Large Structure Dynamics and Entrainment in the Mixing Layer at High Reynolds Number

Observations were made on a turbulent mixing layer in a water channel at Reynolds numbers up to 3 x 10^6 . Flow visualization with dyes revealed (once more) large coherent structures and showed their role in the entrainment process; observations of the reaction of a base and an acid indicator injected on the two sides of the layer, respectively, gave some indication of where molecular mixing occurs. Autocorrelations of streamwise velocity fluctuations, using an LDV, revealed a fundamental periodicity associated with the large structures. The surprisingly long correlation times suggest time scales much longer than had been supposed; it is argued that the mixing layer dynamics at any point is coupled to the large structure further downstream, and some possible consequences about the effects of initial conditions and,. of the influence of apparatus geometry are discussed.