A note on spanwise structure in the two-dimensional mixing layer

The lateral extent of the large-scale features of a high-Reynolds-number, turbulent mixing layer are studied by means of an array of twelve hot wires placed across the span of the wind tunnel. Correlation measurements show that the large-scale structure rapidly approaches an asymptotic state, in which the lateral correlation length becomes proportional to the local mixing-layer thickness. Visualizations of the instantaneous hot-wire outputs show the large structure to extend across the wind tunnel, but to contain spanwise irregularities. It is argued that these spanwise irregularities are produced by the pairing interactions between adjacent vortices.