Measurement of zero-mean-shear turbulence in homogeneous and two-layer fluids

The results of an experimental study carried out to investigate the structure of turbulence in homogeneous fluid and near a shear-free density interface are presented. In the last case the experimental configuration consisted of a two-layer fluid medium in which the lower layer was maintained in a turbulent state by an oscillating grid. The measurements included the root-mean-square (r.m.s.) turbulent velocities, wave-number spectra, Eulerian frequency spectra of velocity and scalar (salinity) fluctuations, dissipation of turbulent kinetic energy and integral lengthscales. It was found that the introduction of a density interface to a turbulent flow can strongly distort the structure of turbulence near the interface. The modification of r.m.s velocities and spectra of both velocity and scalar fields is essentially limited to distances smaller than about an integral lengthscale. The effects due to interfacial waves appear to be important for distances less than about 10-20% of the integral lengthscale.