ON THE ORIGIN OF THE SURFACE CURRENT IN TURBULENT FREE-SURFACE FLOWS

In this study, the interaction with a free surface of an initially axi symmetric jet issuing beneath and parallel to the surface was examined . The purpose was to determine the origin of the 'surface current' - t he large outward velocity which exists in a thin layer adjacent to the surface. Using the equations of mean motion, it is shown that near th e surface, outward acceleration results from the balance between a pos itive contribution from the lateral Reynolds-stress gradients and a ne gative contribution from the lateral pressure gradient. The local pres sure field near the free surface is shown to be largely determined by the local Reynolds-stress field. Combining these results shows that th e lateral acceleration which results in the surface current is related to the Reynolds-stress anisotropy near the surface. The results indic ate that there should be roughly a three-fold increase in the lateral growth rate of the jet near the free surface and a similar increase in the outward velocity, when compared to a deep jet. Comparison to avai lable experimental data showed that the maximum outward velocity was c onsistent with the theory, and that the lateral scale of the surface-c urrent layer was roughly double that of the deep jet, slightly smaller than expected. The near-surface stress anisotropy was shown to be rel ated to the interaction of vorticity with the free surface. This indic ates that the results of this study are consistent with earlier explan ations of the surface current in terms of vortex/free-surface interact ion.