EFFECT OF MOLECULAR DIFFUSION ON
THE STRUCTURE OF A TURBULENT
DENSITY INTERFACE
1. INTROD[ ICTION
The dispersal of pollutants when released into the environment is often
affected by the itction of buoyancy forces either because the pollutant has a
different density from its immediate environment, or because there is
stratification already present in thc environment, or both. Consider, for
example. thc dispersion of smoke from a chimney stack; in general the
smoke is hotter (and therefore less dense) than its surroundings and the
buoyancy force so produced causes the smoke to rise in a turbulent plume,
mixing with the environment as it does so. If there is an inversion present in
the atmosphere above the stack, this rise is inhibited by the stable density
stratification. In thc ocean a similar phenomenon occurs with the oceanic
thermocline tending to prevent the downward turbulent mixing of pollutant
released above it.
On a smaller scale there are many observations of density stratification,
particularly in the ocean (see, e.g., Gregg and Cox, 1972), which show that
density microstructure does exist down to the scale of a few centimeters. A
feature of a large proportion of this microstructure and of the larger scale
structure mentioned above is that there exist relatively deep uniform layers
separated by thin interfaces across which there is a significantly larger
change in density. For structure with a vertical scale of the order of a few
centimeters. it is clear the molecular diffusion may play an important role in
its developnicnt. The determination of the role of molecular diffusion at such
an interface is the aim of this investigation.
This papcr considers the results o f laboratory experiments in which turbulent motions produced by the vertical oscillation of horizontal grids are
THE STRUCTURE OF A TURBULENT
DENSITY INTERFACE
1. INTROD[ ICTION
The dispersal of pollutants when released into the environment is often
affected by the itction of buoyancy forces either because the pollutant has a
different density from its immediate environment, or because there is
stratification already present in thc environment, or both. Consider, for
example. thc dispersion of smoke from a chimney stack; in general the
smoke is hotter (and therefore less dense) than its surroundings and the
buoyancy force so produced causes the smoke to rise in a turbulent plume,
mixing with the environment as it does so. If there is an inversion present in
the atmosphere above the stack, this rise is inhibited by the stable density
stratification. In thc ocean a similar phenomenon occurs with the oceanic
thermocline tending to prevent the downward turbulent mixing of pollutant
released above it.
On a smaller scale there are many observations of density stratification,
particularly in the ocean (see, e.g., Gregg and Cox, 1972), which show that
density microstructure does exist down to the scale of a few centimeters. A
feature of a large proportion of this microstructure and of the larger scale
structure mentioned above is that there exist relatively deep uniform layers
separated by thin interfaces across which there is a significantly larger
change in density. For structure with a vertical scale of the order of a few
centimeters. it is clear the molecular diffusion may play an important role in
its developnicnt. The determination of the role of molecular diffusion at such
an interface is the aim of this investigation.
This papcr considers the results o f laboratory experiments in which turbulent motions produced by the vertical oscillation of horizontal grids are