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*~ V₀-7/11K9/11t2/11 , where K is action, t is time and V 0 is a characteristic velocity of the problem. The experiments also verify Long's theoretical entrainment relation E = α 2 Ri −7/4 , where E is the entrainment coefficient and Ri = D³*Δ b/K² , and Δ b 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.