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 timedependent
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 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.
