Coherent structures in turbulent mixing layers

Coherent structures and the similarity characteristics of the turbulent mixing layer were the subjects of this experimental survey. The results achieved highlighted the fundamental changes in the mixing layer similarity characteristics beyond the mixing transition. The mixing transition was observed to depend on the achievement of a sufficiently high local Reynolds number and the completion of a minimum number of vortex amalgamations. The experiments carried out confirmed and extended to a wider range of experimental conditions the changes observed by Pedley (1990) and Coats (1992) beyond the mixing transition. In this region the growth of the individual structures present in the flow switched from the pre-transition instability driven mechanism to a process in which the individual vortices grow in a continuous and linear manner. In the post transition region of the flow the entrainment of irrotational fluid from the free-streams was seen to occur not in association with amalgamations but continuously as coherent structures grew linearly in a sort of procession along the length of the test section. The experimental results on the entrainment ratio were also obtained from flow visualisation records, applying a developed version of geometrical method, originally suggested by Dimotakis (1986). Comparisons were made between the measured similarity characteristics of the mixing layers studied and predictions from theoretical models. Statistical data on the observed intermittency of coherent structures in the post-transition layer were carried out. The study of the intermittency suggested that two distinctive flow conditions could occur in the post-transition mixing layer, one associated to the coherently structured flow, the other to the unstructured mixing layer. Each of these must have their own similarity characteristics. The key to the understanding of this result was the observed intermittency of the coherent structures. When the mixing layer studied switched from its structured to unstructured condition a shift of the mixing layer centreline towards the high speed stream and a smaller growth were observed.