The growth of a grid-generated turbulent mixed layer in a two-fluid system

This paper describes a laboratory experiment designed to compare measurements with published theoretical ideas of the mixed-layer growth of a two-layer system in which the turbulence is induced by an oscillating grid. Experimental results show excellent agreement with an earlier theory by one of us (Long), in which the mixed-layer depth D_* measured from a virtual origin is given by $D_{*}\sim V_0^{-\frac{7}{11}}K^{\frac{9}{11}}t^{\frac{2}{11}}$, where K is action, t is time and V₀ is a characteristic velocity of the problem. The experiments also verify Long's theoretical entrainment relation E = α₂Ri^−7/4, where E is the entrainment coefficient and $Ri = D^3_{*}\Delta b/K^2$, and Δb is the buoyancy difference between the two layers. The interfacial-layer thickness was observed to be proportional to the depth of the mixed layer, as also predicted by Long. After a certain depth, the entrainment law tends to deviate from Long's theory. The deviation may be due to wall effects.