Appearance and alignment with strain rate of coherent fine scale eddies in turbulent mixing layer

The appearance of coherent fine scale eddies with the transition to turbulence and their alignments with strain rates are investigated in turbulent mixing layers using direct numerical simulations with different Reynolds numbers. With the transition to turbulence, huge numbers of coherent fine scale eddies appear in turbulent mixing layers. The diameter and maximum azimuthal velocity of the coherent fine scale eddies are nine times the Kolmogorov microscale and half the scale of the rms velocity fluctuations for all Reynolds number cases. Although the characteristics of the coherent fine scale eddies seem to be independent of Reynolds number, their spatial distribution, especially for intense coherent fine scale eddies, becomes non-uniform and they make medium scale aggregations with the increase of Reynolds number. At the centre of coherent fine scale eddies, a large strain rate, which is about ten times the mean shear rate, can be observed. With the transition to turbulence, the most expected maximum and minimum eigenvalues of the strain rate tensor slightly decrease and increase, respectively. The eigenvalues at the centre of the fine scale eddies are of the order of mu'(rms)/lambda and the most expected eigenvalue ratio is alpha:beta:gamma = -5:1:4. The eigenvector of the minimum eigenvalue tends to be perpendicular to the rotating axis of the coherent fine scale eddies and the angle between the rotating axis and the eigenvector of the intermediate eigenvalue is less than 45 for about 70% of the fine scale eddies. These alignments at the centre of the coherent fine scale eddies in the fully-developed turbulent mixing layer coincide with those in homogeneous isotropic turbulence.