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INVESTIGATION OF ENSTROPHY PRODUCTION IN THE FAR FIELD OF AN AXISYMMETRIC TURBULENT JET

Cinematographic stereoscopic particle image velocimetry (PIV) was used to extract three dimensional velocity and velocity gradient information on fine scale turbulence in the far field of an axisymmetric jet. The Reynolds number based on the Taylor micro-scale was 150. The interaction between strain and rotation in the production of enstrophy is examined by observing the alignment of the vorticity vector with the principal axes of the rate of strain tensor. The vorticity vector is generally aligned with the intermediate strain rate, consistent with various other studies in the literature. However, in regions of intense vortex stretching, the vorticity vector shows a tendency to align with the extensive strain rate. These regions of intense vortex stretching are found in the vicinity of locally swirling regions (i.e. regions where the local velocity gradient tensor has a complex conjugate pair of eigenvalues). Moreover, instantaneous iso-surfaces of intense vortex stretching regions show the presence of “sheetlike” structures that are characterised by positive (extensive) intermediate strain rate. Statistical results based on probability distributions also confirm this behaviour. A possible physical mechanism is proposed whereby enstrophy is produced in weakly swirling regions and subsequently evolve under the action of the strain rate tensor to form “tube” or “worms”.