Regime transitions and energetics of sustained stratified shear flows

We describe the long-term dynamics of laboratory sustained stratified shear flows relevant to geophysical flows. The stratified inclined duct (SID) experiment sets up a sustained two-layer exchange flow in an inclined duct connecting two reservoirs containing salt solutions of different densities. Varying the two key parameters θ and Re (the tilt angle of the duct and the Reynolds number based on the density difference driving the flow) leads to four qualitatively different regimes: laminar flow; mostly laminar flow with Holmboe waves; spatio-temporally intermittent turbulence; and vigorous interfacial turbulence. In this talk we provide a quantitative basis for this regime classification and explain the power law scaling of the transitions in the (θ,Re) plane. We employ (i) newly-available, state-of-the-art simultaneous volumetric measurements of the density field and the three-component velocity field; (ii) time- and volume-averaged potential and kinetic energy budgets. We show and explain how regime transitions are caused by an increase in the non-dimensional time- and volume-averaged kinetic energy dissipation within the duct, which scales with the non-dimensional group θRe.