How Has the Study of Coherent Structures Contributed to Our Understanding of Turbulent Free Shear Flows? (Keynote Paper)

Recent applications of the ‘slice POD’ to the axisymmetric turbulent wake and jet are reviewed, and the results used to provide a critique of commonly held views about these flows. It is argued that the so-called ‘coherent structures’ are simply artifacts of the source conditions, and have little to do with the far down-stream development of these flows. Also, experimental evidence is presented for the possible presence of Townsend’s large eddies, eddies whose primary role is to warp the mean motion. Finally, classical linear stability analysis which ‘predicted’ that only azimuthal mode-1 could be unstable for these flows is shown to be deficient, with the result that at very least modes-0, 1 and 2 could be important, consistent with the experiments.