Linearly stratified salt solutions of different Prandtl number were subjected to turbulent
stirring by a horizontally oscillating vertical grid in a closed laboratory system.
The experimental set-up allowed the independent direct measurement of a root mean
square turbulent lengthscale Lt, turbulent diffusivity
for mass Kρ, rate of dissipation
of turbulent kinetic energy ε, buoyancy frequency N and viscosity v, as time and
volume averaged quantities. The behaviour of both Lt
and Kρ was characterized over a wide range of the turbulence intensity measure,
ε/vN2, and two regimes were identified.
In the more energetic of these regimes (Regime E, where
300 < ε/vN2 < 105), Lt
was found to be a function of v, κ and N, whilst Kρ
was a function of v, κ and (ε/vN2)1/3.
From these expressions for Lt and Kρ,
a scaling relation for the root mean square turbulent velocity scale Ut was derived,
and this relationship showed good agreement with direct measurements from other data sets.
In the weaker turbulence regime (Regime W, where
10 < ε/vN2 < 300) Kρ was a
function of v, κ and ε/vN2.
For 10 < ε/vN2 < 1000, our directly measured diffusivities,
Kρ, are approximately a factor of 2 different to the diffusivity predicted
by the model of Osborn (1980). For ε/vN2 > 1000, our measured diffusivities
diverge from the model prediction. For example, at ε/vN2 ≈ 104
there is at least an order of magnitude difference between the measured and predicted diffusivities.