Development of quasi two-dimensional structures in a shallow free-surface mixing layer

 The large quasi two-dimensional turbulence structures that emerge in a shallow mixing layer are studied experimentally using Laser Doppler Anemometry. Velocity profiles and turbulence intensities are measured in the first two meters downstream of the splitter plate. In contradistinction with previous experiments, it is shown that the initial growth rate and the turbulence intensities of the shallow mixing layer compare well with deep-water plane mixing layers. Two-point measurements allowed for the determination of spatial correlations of the fluctuating velocity components. The large eddies were found to extend from one tenth of the water depth up to the free surface while the streamwise size of the eddies was found to be three times the mixing layer width. The two-dimensional character of the large structures and the associated reversed energy cascade is inferred from the power spectra of the lateral velocity component. Received: 2 April 1997/Accepted: 25 August 1997