Measurements of diapycnal diffusivities in stratified fluids

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 L t , turbulent diffusivity for mass K ρ , rate of dissipation of turbulent kinetic energy ε, buoyancy frequency N and viscosity v , as time and volume averaged quantities. The behaviour of both L t and K ρ was characterized over a wide range of the turbulence intensity measure, ε/ vN 2 , and two regimes were identified. In the more energetic of these regimes (Regime E, where 300 1000, our measured diffusivities diverge from the model prediction. For example, at ε/ vN 2 ≈ 10 4 there is at least an order of magnitude difference between the measured and predicted diffusivities.