Abstract
The structure and dynamics of turbulence in the presence of a free surface has been investigated by direct numerical simulations of a low Froude number, temporally growing, round turbulent jet which is issued below and parallel to a clean free surface. It is found that the free surface affects the dynamics of turbulence within a ‘surface layer’ of thickness on the order of one lateral Taylor microscale.Within this layer, the vertical velocity fluctuations are inhibited and the turbulence kinetic energy is redistributed to the horizontal components. Analysis of energy and transfer spectra within the the jet show the presence of a nearly two-dimensional turbulent state in the ‘surface layer’ characterized by a net reverse transfer of energy to the large scales and the presence of two distinct k -5/3 and k 3 subranges in the energy spectra. Within this surface layer, the dominant vortical stuctures are in the form of vortex tubes with axis normal to the free surface which are connected to the free surface. The lack of strong vortex stretching in these structures in the vicinity of the free surface leads to their longevity and the establishment of a nearly two-dimensional turbulent state within the ‘surface layer’.
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