also pass the turbulence through a contraction in two directions with an extreme area ratio of 16. Most earlier studies of similar configurations have used hot-wire anemometry, to measure point-wise velocity component time-series, while herein we obtain time-resolved volumetric velocity and vorticity fields, with up to 50,000 consecutive measurement volumes. This allows us to compare the evolution of the rms fluctuations to earlier measurements and to test the alignment and enhancement of coherent vertical structures through the strong extensional strain of the various contractions. Our focus is on how two-dimensionality of the flow is promoted by the strong mean strain. We also track individual structures to follow their stretching/compression and changing orientation with time.