On the Stratification of a Linearly Stratified Liquid under the Influence of Uniform Vertical Mixing (Laboratory Experiment)

Abstract

A series of experiments were carried out on mixing a water layer with an initially constant vertical salinity (density) gradient using oscillating vertical rods, creating a uniform turbulent effect throughout the entire thickness of the water layer. As a result of mixing, in most experiments a stepwise structure was formed, in the form of a sequence of quasi-homogeneous layers separated by highly gradient interlayers. The stepped structure in the density field was observed using a shadow device (schlieren method). In some experiments, the stepped structure was not formed, and stratification, despite mixing, was characterized by a constant vertical density gradient. In each experiment with stratification, measurements of the thicknesses of quasi-homogeneous layers were made. A self-similar dependence of the dimensionless layer thickness on the Richardson number was established. In dimensional form, this dependence indicates that the thickness of the quasi-homogeneous layer is proportional to the product of the amplitude of the oscillation of the rods by the ratio of the kinetic energy of the turbulent effect to the potential energy of stratification. A diagram is presented that allows one to judge the regularity of layer formation at different values of Reynolds and Richardson numbers. The experiments conducted have shown that not only differential-diffusion convection, but also prolonged mechanical mixing under certain conditions can lead to stepwise stratification of a stably stratified aqueous medium.