Two-point LDV measurements in a plane mixing layer

Investigations into the nature of the large structures in a two-dimensional shear layer were carried out using Doppler velocimetry in the GALCIT free-surface water tunnel. By simultaneous measurements of velocity two points outside the turbulent region, above and below the shear layer, it was possible to measure the (total circulation) and location of the vorticity center of the large structures. It was found that structures in the process of pairing convect downstream with the center of their cores close to the ray y/x along which mean velocity is given by Uₘ = l/2 (U₁ + U₁). The determined value of the mean circulation is consistent with independent measurements of the mean spacing between the structures. Results indicate that if the large vorticity distribution is elliptical, the inclination angle of its axis of symmetry with respect to the now is small.