A two-layer salt-stratified tank of water was mixed by turbulence generated by many excursions of a horizontally moving vertical rod. The objective is to observe the time-dependent response of the mean density field for ranges of Richardson number Ri>0.9 and Reynolds Number Re>600. As the density profile of the fluid gradually evolves from a single step to a mixed state over a wide range of time, there is an almost perfect collapse of all the profiles to one universal profile as a function of a similarity variable. Although the turbulent diffusion is not constant, the value in the limit of small stratification has similar magnitude to values found by others.

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