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

The appearance of coherent fine scale eddies with the transition to turbule nce and their alignments with strain rates are investigated in turbulent mi xing layers using direct numerical simulations with different Reynolds numb ers. With the transition to turbulence, huge numbers of coherent fine scale eddies appear in turbulent mixing layers. The diameter and maximum azimuth al velocity of the coherent fine scale eddies are nine times the Kolmogorov microscale and half the scale of the rms velocity fluctuations for all Rey nolds number cases. Although the characteristics of the coherent fine scale eddies seem to be independent of Reynolds number, their spatial distributi on, especially for intense coherent fine scale eddies, becomes non-uniform and they make medium scale aggregations with the increase of Reynolds numbe r. 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 transitio n to turbulence, the most expected maximum and minimum eigenvalues of the s train rate tensor slightly decrease and increase, respectively. The eigenva lues at the centre of the fine scale eddies are of the order of mu'(rms)/la mbda and the most expected eigenvalue ratio is alpha:beta:gamma = -5:1:4. T he eigenvector of the minimum eigenvalue tends to be perpendicular to the r otating axis of the coherent fine scale eddies and the angle between the ro tating 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 l ayer coincide with those in homogeneous isotropic turbulence.