The Stratified Inclined Duct: a new canonical laboratory experiment to study ocean turbulence and mixing

We present a relatively recent laboratory experiment, the Stratified Inclined Duct (SID), that sustains a buoyancy-driven exchange flow and allows to accurately control and measure stratified sheared turbulence. The submesoscale turbulent mixing of momentum, heat, and salinity in the ocean have a leading-order but poorly constrained large-scale impact. We argue that SID may serve as a fruitful testbed for studying these processes, especially near boundaries, such as in estuaries.

First, we introduce SID, which consists of two large (400 litres) reservoirs containing salt solutions connected by a long rectangular duct. The long-lasting  exchange within the duct has a Reynolds number of order Re ~ 1,000-10,000. The apparatus can be tilted at a small angle θ with respect to the horizontal, which energises the flow and increases turbulence levels, due to the emergence of 'hydraulic control'.

Second, we present high-resolution experimental measurements of the three-dimensional velocity and density field within the duct which allow to delve into the energetics of stratified turbulence. SID uniquely allows the experimenter to control the level of turbulent kinetic energy dissipation, and to sweep through increasingly turbulent regimes by varying the key product Re*θ. The levels of turbulent intensity are comparable to those found in moderately turbulent patches in the ocean.

Third, we demonstrate that a data-driven analysis combining automated image analysis, data reduction and unsupervised clustering discovered previously unsuspected patterns in a large SID turbulence dataset. Multiple types of energetic turbulence were found, as well as intermittent turbulence that cycles between these types through distinct transition pathways. We argue that this data-driven identification of turbulence is a stepping stone towards better physics-based parameterizations.