An experimental study of turbulent mixing near a density interface


The entrainment of a stably stratified layer into a turbulent mixed layer in a confined region has been studied in the laboratory for different Richardson numbers, Ri. The results include direct observation of turbulent characteristics and density measurements using two high precision salinity meters. The measurements include turbulence intensity, scale of eddies near the interface and energy spectrum of turbulence. For the step density profile we found a Ri-1 dependence for entrainment speed which is in accordance with the result of others. This changes to Ri-1.3 when the bottom layer has a linear density profile. In this case the internal waves generated at the interface propagate into the stratified fluid that may influence the entrainment process. In these experiments we find that the modal structure of these waves appears to interact with the turbulence processes near the interface creating a non-uniform entrainment rate usually in steps. This may be related to the vertical wave number of the dominant wave which is dependent on the depth of the stratified layer as well as the horizontal cross section of the tank. These are effects that have been considered for the first time here.