On the vortical structure in a round jet

Turbulent vortical structures in a round free jet of water were experimentally visualized by using stereo particle image velocimetry (PIV). A laser light sheet illuminated a cross-sectional plane normal to the axis of the jet, and two charge-coupled device cameras captured particle images in the same region of interest but from different directions. The stereo-PIV algorithm had been applied to obtain two-dimensional, three-component (2D-3C) velocity distributions on various cross-sectional planes along the axis downstream. All nine components of the velocity gradient tensor were reconstructed from time-dependent 2D-3C velocity data by locally assuming Taylor's frozen field hypothesis based on the convective velocity evaluated from the mean flow profile. Isosurfaces of the swirling strength lambda(i) revealed that the existence of a group of hairpinlike vortex structures was quite evident around the rim of the shear layer of the jet. The center of curvature of the head of the hairpin was typically observed around r/b=1.5, and the azimuthal spacing between the legs of the hairpin was roughly 0.9b. A similar hairpin structure was estimated by linear stochastic estimation. The typical spacing between the legs of the estimated hairpin was 0.65b, which is generally constant over the range of Re=1500-5000. (C) 2005 American Institute of Physics.