THREE DIMENSIONALITY IN THE NEAR FIELD OF A ROUND JET

A round air jet (Re = 16000) was studied using Stereo PIV techniques to understand the characteristics of all three velocity components in the developing shear layer and to study the evolution of three dimensionality with increasing axial distance. Pulsed laser sheets were aligned to illuminate the centerline plane of the jet. The stereo PIV setup was suitably calibrated to translate the resulting pixel displacements into axial, radial, and azimuthal velocity components. The RMS of the azimuthal velocity is of the order of 0.1Vo in the shear layer downstream of Y/D ~ 0.5. This is significantly earlier than reported in previous studies. Individual fields show that both vortex cores and braids are three dimensional. However, the initial azimuthal perturbations typically were associated with straining regions immediately upstream of the first vortex ring that formed.